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Calculated Shard: 14 (from laksa192)

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curl -X POST \
  'http://laksa014.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://www.redhat.com/en/blog/bash-scripting-loops\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.redhat.com/en/blog/bash-scripting-loops\'))) FORMAT JSONEachRow'

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{"found_url":"https:\/\/www.redhat.com\/en\/blog\/bash-scripting-loops","crawl_time":1772815079,"first_indexed_time":1733211744,"http_code":200,"src_unparsed":"com,redhat!www,\/en\/blog\/bash-scripting-loops s443","src_root_hash":"4780968593380432814","history_drop_reason":null,"meta_title":"Introduction to Linux Bash programming: 5 `for` loop tips","meta_descriptions":["Every sysadmin probably has some skill they've learned over the years that they can point at and say, \"That changed my world.\" That skill, or that bit of inf..."],"attrs_boilerpipe_text":"Every sysadmin probably has some skill they've learned over the years that they can point at and say, \"That changed my world.\" That skill, or that bit of information, or that technique just changed how I do things. For many of us, that thing is\nlooping\nin Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the\nfor\nloop, though.\nIf you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the\nfor\nloop is definitely the right tool for the job.\n[ You might also like: \nMastering loops with Jinja templates in Ansible\n]\nIf you already have a programming or scripting background, you're probably familiar with what\nfor\nloops do. If you're not, I'll try to break it down in plain English for you.\nThe basic concept is:\nFOR\na given set of items,\nDO\na thing.\nThe given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a\nfor\nloop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script.\nBasic structure of the for loop\nFirst, let's talk about the basic structure of a\nfor\nloop, and then we'll get into some examples.\nThe basic syntax of a\nfor\nloop is:\nfor <variable name> in <a list of items>;do <some command> $<variable name>;done;\nThe\nvariable name\nwill be the variable you specify in the\ndo\nsection and will contain the item in the loop that you're on.\nThe\nlist of items\ncan be anything that returns a space or newline-separated list.\nHere's an example:\n$ for name in joey suzy bobby;do echo $name;done\nThat's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable\n$name\nwill contain the item in the list that the loop is currently operating on, and once the command (or commands) in the\ndo\nsection are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between\ndo\n and\ndone\n will be executed. New commands just need a\n;\ndelimiting them.\n$ for name in joey suzy bobby; do echo first $name;echo second $name;done;\nfirst joey\nsecond joey\nfirst suzy\nsecond suzy\nfirst bobby\nsecond bobby\nNow for some\nreal\nexamples.\nRenaming files\nThis loop takes the output of the Bash command\nls *.pdf\nand performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension).\nfor i in $(ls *.pdf); do\nmv $i $(basename $i .pdf)_$(date +%Y%m%d).pdf\ndone\nTo illustrate, run this loop in a directory containing these files:\nfile1.pdf\nfile2.pdf\n...\nfileN.pdf\nThe files will be renamed like this:\nfile1_20210210.pdf\nfile2_20210210.pdf\n...\nfileN_20210210.pdf\nIn a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them.\nExtrapolating lists of items\nImagine that you have a file that you want to\nscp\nto several servers. Remember that you can combine the\nfor\nloop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example:\n$ echo {0..10}\n0 1 2 3 4 5 6 7 8 9 10\nAssuming your servers are named in some sort of pattern like,\nweb0\n,\nweb1\n,\nweb2\n,\nweb3\n, you can have Bash iterate the series of numbers like this:\n$ for i in web{0..10};do scp somefile.txt ${i}:;done;\nThis will iterate through\nweb0\n,\nweb1\n,\nweb2\n,\nweb3\n, and so forth, executing your command on each item.\nYou can also define a few iterations. For example:\n$ for i in web{0..10} db{0..2} balance_{a..c};do echo $i;done\nweb0\nweb1\nweb2\nweb3\nweb4\nweb5\nweb6\nweb7\nweb8\nweb9\nweb10\ndb0\ndb1\ndb2\nbalance_a\nbalance_b\nbalance_c\nYou can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example,\nweb-us-0\nwould be the first web server in the US data center, while\nweb-ca-0\nwould be\nweb 0's\ncounterpart in the CA data center. To execute something on both systems, you can use a sequence like this:\n$ for i in web-{us,ca}-{0..3};do echo $i;done\nweb-us-0\nweb-us-1\nweb-us-2\nweb-us-3\nweb-ca-0\nweb-ca-1\nweb-ca-2\nweb-ca-3\nIn case your server names are not easy to iterate through, you can provide a list of names to the\nfor\nloop:\n$ cat somelist\nfirst_item\nmiddle_things\nfoo\nbar\nbaz\nlast_item\n\n$ for i in `cat somelist`;do echo \"ITEM: $i\";done\nITEM: first_item\nITEM: middle_things\nITEM: foo\nITEM: bar\nITEM: baz\nITEM: last_item\nNesting\nYou can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example.\nYou can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested\nfor\nloop gets executed on every iteration of the parent\nfor\nloop. Be sure to specify different variable names for each loop.\nTo copy the list of files\nfile1.txt, file2.txt,\nand\nfile3.txt\nto the web servers, use this nested loop:\n$ for i in file{1..3};do for x in web{0..3};do echo \"Copying $i to server $x\"; scp $i $x; done; done\nCopying file1 to server web0\nCopying file1 to server web1\nCopying file1 to server web2\nCopying file1 to server web3\nCopying file2 to server web0\nCopying file2 to server web1\nCopying file2 to server web2\nCopying file2 to server web3\nCopying file3 to server web0\nCopying file3 to server web1\nCopying file3 to server web2\nCopying file3 to server web3\nMore creative renaming\nThere might be other ways to get this done, but remember, this is just an example of things you\ncan\ndo with a\nfor\nloop. What if you have a mountain of files named something like\nFILE002.txt\n, and you want to replace\nFILE\nwith something like\nTEXT\n. Remember that in addition to Bash itself, you also have other open source tools at your disposal, like\nsed\n,\ngrep\n, and more. You can combine those tools with the\nfor\nloop, like this:\n$ ls FILE*.txt\nFILE0.txt  FILE10.txt  FILE1.txt  FILE2.txt  FILE3.txt  FILE4.txt  FILE5.txt  FILE6.txt  FILE7.txt  FILE8.txt  FILE9.txt\n\n$ for i in $(ls FILE*.txt);do mv $i `echo $i | sed s\/FILE\/TEXT\/`;done\n\n$ ls FILE*.txt\nls: cannot access 'FILE*.txt': No such file or directory\n\n$ ls TEXT*.txt\nTEXT0.txt  TEXT10.txt  TEXT1.txt  TEXT2.txt  TEXT3.txt  TEXT4.txt  TEXT5.txt  TEXT6.txt  TEXT7.txt  TEXT8.txt  TEXT9.txt\nAdding a for loop to a Bash script\nRunning\nfor\nloops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include\nfor\nloops as part of your Bash scripts for increased power, readability, and flexibility.\nFor example, you can add the nested loop example to a Bash script to improve its readability, like this:\n$ vim copy_web_files.sh\n# !\/bin\/bash\n\nfor i in file{1..3};do\n  for x in web{0..3};do\n    echo \"Copying $i to server $x\"\n    scp $i $x\n  done\ndone\nWhen you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain.\n$ bash copy_web_files.sh\nCopying file1 to server web0\nCopying file1 to server web1\n... TRUNCATED ...\nCopying file3 to server web3\nYou can also increase the flexibility and reusability of your\nfor\nloops by including them in Bash scripts that allow parameter input. For example, to rename files like the example\nMore creative renaming\nabove allowing the user to specify the name suffix, use this script:\n$ vim rename_files.sh\n# !\/bin\/bash\n\nsource_prefix=$1\nsuffix=$2\ndestination_prefix=$3\n\nfor i in $(ls ${source_prefix}*.${suffix});do\n  mv $i $(echo $i | sed s\/${source_prefix}\/${destination_prefix}\/)\ndone\nIn this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with\nFILE\n, of type\n.txt\nto\nTEXT\n, execute the script like this :\n$ ls FILE*.txt\nFILE0.txt  FILE10.txt  FILE1.txt  FILE2.txt  FILE3.txt  FILE4.txt  FILE5.txt  FILE6.txt  FILE7.txt  FILE8.txt  FILE9.txt\n\n$ bash rename_files.sh FILE txt TEXT\n\n$ ls TEXT*.txt\nTEXT0.txt  TEXT10.txt  TEXT1.txt  TEXT2.txt  TEXT3.txt  TEXT4.txt  TEXT5.txt  TEXT6.txt  TEXT7.txt  TEXT8.txt  TEXT9.txt\nThis is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with\nTEXT\nto\nNEW\n, use the following:\n$ bash rename_files.sh TEXT txt NEW\n\n$ ls NEW*.txt\nNEW0.txt  NEW10.txt  NEW1.txt  NEW2.txt  NEW3.txt  NEW4.txt  NEW5.txt  NEW6.txt  NEW7.txt  NEW8.txt  NEW9.txt\n[ A free course for you: Virtualization and Infrastructure Migration Technical Overview. ] \nConclusion\nHopefully, these examples have demonstrated the power of a\nfor\nloop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks.\nWe hope that Bash\nfor\nloops change your world the same way they changed ours.","attrs_markdown":"[Skip to content](https:\/\/www.redhat.com\/en\/blog\/bash-scripting-loops#rhdc-main-content)\n\n## Navigation\nMenu\n\nAI\n\n- ### Our approach\n  - [News and insights](https:\/\/www.redhat.com\/en\/blog\/channel\/artificial-intelligence)\n  - [Technical blog](https:\/\/developers.redhat.com\/aiml#ai-ml-main-tab-recent-articles-100491)\n  - [Research](https:\/\/www.redhat.com\/en\/artificial-intelligence\/research)\n  - [Live AI events](https:\/\/www.redhat.com\/en\/events\/ai)\n  - [Get an overview](https:\/\/www.redhat.com\/en\/artificial-intelligence)\n\n- ### Products\n  - [Red Hat AI Enterprise](https:\/\/www.redhat.com\/en\/products\/ai\/enterprise)\n  - [Red Hat AI Inference Server](https:\/\/www.redhat.com\/en\/products\/ai\/inference-server)\n  - 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For many of us, that thing is *looping* in Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the `for` loop, though.\n\nIf you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the `for` loop is definitely the right tool for the job.\n\n***\\[ You might also like:*** [***Mastering loops with Jinja templates in Ansible***](https:\/\/www.redhat.com\/sysadmin\/ansible-jinja) ***\\]***\n\nIf you already have a programming or scripting background, you're probably familiar with what `for` loops do. If you're not, I'll try to break it down in plain English for you.\n\n> The basic concept is: **FOR** a given set of items, **DO** a thing.\n\nThe given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a `for` loop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script.\n\n## Basic structure of the for loop\nFirst, let's talk about the basic structure of a `for` loop, and then we'll get into some examples.\n\nThe basic syntax of a `for` loop is:\n```\nfor <variable name> in <a list of items>;do <some command> $<variable name>;done;\n```\nThe **variable name** will be the variable you specify in the `do` section and will contain the item in the loop that you're on.\n\nThe **list of items** can be anything that returns a space or newline-separated list.\n\nHere's an example:\n```\n$ for name in joey suzy bobby;do echo $name;done\n```\nThat's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable **\\$name** will contain the item in the list that the loop is currently operating on, and once the command (or commands) in the `do` section are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between `do` and `done` will be executed. New commands just need a `;` delimiting them.\n```\n$ for name in joey suzy bobby; do echo first $name;echo second $name;done;\nfirst joey\nsecond joey\nfirst suzy\nsecond suzy\nfirst bobby\nsecond bobby\n```\nNow for some *real* examples.\n\n## Renaming files\nThis loop takes the output of the Bash command `ls *.pdf` and performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension).\n```\nfor i in $(ls *.pdf); do\nmv $i $(basename $i .pdf)_$(date +%Y%m%d).pdf\ndone\n```\nTo illustrate, run this loop in a directory containing these files:\n```\nfile1.pdf\nfile2.pdf\n...\nfileN.pdf\n```\nThe files will be renamed like this:\n```\nfile1_20210210.pdf\nfile2_20210210.pdf\n...\nfileN_20210210.pdf\n```\nIn a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them.\n\n## Extrapolating lists of items\nImagine that you have a file that you want to `scp` to several servers. Remember that you can combine the `for` loop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example:\n```\n$ echo {0..10}\n0 1 2 3 4 5 6 7 8 9 10\n```\nAssuming your servers are named in some sort of pattern like, **web0**, **web1**, **web2**, **web3**, you can have Bash iterate the series of numbers like this:\n```\n$ for i in web{0..10};do scp somefile.txt ${i}:;done;\n```\nThis will iterate through **web0**, **web1**, **web2**, **web3**, and so forth, executing your command on each item.\n\nYou can also define a few iterations. For example:\n```\n$ for i in web{0..10} db{0..2} balance_{a..c};do echo $i;done\nweb0\nweb1\nweb2\nweb3\nweb4\nweb5\nweb6\nweb7\nweb8\nweb9\nweb10\ndb0\ndb1\ndb2\nbalance_a\nbalance_b\nbalance_c\n```\nYou can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example, **web-us-0** would be the first web server in the US data center, while **web-ca-0** would be **web 0's** counterpart in the CA data center. To execute something on both systems, you can use a sequence like this:\n```\n$ for i in web-{us,ca}-{0..3};do echo $i;done\nweb-us-0\nweb-us-1\nweb-us-2\nweb-us-3\nweb-ca-0\nweb-ca-1\nweb-ca-2\nweb-ca-3\n```\nIn case your server names are not easy to iterate through, you can provide a list of names to the `for` loop:\n```\n$ cat somelist\nfirst_item\nmiddle_things\nfoo\nbar\nbaz\nlast_item\n\n$ for i in `cat somelist`;do echo \"ITEM: $i\";done\nITEM: first_item\nITEM: middle_things\nITEM: foo\nITEM: bar\nITEM: baz\nITEM: last_item\n```\n## Nesting\nYou can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example.\n\nYou can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested `for` loop gets executed on every iteration of the parent `for` loop. Be sure to specify different variable names for each loop.\n\nTo copy the list of files **file1.txt, file2.txt,** and **file3.txt** to the web servers, use this nested loop:\n```\n$ for i in file{1..3};do for x in web{0..3};do echo \"Copying $i to server $x\"; scp $i $x; done; done\nCopying file1 to server web0\nCopying file1 to server web1\nCopying file1 to server web2\nCopying file1 to server web3\nCopying file2 to server web0\nCopying file2 to server web1\nCopying file2 to server web2\nCopying file2 to server web3\nCopying file3 to server web0\nCopying file3 to server web1\nCopying file3 to server web2\nCopying file3 to server web3\n```\n## More creative renaming\nThere might be other ways to get this done, but remember, this is just an example of things you *can* do with a `for` loop. What if you have a mountain of files named something like **FILE002.txt**, and you want to replace **FILE** with something like **TEXT**. Remember that in addition to Bash itself, you also have other open source tools at your disposal, like `sed`, `grep`, and more. You can combine those tools with the `for` loop, like this:\n```\n$ ls FILE*.txt\nFILE0.txt  FILE10.txt  FILE1.txt  FILE2.txt  FILE3.txt  FILE4.txt  FILE5.txt  FILE6.txt  FILE7.txt  FILE8.txt  FILE9.txt\n\n$ for i in $(ls FILE*.txt);do mv $i `echo $i | sed s\/FILE\/TEXT\/`;done\n\n$ ls FILE*.txt\nls: cannot access 'FILE*.txt': No such file or directory\n\n$ ls TEXT*.txt\nTEXT0.txt  TEXT10.txt  TEXT1.txt  TEXT2.txt  TEXT3.txt  TEXT4.txt  TEXT5.txt  TEXT6.txt  TEXT7.txt  TEXT8.txt  TEXT9.txt\n```\n## Adding a for loop to a Bash script\nRunning `for` loops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include `for` loops as part of your Bash scripts for increased power, readability, and flexibility.\n\nFor example, you can add the nested loop example to a Bash script to improve its readability, like this:\n```\n$ vim copy_web_files.sh\n# !\/bin\/bash\n\nfor i in file{1..3};do\n  for x in web{0..3};do\n    echo \"Copying $i to server $x\"\n    scp $i $x\n  done\ndone\n```\nWhen you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain.\n```\n$ bash copy_web_files.sh\nCopying file1 to server web0\nCopying file1 to server web1\n... TRUNCATED ...\nCopying file3 to server web3\n```\nYou can also increase the flexibility and reusability of your `for` loops by including them in Bash scripts that allow parameter input. For example, to rename files like the example **More creative renaming** above allowing the user to specify the name suffix, use this script:\n```\n$ vim rename_files.sh\n# !\/bin\/bash\n\nsource_prefix=$1\nsuffix=$2\ndestination_prefix=$3\n\nfor i in $(ls ${source_prefix}*.${suffix});do\n  mv $i $(echo $i | sed s\/${source_prefix}\/${destination_prefix}\/)\ndone\n```\nIn this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with **FILE**, of type **.txt** to **TEXT**, execute the script like this :\n```\n$ ls FILE*.txt\nFILE0.txt  FILE10.txt  FILE1.txt  FILE2.txt  FILE3.txt  FILE4.txt  FILE5.txt  FILE6.txt  FILE7.txt  FILE8.txt  FILE9.txt\n\n$ bash rename_files.sh FILE txt TEXT\n\n$ ls TEXT*.txt\nTEXT0.txt  TEXT10.txt  TEXT1.txt  TEXT2.txt  TEXT3.txt  TEXT4.txt  TEXT5.txt  TEXT6.txt  TEXT7.txt  TEXT8.txt  TEXT9.txt\n```\nThis is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with **TEXT** to **NEW**, use the following:\n```\n$ bash rename_files.sh TEXT txt NEW\n\n$ ls NEW*.txt\nNEW0.txt  NEW10.txt  NEW1.txt  NEW2.txt  NEW3.txt  NEW4.txt  NEW5.txt  NEW6.txt  NEW7.txt  NEW8.txt  NEW9.txt\n```\n***\\[ A free course for you: Virtualization and Infrastructure Migration Technical Overview. \\]***\n\n## Conclusion\nHopefully, these examples have demonstrated the power of a `for` loop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks.\n\nWe hope that Bash `for` loops change your world the same way they changed ours.\n***\n### About the authors\n[![Nathan Lager](https:\/\/www.redhat.com\/rhdc\/managed-files\/styles\/media_thumbnail\/private\/sysadmin\/pictures\/2019-10\/IMG_1476.png?itok=m_lcTmoX)](https:\/\/www.redhat.com\/en\/authors\/nathan-lager)\n\n[Nathan Lager](https:\/\/www.redhat.com\/en\/authors\/nathan-lager)\n\nNate is a Technical Account Manager with Red Hat and an experienced sysadmin with 20 years in the industry. He first encountered Linux (Red Hat 5.0) as a teenager, after deciding that software licensing was too expensive for a kid with no income, in the late 90’s. Since then he’s run everything from BBS’s (remember those?) to derby hat’s containing raspberry pi’s, to Linux systems in his basement, or in enterprise-class data-centers.\n\nHe runs his own blog at [undrground.org](https:\/\/www.undrground.org\/), hosts the [Iron Sysadmin Podcast](https:\/\/www.ironsysadmin.com\/), and when he’s not at a command line, he’s probably in the garage tinkering on his Jeep, or out on the trails.\n\n[Read full bio](https:\/\/www.redhat.com\/en\/authors\/nathan-lager)\n\n[![Ricardo Gerardi](https:\/\/www.redhat.com\/rhdc\/managed-files\/styles\/media_thumbnail\/private\/sysadmin\/pictures\/2020-08\/ricardo_tiniest.png?itok=CKw79_bE)](https:\/\/www.redhat.com\/en\/authors\/rgerardi)\n\n[Ricardo GerardiPrincipal Consultant & Author](https:\/\/www.redhat.com\/en\/authors\/rgerardi)\n\nRicardo Gerardi is a Principal Consultant at Red Hat, having transitioned from his previous role as a Technical Community Advocate for Enable Sysadmin. He's been at Red Hat since 2018, specializing in IT automation using Ansible and OpenShift.\n\nWith over 25 years of industry experience and 20+ years as a Linux and open source enthusiast and contributor, Ricardo is passionate about technology. He is particularly interested in hacking with the Go programming language and is the author of Powerful Command-Line Applications in Go and Automate Your Home Using Go. Ricardo also writes regularly for Red Hat and other blogs, covering topics like Linux, Vim, Ansible, Containers, Kubernetes, and command-line applications.\n\nOutside of work, Ricardo enjoys spending time with his daughters, reading science fiction books, and playing video games.\n\n[Read full bio](https:\/\/www.redhat.com\/en\/authors\/rgerardi)\n\n## More like this\nBlog post\n\n### [Strategic momentum: The new era of Red Hat and HPE Juniper network automation](https:\/\/www.redhat.com\/en\/blog\/strategic-momentum-new-era-red-hat-and-hpe-juniper-network-automation)\n\nBlog post\n\n### [Redefining automation governance: From execution to observability at Bradesco](https:\/\/www.redhat.com\/en\/blog\/redefining-automation-governance-execution-observability-bradesco)\n\nOriginal podcast\n\n### [Technically Speaking \\| Taming AI agents with observability](https:\/\/www.redhat.com\/en\/technically-speaking\/taming-ai-agents-observability)\n\nOriginal podcast\n\n### [Ready To Launch \\| 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As a [trusted adviser to the Fortune 500](https:\/\/www.redhat.com\/en\/about\/company), Red Hat offers cloud, developer, Linux, automation, and application platform technologies, as well as [award-winning](https:\/\/access.redhat.com\/recognition) services.\n\n- [Our company](https:\/\/www.redhat.com\/en\/about\/company)\n- [How we work](https:\/\/www.redhat.com\/en\/about\/our-culture)\n- [Customer success stories](https:\/\/www.redhat.com\/en\/success-stories)\n- [Analyst relations](https:\/\/www.redhat.com\/en\/about\/analysts)\n- [Newsroom](https:\/\/www.redhat.com\/en\/about\/newsroom)\n- [Open source commitments](https:\/\/www.redhat.com\/en\/about\/open-source)\n- [Our social impact](https:\/\/www.redhat.com\/en\/about\/community-social-responsibility)\n- [Jobs](https:\/\/www.redhat.com\/en\/jobs)\n### Change page language\n### Red Hat legal and privacy links\n- [About Red Hat](https:\/\/www.redhat.com\/en\/about\/company)\n- [Jobs](https:\/\/www.redhat.com\/en\/jobs)\n- [Events](https:\/\/www.redhat.com\/en\/events)\n- [Locations](https:\/\/www.redhat.com\/en\/about\/office-locations)\n- [Contact Red Hat](https:\/\/www.redhat.com\/en\/contact)\n- [Red Hat Blog](https:\/\/www.redhat.com\/en\/blog)\n- [Inclusion at Red Hat](https:\/\/www.redhat.com\/en\/about\/our-culture\/inclusion)\n- [Cool Stuff Store](https:\/\/coolstuff.redhat.com\/)\n- [Red Hat Summit](https:\/\/www.redhat.com\/en\/summit)\n\n© 2026 Red Hat\n\n### Red Hat legal and privacy links\n- [Privacy statement](https:\/\/www.redhat.com\/en\/about\/privacy-policy)\n- [Terms of use](https:\/\/www.redhat.com\/en\/about\/terms-use)\n- [All policies and guidelines](https:\/\/www.redhat.com\/en\/about\/all-policies-guidelines)\n- [Digital accessibility](https:\/\/www.redhat.com\/en\/about\/digital-accessibility)","attrs_readable_markdown":"Every sysadmin probably has some skill they've learned over the years that they can point at and say, \"That changed my world.\" That skill, or that bit of information, or that technique just changed how I do things. For many of us, that thing is *looping* in Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the `for` loop, though.\n\nIf you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the `for` loop is definitely the right tool for the job.\n\n***\\[ You might also like:*** [***Mastering loops with Jinja templates in Ansible***](https:\/\/www.redhat.com\/sysadmin\/ansible-jinja) ***\\]***\n\nIf you already have a programming or scripting background, you're probably familiar with what `for` loops do. If you're not, I'll try to break it down in plain English for you.\n\n> The basic concept is: **FOR** a given set of items, **DO** a thing.\n\nThe given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a `for` loop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script.\n\n## Basic structure of the for loop\nFirst, let's talk about the basic structure of a `for` loop, and then we'll get into some examples.\n\nThe basic syntax of a `for` loop is:\n```\nfor <variable name> in <a list of items>;do <some command> $<variable name>;done;\n```\nThe **variable name** will be the variable you specify in the `do` section and will contain the item in the loop that you're on.\n\nThe **list of items** can be anything that returns a space or newline-separated list.\n\nHere's an example:\n```\n$ for name in joey suzy bobby;do echo $name;done\n```\nThat's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable **\\$name** will contain the item in the list that the loop is currently operating on, and once the command (or commands) in the `do` section are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between `do` and `done` will be executed. New commands just need a `;` delimiting them.\n```\n$ for name in joey suzy bobby; do echo first $name;echo second $name;done;\nfirst joey\nsecond joey\nfirst suzy\nsecond suzy\nfirst bobby\nsecond bobby\n```\nNow for some *real* examples.\n\n## Renaming files\nThis loop takes the output of the Bash command `ls *.pdf` and performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension).\n```\nfor i in $(ls *.pdf); do\nmv $i $(basename $i .pdf)_$(date +%Y%m%d).pdf\ndone\n```\nTo illustrate, run this loop in a directory containing these files:\n```\nfile1.pdf\nfile2.pdf\n...\nfileN.pdf\n```\nThe files will be renamed like this:\n```\nfile1_20210210.pdf\nfile2_20210210.pdf\n...\nfileN_20210210.pdf\n```\nIn a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them.\n\n## Extrapolating lists of items\nImagine that you have a file that you want to `scp` to several servers. Remember that you can combine the `for` loop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example:\n```\n$ echo {0..10}\n0 1 2 3 4 5 6 7 8 9 10\n```\nAssuming your servers are named in some sort of pattern like, **web0**, **web1**, **web2**, **web3**, you can have Bash iterate the series of numbers like this:\n```\n$ for i in web{0..10};do scp somefile.txt ${i}:;done;\n```\nThis will iterate through **web0**, **web1**, **web2**, **web3**, and so forth, executing your command on each item.\n\nYou can also define a few iterations. For example:\n```\n$ for i in web{0..10} db{0..2} balance_{a..c};do echo $i;done\nweb0\nweb1\nweb2\nweb3\nweb4\nweb5\nweb6\nweb7\nweb8\nweb9\nweb10\ndb0\ndb1\ndb2\nbalance_a\nbalance_b\nbalance_c\n```\nYou can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example, **web-us-0** would be the first web server in the US data center, while **web-ca-0** would be **web 0's** counterpart in the CA data center. To execute something on both systems, you can use a sequence like this:\n```\n$ for i in web-{us,ca}-{0..3};do echo $i;done\nweb-us-0\nweb-us-1\nweb-us-2\nweb-us-3\nweb-ca-0\nweb-ca-1\nweb-ca-2\nweb-ca-3\n```\nIn case your server names are not easy to iterate through, you can provide a list of names to the `for` loop:\n```\n$ cat somelist\nfirst_item\nmiddle_things\nfoo\nbar\nbaz\nlast_item\n\n$ for i in `cat somelist`;do echo \"ITEM: $i\";done\nITEM: first_item\nITEM: middle_things\nITEM: foo\nITEM: bar\nITEM: baz\nITEM: last_item\n```\n## Nesting\nYou can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example.\n\nYou can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested `for` loop gets executed on every iteration of the parent `for` loop. Be sure to specify different variable names for each loop.\n\nTo copy the list of files **file1.txt, file2.txt,** and **file3.txt** to the web servers, use this nested loop:\n```\n$ for i in file{1..3};do for x in web{0..3};do echo \"Copying $i to server $x\"; scp $i $x; done; done\nCopying file1 to server web0\nCopying file1 to server web1\nCopying file1 to server web2\nCopying file1 to server web3\nCopying file2 to server web0\nCopying file2 to server web1\nCopying file2 to server web2\nCopying file2 to server web3\nCopying file3 to server web0\nCopying file3 to server web1\nCopying file3 to server web2\nCopying file3 to server web3\n```\n## More creative renaming\nThere might be other ways to get this done, but remember, this is just an example of things you *can* do with a `for` loop. What if you have a mountain of files named something like **FILE002.txt**, and you want to replace **FILE** with something like **TEXT**. Remember that in addition to Bash itself, you also have other open source tools at your disposal, like `sed`, `grep`, and more. You can combine those tools with the `for` loop, like this:\n```\n$ ls FILE*.txt\nFILE0.txt  FILE10.txt  FILE1.txt  FILE2.txt  FILE3.txt  FILE4.txt  FILE5.txt  FILE6.txt  FILE7.txt  FILE8.txt  FILE9.txt\n\n$ for i in $(ls FILE*.txt);do mv $i `echo $i | sed s\/FILE\/TEXT\/`;done\n\n$ ls FILE*.txt\nls: cannot access 'FILE*.txt': No such file or directory\n\n$ ls TEXT*.txt\nTEXT0.txt  TEXT10.txt  TEXT1.txt  TEXT2.txt  TEXT3.txt  TEXT4.txt  TEXT5.txt  TEXT6.txt  TEXT7.txt  TEXT8.txt  TEXT9.txt\n```\n## Adding a for loop to a Bash script\nRunning `for` loops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include `for` loops as part of your Bash scripts for increased power, readability, and flexibility.\n\nFor example, you can add the nested loop example to a Bash script to improve its readability, like this:\n```\n$ vim copy_web_files.sh\n# !\/bin\/bash\n\nfor i in file{1..3};do\n  for x in web{0..3};do\n    echo \"Copying $i to server $x\"\n    scp $i $x\n  done\ndone\n```\nWhen you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain.\n```\n$ bash copy_web_files.sh\nCopying file1 to server web0\nCopying file1 to server web1\n... TRUNCATED ...\nCopying file3 to server web3\n```\nYou can also increase the flexibility and reusability of your `for` loops by including them in Bash scripts that allow parameter input. For example, to rename files like the example **More creative renaming** above allowing the user to specify the name suffix, use this script:\n```\n$ vim rename_files.sh\n# !\/bin\/bash\n\nsource_prefix=$1\nsuffix=$2\ndestination_prefix=$3\n\nfor i in $(ls ${source_prefix}*.${suffix});do\n  mv $i $(echo $i | sed s\/${source_prefix}\/${destination_prefix}\/)\ndone\n```\nIn this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with **FILE**, of type **.txt** to **TEXT**, execute the script like this :\n```\n$ ls FILE*.txt\nFILE0.txt  FILE10.txt  FILE1.txt  FILE2.txt  FILE3.txt  FILE4.txt  FILE5.txt  FILE6.txt  FILE7.txt  FILE8.txt  FILE9.txt\n\n$ bash rename_files.sh FILE txt TEXT\n\n$ ls TEXT*.txt\nTEXT0.txt  TEXT10.txt  TEXT1.txt  TEXT2.txt  TEXT3.txt  TEXT4.txt  TEXT5.txt  TEXT6.txt  TEXT7.txt  TEXT8.txt  TEXT9.txt\n```\nThis is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with **TEXT** to **NEW**, use the following:\n```\n$ bash rename_files.sh TEXT txt NEW\n\n$ ls NEW*.txt\nNEW0.txt  NEW10.txt  NEW1.txt  NEW2.txt  NEW3.txt  NEW4.txt  NEW5.txt  NEW6.txt  NEW7.txt  NEW8.txt  NEW9.txt\n```\n***\\[ A free course for you: Virtualization and Infrastructure Migration Technical Overview. \\]***\n\n## Conclusion\nHopefully, these examples have demonstrated the power of a `for` loop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks.\n\nWe hope that Bash `for` loops change your world the same way they changed ours.","meta_canonical":null}

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Boilerpipe Text	Every sysadmin probably has some skill they've learned over the years that they can point at and say, "That changed my world." That skill, or that bit of information, or that technique just changed how I do things. For many of us, that thing is looping in Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the for loop, though. If you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the for loop is definitely the right tool for the job. [ You might also like: Mastering loops with Jinja templates in Ansible ] If you already have a programming or scripting background, you're probably familiar with what for loops do. If you're not, I'll try to break it down in plain English for you. The basic concept is: FOR a given set of items, DO a thing. The given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a for loop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script. Basic structure of the for loop First, let's talk about the basic structure of a for loop, and then we'll get into some examples. The basic syntax of a for loop is: for <variable name> in <a list of items>;do <some command> $<variable name>;done; The variable name will be the variable you specify in the do section and will contain the item in the loop that you're on. The list of items can be anything that returns a space or newline-separated list. Here's an example: $ for name in joey suzy bobby;do echo $name;done That's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable $name will contain the item in the list that the loop is currently operating on, and once the command (or commands) in the do section are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between do and done will be executed. New commands just need a ; delimiting them. $ for name in joey suzy bobby; do echo first $name;echo second $name;done; first joey second joey first suzy second suzy first bobby second bobby Now for some real examples. Renaming files This loop takes the output of the Bash command ls .pdf and performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension). for i in $(ls .pdf); do mv $i $(basename $i .pdf)_$(date +%Y%m%d).pdf done To illustrate, run this loop in a directory containing these files: file1.pdf file2.pdf ... fileN.pdf The files will be renamed like this: file1_20210210.pdf file2_20210210.pdf ... fileN_20210210.pdf In a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them. Extrapolating lists of items Imagine that you have a file that you want to scp to several servers. Remember that you can combine the for loop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example: $ echo {0..10} 0 1 2 3 4 5 6 7 8 9 10 Assuming your servers are named in some sort of pattern like, web0 , web1 , web2 , web3 , you can have Bash iterate the series of numbers like this: $ for i in web{0..10};do scp somefile.txt ${i}:;done; This will iterate through web0 , web1 , web2 , web3 , and so forth, executing your command on each item. You can also define a few iterations. For example: $ for i in web{0..10} db{0..2} balance_{a..c};do echo $i;done web0 web1 web2 web3 web4 web5 web6 web7 web8 web9 web10 db0 db1 db2 balance_a balance_b balance_c You can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example, web-us-0 would be the first web server in the US data center, while web-ca-0 would be web 0's counterpart in the CA data center. To execute something on both systems, you can use a sequence like this: $ for i in web-{us,ca}-{0..3};do echo $i;done web-us-0 web-us-1 web-us-2 web-us-3 web-ca-0 web-ca-1 web-ca-2 web-ca-3 In case your server names are not easy to iterate through, you can provide a list of names to the for loop: $ cat somelist first_item middle_things foo bar baz last_item $ for i in `cat somelist`;do echo "ITEM: $i";done ITEM: first_item ITEM: middle_things ITEM: foo ITEM: bar ITEM: baz ITEM: last_item Nesting You can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example. You can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested for loop gets executed on every iteration of the parent for loop. Be sure to specify different variable names for each loop. To copy the list of files file1.txt, file2.txt, and file3.txt to the web servers, use this nested loop: $ for i in file{1..3};do for x in web{0..3};do echo "Copying $i to server $x"; scp $i $x; done; done Copying file1 to server web0 Copying file1 to server web1 Copying file1 to server web2 Copying file1 to server web3 Copying file2 to server web0 Copying file2 to server web1 Copying file2 to server web2 Copying file2 to server web3 Copying file3 to server web0 Copying file3 to server web1 Copying file3 to server web2 Copying file3 to server web3 More creative renaming There might be other ways to get this done, but remember, this is just an example of things you can do with a for loop. What if you have a mountain of files named something like FILE002.txt , and you want to replace FILE with something like TEXT . Remember that in addition to Bash itself, you also have other open source tools at your disposal, like sed , grep , and more. You can combine those tools with the for loop, like this: $ ls FILE.txt FILE0.txt FILE10.txt FILE1.txt FILE2.txt FILE3.txt FILE4.txt FILE5.txt FILE6.txt FILE7.txt FILE8.txt FILE9.txt $ for i in $(ls FILE.txt);do mv $i `echo $i \| sed s/FILE/TEXT/`;done $ ls FILE.txt ls: cannot access 'FILE.txt': No such file or directory $ ls TEXT.txt TEXT0.txt TEXT10.txt TEXT1.txt TEXT2.txt TEXT3.txt TEXT4.txt TEXT5.txt TEXT6.txt TEXT7.txt TEXT8.txt TEXT9.txt Adding a for loop to a Bash script Running for loops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include for loops as part of your Bash scripts for increased power, readability, and flexibility. For example, you can add the nested loop example to a Bash script to improve its readability, like this: $ vim copy_web_files.sh # !/bin/bash for i in file{1..3};do for x in web{0..3};do echo "Copying $i to server $x" scp $i $x done done When you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain. $ bash copy_web_files.sh Copying file1 to server web0 Copying file1 to server web1 ... TRUNCATED ... Copying file3 to server web3 You can also increase the flexibility and reusability of your for loops by including them in Bash scripts that allow parameter input. For example, to rename files like the example More creative renaming above allowing the user to specify the name suffix, use this script: $ vim rename_files.sh # !/bin/bash source_prefix=$1 suffix=$2 destination_prefix=$3 for i in $(ls ${source_prefix}.${suffix});do mv $i $(echo $i \| sed s/${source_prefix}/${destination_prefix}/) done In this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with FILE , of type .txt to TEXT , execute the script like this : $ ls FILE.txt FILE0.txt FILE10.txt FILE1.txt FILE2.txt FILE3.txt FILE4.txt FILE5.txt FILE6.txt FILE7.txt FILE8.txt FILE9.txt $ bash rename_files.sh FILE txt TEXT $ ls TEXT.txt TEXT0.txt TEXT10.txt TEXT1.txt TEXT2.txt TEXT3.txt TEXT4.txt TEXT5.txt TEXT6.txt TEXT7.txt TEXT8.txt TEXT9.txt This is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with TEXT to NEW , use the following: $ bash rename_files.sh TEXT txt NEW $ ls NEW*.txt NEW0.txt NEW10.txt NEW1.txt NEW2.txt NEW3.txt NEW4.txt NEW5.txt NEW6.txt NEW7.txt NEW8.txt NEW9.txt [ A free course for you: Virtualization and Infrastructure Migration Technical Overview. ] Conclusion Hopefully, these examples have demonstrated the power of a for loop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks. We hope that Bash for loops change your world the same way they changed ours.
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[Log in or register](https://sso.redhat.com/) Change page language [Contact us](https://www.redhat.com/en/contact) ### \[\[name\]\] [Edit avatar](https://access.redhat.com/user/edit) Login: \[\[login\]\] Account number: \[\[account\_number\]\] \[\[email\]\] Change page language Log out [Red Hat Blog](https://www.redhat.com/en/blog) - [By product]() - [Red Hat AI](https://www.redhat.com/en/blog/channel/red-hat-ai "Red Hat AI") - [Red Hat Ansible Automation Platform](https://www.redhat.com/en/blog/channel/red-hat-ansible-automation "Red Hat Ansible Automation Platform") - [Red Hat Enterprise Linux](https://www.redhat.com/en/blog/channel/red-hat-enterprise-linux "Red Hat Enterprise Linux") - [Red Hat OpenShift](https://www.redhat.com/en/blog/channel/red-hat-openshift "Red Hat OpenShift") * [More products](https://www.redhat.com/en/blog/products "More products") - [By topic]() - [AI](https://www.redhat.com/en/blog/channel/artificial-intelligence "AI") - [Virtualization](https://www.redhat.com/en/blog/channel/red-hat-virtualization "Virtualization") - [Digital sovereignty](https://www.redhat.com/en/blog/channel/digital-sovereignty "Digital sovereignty") - [Applications](https://www.redhat.com/en/blog/channel/applications "Applications") - [Automation](https://www.redhat.com/en/blog/channel/management-and-automation "Automation") - [Cloud services](https://www.redhat.com/en/blog/channel/cloud-services "Cloud services") - [Edge computing](https://www.redhat.com/en/blog/channel/edge-computing "Edge computing") - [Infrastructure](https://www.redhat.com/en/blog/channel/infrastructure "Infrastructure") - [Open hybrid cloud](https://www.redhat.com/en/blog/channel/hybrid-cloud-infrastructure "Open hybrid cloud") - [Original shows](https://www.redhat.com/en/red-hat-original-series "Original shows") - [Security](https://www.redhat.com/en/blog/channel/security "Security") * [All topics](https://www.redhat.com/en/blog/channels "All topics") - [Podcasts]() - [Technically Speaking with Chris Wright](https://www.redhat.com/en/technically-speaking "Technically Speaking with Chris Wright") - [Code Comments](https://www.redhat.com/en/code-comments-podcast "Code Comments") - [Command Line Heroes](https://www.redhat.com/en/command-line-heroes "Command Line Heroes") - [Compiler](https://www.redhat.com/en/compiler-podcast "Compiler") - [More blogs]() - [Red Hat Developer blog](https://developers.redhat.com/blog "Red Hat Developer blog") - [Red Hat Partner Connect blog](https://connect.redhat.com/en/blog "Red Hat Partner Connect blog") # Introduction to Linux Bash programming: 5 \`for\` loop tips March 22, 2021[Nathan Lager](https://www.redhat.com/en/authors/nathan-lager "See more by Nathan Lager"), [Ricardo Gerardi](https://www.redhat.com/en/authors/rgerardi "See more by Ricardo Gerardi")6\-minute read [Automation and management](https://www.redhat.com/en/blog?f[0]=taxonomy_topic_tid:27011#rhdc-search-listing) [Linux](https://www.redhat.com/en/blog?f[0]=taxonomy_topic_tid:27061#rhdc-search-listing) Share Subscribe to RSS - [Back to all posts](https://www.redhat.com/en/blog) *** Every sysadmin probably has some skill they've learned over the years that they can point at and say, "That changed my world." That skill, or that bit of information, or that technique just changed how I do things. For many of us, that thing is looping in Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the `for` loop, though. If you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the `for` loop is definitely the right tool for the job. *\[ You might also like:* [*Mastering loops with Jinja templates in Ansible](https://www.redhat.com/sysadmin/ansible-jinja) \]* If you already have a programming or scripting background, you're probably familiar with what `for` loops do. If you're not, I'll try to break it down in plain English for you. > The basic concept is: FOR a given set of items, DO a thing. The given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a `for` loop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script. ## Basic structure of the for loop First, let's talk about the basic structure of a `for` loop, and then we'll get into some examples. The basic syntax of a `for` loop is: ``` for <variable name> in <a list of items>;do <some command> $<variable name>;done; ``` The variable name will be the variable you specify in the `do` section and will contain the item in the loop that you're on. The list of items can be anything that returns a space or newline-separated list. Here's an example: ``` $ for name in joey suzy bobby;do echo $name;done ``` That's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable \$name will contain the item in the list that the loop is currently operating on, and once the command (or commands) in the `do` section are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between `do` and `done` will be executed. New commands just need a `;` delimiting them. ``` $ for name in joey suzy bobby; do echo first $name;echo second $name;done; first joey second joey first suzy second suzy first bobby second bobby ``` Now for some real examples. ## Renaming files This loop takes the output of the Bash command `ls .pdf` and performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension). ``` for i in $(ls .pdf); do mv $i $(basename $i .pdf)_$(date +%Y%m%d).pdf done ``` To illustrate, run this loop in a directory containing these files: ``` file1.pdf file2.pdf ... fileN.pdf ``` The files will be renamed like this: ``` file1_20210210.pdf file2_20210210.pdf ... fileN_20210210.pdf ``` In a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them. ## Extrapolating lists of items Imagine that you have a file that you want to `scp` to several servers. Remember that you can combine the `for` loop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example: ``` $ echo {0..10} 0 1 2 3 4 5 6 7 8 9 10 ``` Assuming your servers are named in some sort of pattern like, web0, web1, web2, web3, you can have Bash iterate the series of numbers like this: ``` $ for i in web{0..10};do scp somefile.txt ${i}:;done; ``` This will iterate through web0, web1, web2, web3, and so forth, executing your command on each item. You can also define a few iterations. For example: ``` $ for i in web{0..10} db{0..2} balance_{a..c};do echo $i;done web0 web1 web2 web3 web4 web5 web6 web7 web8 web9 web10 db0 db1 db2 balance_a balance_b balance_c ``` You can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example, web-us-0 would be the first web server in the US data center, while web-ca-0 would be web 0's counterpart in the CA data center. To execute something on both systems, you can use a sequence like this: ``` $ for i in web-{us,ca}-{0..3};do echo $i;done web-us-0 web-us-1 web-us-2 web-us-3 web-ca-0 web-ca-1 web-ca-2 web-ca-3 ``` In case your server names are not easy to iterate through, you can provide a list of names to the `for` loop: ``` $ cat somelist first_item middle_things foo bar baz last_item $ for i in `cat somelist`;do echo "ITEM: $i";done ITEM: first_item ITEM: middle_things ITEM: foo ITEM: bar ITEM: baz ITEM: last_item ``` ## Nesting You can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example. You can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested `for` loop gets executed on every iteration of the parent `for` loop. Be sure to specify different variable names for each loop. To copy the list of files file1.txt, file2.txt, and file3.txt to the web servers, use this nested loop: ``` $ for i in file{1..3};do for x in web{0..3};do echo "Copying $i to server $x"; scp $i $x; done; done Copying file1 to server web0 Copying file1 to server web1 Copying file1 to server web2 Copying file1 to server web3 Copying file2 to server web0 Copying file2 to server web1 Copying file2 to server web2 Copying file2 to server web3 Copying file3 to server web0 Copying file3 to server web1 Copying file3 to server web2 Copying file3 to server web3 ``` ## More creative renaming There might be other ways to get this done, but remember, this is just an example of things you can do with a `for` loop. What if you have a mountain of files named something like FILE002.txt, and you want to replace FILE with something like TEXT. Remember that in addition to Bash itself, you also have other open source tools at your disposal, like `sed`, `grep`, and more. You can combine those tools with the `for` loop, like this: ``` $ ls FILE.txt FILE0.txt FILE10.txt FILE1.txt FILE2.txt FILE3.txt FILE4.txt FILE5.txt FILE6.txt FILE7.txt FILE8.txt FILE9.txt $ for i in $(ls FILE.txt);do mv $i `echo $i \| sed s/FILE/TEXT/`;done $ ls FILE.txt ls: cannot access 'FILE.txt': No such file or directory $ ls TEXT.txt TEXT0.txt TEXT10.txt TEXT1.txt TEXT2.txt TEXT3.txt TEXT4.txt TEXT5.txt TEXT6.txt TEXT7.txt TEXT8.txt TEXT9.txt ``` ## Adding a for loop to a Bash script Running `for` loops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include `for` loops as part of your Bash scripts for increased power, readability, and flexibility. For example, you can add the nested loop example to a Bash script to improve its readability, like this: ``` $ vim copy_web_files.sh # !/bin/bash for i in file{1..3};do for x in web{0..3};do echo "Copying $i to server $x" scp $i $x done done ``` When you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain. ``` $ bash copy_web_files.sh Copying file1 to server web0 Copying file1 to server web1 ... TRUNCATED ... Copying file3 to server web3 ``` You can also increase the flexibility and reusability of your `for` loops by including them in Bash scripts that allow parameter input. For example, to rename files like the example More creative renaming* above allowing the user to specify the name suffix, use this script: ``` $ vim rename_files.sh # !/bin/bash source_prefix=$1 suffix=$2 destination_prefix=$3 for i in $(ls ${source_prefix}.${suffix});do mv $i $(echo $i \| sed s/${source_prefix}/${destination_prefix}/) done ``` In this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with FILE, of type .txt* to TEXT, execute the script like this : ``` $ ls FILE.txt FILE0.txt FILE10.txt FILE1.txt FILE2.txt FILE3.txt FILE4.txt FILE5.txt FILE6.txt FILE7.txt FILE8.txt FILE9.txt $ bash rename_files.sh FILE txt TEXT $ ls TEXT.txt TEXT0.txt TEXT10.txt TEXT1.txt TEXT2.txt TEXT3.txt TEXT4.txt TEXT5.txt TEXT6.txt TEXT7.txt TEXT8.txt TEXT9.txt ``` This is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with TEXT to NEW, use the following: ``` $ bash rename_files.sh TEXT txt NEW $ ls NEW.txt NEW0.txt NEW10.txt NEW1.txt NEW2.txt NEW3.txt NEW4.txt NEW5.txt NEW6.txt NEW7.txt NEW8.txt NEW9.txt ``` \[ A free course for you: Virtualization and Infrastructure Migration Technical Overview. \]* ## Conclusion Hopefully, these examples have demonstrated the power of a `for` loop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks. We hope that Bash `for` loops change your world the same way they changed ours. *** ### About the authors [![Nathan Lager](https://www.redhat.com/rhdc/managed-files/styles/media_thumbnail/private/sysadmin/pictures/2019-10/IMG_1476.png?itok=m_lcTmoX)](https://www.redhat.com/en/authors/nathan-lager) [Nathan Lager](https://www.redhat.com/en/authors/nathan-lager) Nate is a Technical Account Manager with Red Hat and an experienced sysadmin with 20 years in the industry. He first encountered Linux (Red Hat 5.0) as a teenager, after deciding that software licensing was too expensive for a kid with no income, in the late 90’s. Since then he’s run everything from BBS’s (remember those?) to derby hat’s containing raspberry pi’s, to Linux systems in his basement, or in enterprise-class data-centers. He runs his own blog at [undrground.org](https://www.undrground.org/), hosts the [Iron Sysadmin Podcast](https://www.ironsysadmin.com/), and when he’s not at a command line, he’s probably in the garage tinkering on his Jeep, or out on the trails. [Read full bio](https://www.redhat.com/en/authors/nathan-lager) [![Ricardo Gerardi](https://www.redhat.com/rhdc/managed-files/styles/media_thumbnail/private/sysadmin/pictures/2020-08/ricardo_tiniest.png?itok=CKw79_bE)](https://www.redhat.com/en/authors/rgerardi) [Ricardo GerardiPrincipal Consultant & Author](https://www.redhat.com/en/authors/rgerardi) Ricardo Gerardi is a Principal Consultant at Red Hat, having transitioned from his previous role as a Technical Community Advocate for Enable Sysadmin. He's been at Red Hat since 2018, specializing in IT automation using Ansible and OpenShift. With over 25 years of industry experience and 20+ years as a Linux and open source enthusiast and contributor, Ricardo is passionate about technology. He is particularly interested in hacking with the Go programming language and is the author of Powerful Command-Line Applications in Go and Automate Your Home Using Go. Ricardo also writes regularly for Red Hat and other blogs, covering topics like Linux, Vim, Ansible, Containers, Kubernetes, and command-line applications. Outside of work, Ricardo enjoys spending time with his daughters, reading science fiction books, and playing video games. 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Readable Markdown	Every sysadmin probably has some skill they've learned over the years that they can point at and say, "That changed my world." That skill, or that bit of information, or that technique just changed how I do things. For many of us, that thing is looping in Bash. There are other approaches to automation that are certainly more robust or scalable. Most of them do not compare to the simplicity and ready usability of the `for` loop, though. If you want to automate the configuration of thousands of systems, you should probably use Ansible. However, if you're trying to rename a thousand files, or execute the same command several times, then the `for` loop is definitely the right tool for the job. *\[ You might also like:* [*Mastering loops with Jinja templates in Ansible](https://www.redhat.com/sysadmin/ansible-jinja) \]* If you already have a programming or scripting background, you're probably familiar with what `for` loops do. If you're not, I'll try to break it down in plain English for you. > The basic concept is: FOR a given set of items, DO a thing. The given set of items can be a literal set of objects or anything that Bash can extrapolate to a list. For example, text pulled from a file, the output of another Bash command, or parameters passed via the command line. Converting this loop structure into a Bash script is also trivial. In this article, we show you some examples of how a `for` loop can make you look like a command line hero, and then we take some of those examples and put them inside a more structured Bash script. ## Basic structure of the for loop First, let's talk about the basic structure of a `for` loop, and then we'll get into some examples. The basic syntax of a `for` loop is: ``` for <variable name> in <a list of items>;do <some command> $<variable name>;done; ``` The variable name will be the variable you specify in the `do` section and will contain the item in the loop that you're on. The list of items can be anything that returns a space or newline-separated list. Here's an example: ``` $ for name in joey suzy bobby;do echo $name;done ``` That's about as simple as it gets and there isn't a whole lot going on there, but it gets you started. The variable \$name will contain the item in the list that the loop is currently operating on, and once the command (or commands) in the `do` section are carried out, the loop will move to the next item. You can also perform more than one action per loop. Anything between `do` and `done` will be executed. New commands just need a `;` delimiting them. ``` $ for name in joey suzy bobby; do echo first $name;echo second $name;done; first joey second joey first suzy second suzy first bobby second bobby ``` Now for some real examples. ## Renaming files This loop takes the output of the Bash command `ls .pdf` and performs an action on each returned file name. In this case, we're adding today's date to the end of the file name (but before the file extension). ``` for i in $(ls .pdf); do mv $i $(basename $i .pdf)_$(date +%Y%m%d).pdf done ``` To illustrate, run this loop in a directory containing these files: ``` file1.pdf file2.pdf ... fileN.pdf ``` The files will be renamed like this: ``` file1_20210210.pdf file2_20210210.pdf ... fileN_20210210.pdf ``` In a directory with hundreds of files, this loop saves you a considerable amount of time in renaming all of them. ## Extrapolating lists of items Imagine that you have a file that you want to `scp` to several servers. Remember that you can combine the `for` loop with other Bash features, such as shell expansion, which allows Bash to expand a list of items that are in a series. This can work for letters and numbers. For example: ``` $ echo {0..10} 0 1 2 3 4 5 6 7 8 9 10 ``` Assuming your servers are named in some sort of pattern like, web0, web1, web2, web3, you can have Bash iterate the series of numbers like this: ``` $ for i in web{0..10};do scp somefile.txt ${i}:;done; ``` This will iterate through web0, web1, web2, web3, and so forth, executing your command on each item. You can also define a few iterations. For example: ``` $ for i in web{0..10} db{0..2} balance_{a..c};do echo $i;done web0 web1 web2 web3 web4 web5 web6 web7 web8 web9 web10 db0 db1 db2 balance_a balance_b balance_c ``` You can also combine iterations. Imagine that you have two data centers, one in the United States, another in Canada, and the server's naming convention identifies which data center a server HA pair lived in. For example, web-us-0 would be the first web server in the US data center, while web-ca-0 would be web 0's counterpart in the CA data center. To execute something on both systems, you can use a sequence like this: ``` $ for i in web-{us,ca}-{0..3};do echo $i;done web-us-0 web-us-1 web-us-2 web-us-3 web-ca-0 web-ca-1 web-ca-2 web-ca-3 ``` In case your server names are not easy to iterate through, you can provide a list of names to the `for` loop: ``` $ cat somelist first_item middle_things foo bar baz last_item $ for i in `cat somelist`;do echo "ITEM: $i";done ITEM: first_item ITEM: middle_things ITEM: foo ITEM: bar ITEM: baz ITEM: last_item ``` ## Nesting You can also combine some of these ideas for more complex use cases. For example, imagine that you want to copy a list of files to your web servers that follow the numbered naming convention you used in the previous example. You can accomplish that by iterating a second list based on your first list through nested loops. This gets a little hard to follow when you're doing it as a one-liner, but it can definitely be done. Your nested `for` loop gets executed on every iteration of the parent `for` loop. Be sure to specify different variable names for each loop. To copy the list of files file1.txt, file2.txt, and file3.txt to the web servers, use this nested loop: ``` $ for i in file{1..3};do for x in web{0..3};do echo "Copying $i to server $x"; scp $i $x; done; done Copying file1 to server web0 Copying file1 to server web1 Copying file1 to server web2 Copying file1 to server web3 Copying file2 to server web0 Copying file2 to server web1 Copying file2 to server web2 Copying file2 to server web3 Copying file3 to server web0 Copying file3 to server web1 Copying file3 to server web2 Copying file3 to server web3 ``` ## More creative renaming There might be other ways to get this done, but remember, this is just an example of things you can do with a `for` loop. What if you have a mountain of files named something like FILE002.txt, and you want to replace FILE with something like TEXT. Remember that in addition to Bash itself, you also have other open source tools at your disposal, like `sed`, `grep`, and more. You can combine those tools with the `for` loop, like this: ``` $ ls FILE.txt FILE0.txt FILE10.txt FILE1.txt FILE2.txt FILE3.txt FILE4.txt FILE5.txt FILE6.txt FILE7.txt FILE8.txt FILE9.txt $ for i in $(ls FILE.txt);do mv $i `echo $i \| sed s/FILE/TEXT/`;done $ ls FILE.txt ls: cannot access 'FILE.txt': No such file or directory $ ls TEXT.txt TEXT0.txt TEXT10.txt TEXT1.txt TEXT2.txt TEXT3.txt TEXT4.txt TEXT5.txt TEXT6.txt TEXT7.txt TEXT8.txt TEXT9.txt ``` ## Adding a for loop to a Bash script Running `for` loops directly on the command line is great and saves you a considerable amount of time for some tasks. In addition, you can include `for` loops as part of your Bash scripts for increased power, readability, and flexibility. For example, you can add the nested loop example to a Bash script to improve its readability, like this: ``` $ vim copy_web_files.sh # !/bin/bash for i in file{1..3};do for x in web{0..3};do echo "Copying $i to server $x" scp $i $x done done ``` When you save and execute this script, the result is the same as running the nested loop example above, but it's more readable, plus it's easier to change and maintain. ``` $ bash copy_web_files.sh Copying file1 to server web0 Copying file1 to server web1 ... TRUNCATED ... Copying file3 to server web3 ``` You can also increase the flexibility and reusability of your `for` loops by including them in Bash scripts that allow parameter input. For example, to rename files like the example More creative renaming* above allowing the user to specify the name suffix, use this script: ``` $ vim rename_files.sh # !/bin/bash source_prefix=$1 suffix=$2 destination_prefix=$3 for i in $(ls ${source_prefix}.${suffix});do mv $i $(echo $i \| sed s/${source_prefix}/${destination_prefix}/) done ``` In this script, the user provides the source file's prefix as the first parameter, the file suffix as the second, and the new prefix as the third parameter. For example, to rename all files starting with FILE, of type .txt* to TEXT, execute the script like this : ``` $ ls FILE.txt FILE0.txt FILE10.txt FILE1.txt FILE2.txt FILE3.txt FILE4.txt FILE5.txt FILE6.txt FILE7.txt FILE8.txt FILE9.txt $ bash rename_files.sh FILE txt TEXT $ ls TEXT.txt TEXT0.txt TEXT10.txt TEXT1.txt TEXT2.txt TEXT3.txt TEXT4.txt TEXT5.txt TEXT6.txt TEXT7.txt TEXT8.txt TEXT9.txt ``` This is similar to the original example, but now your users can specify other parameters to change the script behavior. For example, to rename all files now starting with TEXT to NEW, use the following: ``` $ bash rename_files.sh TEXT txt NEW $ ls NEW.txt NEW0.txt NEW10.txt NEW1.txt NEW2.txt NEW3.txt NEW4.txt NEW5.txt NEW6.txt NEW7.txt NEW8.txt NEW9.txt ``` \[ A free course for you: Virtualization and Infrastructure Migration Technical Overview. \]* ## Conclusion Hopefully, these examples have demonstrated the power of a `for` loop at the Bash command line. You really can save a lot of time and perform tasks in a less error-prone way with loops. Just be careful. Your loops will do what you ask them to, even if you ask them to do something destructive by accident, like creating (or deleting) logical volumes or virtual disks. We hope that Bash `for` loops change your world the same way they changed ours.
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