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Calculated Shard: 14 (from laksa146)

2. Crawled Status Check

Query:

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-while-loops\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.redhat.com/en/blog/bash-scripting-while-loops\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/www.redhat.com\/en\/blog\/bash-scripting-while-loops","crawl_time":1773722256,"first_indexed_time":1734427940,"http_code":200,"src_unparsed":"com,redhat!www,\/en\/blog\/bash-scripting-while-loops s443","src_root_hash":"4780968593380432814","history_drop_reason":null,"meta_title":"Linux scripting: 3 how-tos for while loops in Bash","meta_descriptions":["Shell scripting, specifically Bash scripting, is a great way to automate repetitive or tedious tasks on your systems. Why type when you can schedule and run ..."],"attrs_boilerpipe_text":"Shell scripting, specifically Bash scripting, is a great way to automate repetitive or tedious tasks on your systems. Why type when you can schedule and run scripts in a hands-free manner? One of the many scripting constructs is the loop. A loop is a section of code that picks up data or generates data and then performs some operation on the data and then begins the process over again until some condition is met, the script is disrupted, or when input data is exhausted. When you use\nBash scripting\n, sometimes it is useful to perform actions using some form of loops.\n[ Readers also liked: \nFive ways to use redirect operators in bash\n]\nThe basic loop commands in Bash scripts are\nfor\nand\nwhile\n.\nfor\nloops are typically used when you have a known, finite list, like a series of numbers, a list of items, or counters.\nwhile\nloops can be used with lists but are also useful for conditions that do\nnot\nhave a known limit. You can use it to run commands\nwhile\na condition is true (or invert the logic and run it while the condition is false).\nSo, while (pun intended) there are different ways to perform loops, in this article, I focus on examples using\nwhile\nloops.\n1. A simple while loop\nImagine that you're installing a huge application, and you're concerned that it may eventually use too much space on the file system.\nInstead of running\ndf\nin a separate terminal, you can run a simple command to monitor the disk utilization every second. This allows you to interrupt the installation process if you see that it will hit that limit.\nwhile true\ndo\n  df -k | grep home\n  sleep 1\ndone\nIn this case, you're running the loop with a\ntrue\ncondition, which means it will run forever or until you hit\nCTRL-C.\nTherefore, you need to keep an eye on it (otherwise, it will remain using the system's resources).\nNote\n: If you use a loop like this, you need to include a command like\nsleep\nto give the system some time to breathe between executions. Running anything non-stop could become a performance issue, especially if the commands inside the loop involve I\/O operations.\n2. Waiting for a condition to become true\nThere are variations of this scenario. For example, you know that at some point, the process will create a directory, and you are just waiting for that moment to perform other validations.\nYou can have a\nwhile\nloop to keep checking for that directory's existence and only write a message while the directory does not exist.\nIf you want to do something more elaborate, you could create a script and show a more clear indication that the loop condition became true:\n#!\/bin\/bash\n\nwhile [ ! -d directory_expected ]\ndo\n   echo \"`date` - Still waiting\" \n   sleep 1\ndone\n\necho \"DIRECTORY IS THERE!!!\"\n3. Using a while loop to manipulate a file\nAnother useful application of a\nwhile\nloop is to combine it with the\nread\ncommand to have access to columns (or fields) quickly from a text file and perform some actions on them.\nIn the following example, you are simply picking the columns from a text file with a predictable format and printing the values that you want to use to populate an\n\/etc\/hosts\nfile.\nHere the assumption is that the file has columns delimited by spaces or tabs and that there are\nno\nspaces in the content of the columns. That could shift the content of the fields and not give you what you needed.\nNotice that you're just doing a simple operation to extract and manipulate information and not concerned about the command's reusability. I would classify this as one of those \"quick and dirty tricks.\"\nOf course, if this was something that you would repeatedly do, you should run it from a script, use proper names for the variables, and all those good practices (including transforming the filename in an argument and defining where to send the output, but today, the topic is\nwhile\nloops).\n#!\/bin\/bash\n\ncat servers.txt | grep -v CPU | while read servername cpu ram ip\ndo\n   echo $ip $servername\ndone\n[ Learn the basics of using Kubernetes in this\nfree cheat sheet\n. ] \nWrap up\nwhile\nloops are useful in scripts that depend on a certain condition to be true or false, but you can also use them interactively to monitor some condition.\nThe important things to remember are:\nAlways be clear about what is the condition to\nend\nthe loop. If that is something that you expect to do yourself with a\nCTRL-C\n, then keep an eye on your command. Think about\nhow fast\nyou want each loop cycle to be repeated. If you do any operation that involves I\/O inside the loop, that becomes a critical aspect of planning.\nWith that in mind, have fun\nwhile\nyou can.","attrs_markdown":"[Skip to content](https:\/\/www.redhat.com\/en\/blog\/bash-scripting-while-loops#rhdc-main-content)\n\n## Navigation\nMenu\n\nAI\n\n- ### Overview\n  - [AI news](https:\/\/www.redhat.com\/en\/blog\/channel\/artificial-intelligence)\n  - [Technical blog](https:\/\/developers.redhat.com\/blog?field_tax_product_target_id%5B37288%5D=37288&extIdCarryOver=true&intcmp=7013a000003Sl59AAC&sc_cid=RHCTG0250000454096&_gl=1%2A1jal9i8%2A_gcl_au%2ANjU2NDY1NTAwLjE3NjQ2MTY4NTU.&percmp=RHCTG0250000466729)\n  - [Live AI events](https:\/\/www.redhat.com\/en\/events\/ai)\n  - [Inference explained](https:\/\/www.redhat.com\/en\/ai\/inference\/what-you-should-know-about-inference)\n  - [See our approach](https:\/\/www.redhat.com\/en\/artificial-intelligence)\n\n- ### Products\n  - [Red Hat AI Enterprise](https:\/\/www.redhat.com\/en\/products\/ai\/enterprise)\n  - 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Why type when you can schedule and run scripts in a hands-free manner? One of the many scripting constructs is the loop. A loop is a section of code that picks up data or generates data and then performs some operation on the data and then begins the process over again until some condition is met, the script is disrupted, or when input data is exhausted. When you use [Bash scripting](https:\/\/www.redhat.com\/sysadmin\/using-bash-automation), sometimes it is useful to perform actions using some form of loops.\n\n***\\[ Readers also liked: [Five ways to use redirect operators in bash](https:\/\/www.redhat.com\/sysadmin\/redirect-operators-bash) \\]***\n\nThe basic loop commands in Bash scripts are `for` and `while`.\n\n- `for` loops are typically used when you have a known, finite list, like a series of numbers, a list of items, or counters.\n- `while` loops can be used with lists but are also useful for conditions that do *not* have a known limit. You can use it to run commands *while* a condition is true (or invert the logic and run it while the condition is false).\n\nSo, while (pun intended) there are different ways to perform loops, in this article, I focus on examples using `while` loops.\n\n## 1\\. A simple while loop\nImagine that you're installing a huge application, and you're concerned that it may eventually use too much space on the file system.\n\nInstead of running `df` in a separate terminal, you can run a simple command to monitor the disk utilization every second. This allows you to interrupt the installation process if you see that it will hit that limit.\n```\nwhile true\ndo\n  df -k | grep home\n  sleep 1\ndone\n```\nIn this case, you're running the loop with a **true** condition, which means it will run forever or until you hit **CTRL-C.** Therefore, you need to keep an eye on it (otherwise, it will remain using the system's resources).\n\n**Note**: If you use a loop like this, you need to include a command like `sleep` to give the system some time to breathe between executions. Running anything non-stop could become a performance issue, especially if the commands inside the loop involve I\/O operations.\n\n## 2\\. Waiting for a condition to become true\nThere are variations of this scenario. For example, you know that at some point, the process will create a directory, and you are just waiting for that moment to perform other validations.\n\nYou can have a `while` loop to keep checking for that directory's existence and only write a message while the directory does not exist.\n\nIf you want to do something more elaborate, you could create a script and show a more clear indication that the loop condition became true:\n```\n#!\/bin\/bash\n\nwhile [ ! -d directory_expected ]\ndo\n   echo \"`date` - Still waiting\" \n   sleep 1\ndone\n\necho \"DIRECTORY IS THERE!!!\"\n```\n## 3\\. Using a while loop to manipulate a file\nAnother useful application of a `while` loop is to combine it with the `read` command to have access to columns (or fields) quickly from a text file and perform some actions on them.\n\nIn the following example, you are simply picking the columns from a text file with a predictable format and printing the values that you want to use to populate an `\/etc\/hosts` file.\n\nHere the assumption is that the file has columns delimited by spaces or tabs and that there are *no* spaces in the content of the columns. That could shift the content of the fields and not give you what you needed.\n\nNotice that you're just doing a simple operation to extract and manipulate information and not concerned about the command's reusability. I would classify this as one of those \"quick and dirty tricks.\"\n\nOf course, if this was something that you would repeatedly do, you should run it from a script, use proper names for the variables, and all those good practices (including transforming the filename in an argument and defining where to send the output, but today, the topic is `while` loops).\n```\n#!\/bin\/bash\n\ncat servers.txt | grep -v CPU | while read servername cpu ram ip\ndo\n   echo $ip $servername\ndone\n```\n***\\[ Learn the basics of using Kubernetes in this [free cheat sheet](https:\/\/developers.redhat.com\/promotions\/kubernetes-cheatsheet?intcmp=701f20000012ngPAAQ). \\]***\n\n## Wrap up\n`while` loops are useful in scripts that depend on a certain condition to be true or false, but you can also use them interactively to monitor some condition.\n\nThe important things to remember are:\n\nAlways be clear about what is the condition to *end* the loop. If that is something that you expect to do yourself with a **CTRL-C**, then keep an eye on your command. Think about *how fast* you want each loop cycle to be repeated. If you do any operation that involves I\/O inside the loop, that becomes a critical aspect of planning.\n\nWith that in mind, have fun `while` you can.\n***\n### About the author\n[![Roberto Nozaki](https:\/\/www.redhat.com\/rhdc\/managed-files\/styles\/media_thumbnail\/private\/sysadmin\/pictures\/2022-11\/RoberoNozaki.jpeg?itok=WBbw9DVe)](https:\/\/www.redhat.com\/en\/authors\/roberto-nozaki)\n\n[Roberto Nozaki](https:\/\/www.redhat.com\/en\/authors\/roberto-nozaki)\n\nRoberto Nozaki (RHCSA\/RHCE\/RHCA) is an Automation Principal Consultant at Red Hat Canada where he specializes in IT automation with Ansible. He has experience in the financial, retail, and telecommunications sectors, having performed different roles in his career, from programming in mainframe environments to delivering IBM\/Tivoli and Netcool products as a pre-sales and post-sales consultant.\n\nRoberto has been a computer and software programming enthusiast for over 35 years. He is currently interested in hacking what he considers to be the ultimate hardware and software: our bodies and our minds.\n\nRoberto lives in Toronto, and when he is not studying and working with Linux and Ansible, he likes to meditate, play the electric guitar, and research neuroscience, altered states of consciousness, biohacking, and spirituality.\n\n[Read full bio](https:\/\/www.redhat.com\/en\/authors\/roberto-nozaki)\n\n## More like this\nBlog post\n\n### [Enable intelligent insights with Red Hat Satellite MCP Server](https:\/\/www.redhat.com\/en\/blog\/enable-intelligent-insights-red-hat-satellite-mcp-server)\n\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\nOriginal podcast\n\n### [Technically Speaking \\| Taming AI agents with 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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":"Shell scripting, specifically Bash scripting, is a great way to automate repetitive or tedious tasks on your systems. Why type when you can schedule and run scripts in a hands-free manner? One of the many scripting constructs is the loop. A loop is a section of code that picks up data or generates data and then performs some operation on the data and then begins the process over again until some condition is met, the script is disrupted, or when input data is exhausted. When you use [Bash scripting](https:\/\/www.redhat.com\/sysadmin\/using-bash-automation), sometimes it is useful to perform actions using some form of loops.\n\n***\\[ Readers also liked: [Five ways to use redirect operators in bash](https:\/\/www.redhat.com\/sysadmin\/redirect-operators-bash) \\]***\n\nThe basic loop commands in Bash scripts are `for` and `while`.\n\n- `for` loops are typically used when you have a known, finite list, like a series of numbers, a list of items, or counters.\n- `while` loops can be used with lists but are also useful for conditions that do *not* have a known limit. You can use it to run commands *while* a condition is true (or invert the logic and run it while the condition is false).\n\nSo, while (pun intended) there are different ways to perform loops, in this article, I focus on examples using `while` loops.\n\n## 1\\. A simple while loop\nImagine that you're installing a huge application, and you're concerned that it may eventually use too much space on the file system.\n\nInstead of running `df` in a separate terminal, you can run a simple command to monitor the disk utilization every second. This allows you to interrupt the installation process if you see that it will hit that limit.\n```\nwhile true\ndo\n  df -k | grep home\n  sleep 1\ndone\n```\nIn this case, you're running the loop with a **true** condition, which means it will run forever or until you hit **CTRL-C.** Therefore, you need to keep an eye on it (otherwise, it will remain using the system's resources).\n\n**Note**: If you use a loop like this, you need to include a command like `sleep` to give the system some time to breathe between executions. Running anything non-stop could become a performance issue, especially if the commands inside the loop involve I\/O operations.\n\n## 2\\. Waiting for a condition to become true\nThere are variations of this scenario. For example, you know that at some point, the process will create a directory, and you are just waiting for that moment to perform other validations.\n\nYou can have a `while` loop to keep checking for that directory's existence and only write a message while the directory does not exist.\n\nIf you want to do something more elaborate, you could create a script and show a more clear indication that the loop condition became true:\n```\n#!\/bin\/bash\n\nwhile [ ! -d directory_expected ]\ndo\n   echo \"`date` - Still waiting\" \n   sleep 1\ndone\n\necho \"DIRECTORY IS THERE!!!\"\n```\n## 3\\. Using a while loop to manipulate a file\nAnother useful application of a `while` loop is to combine it with the `read` command to have access to columns (or fields) quickly from a text file and perform some actions on them.\n\nIn the following example, you are simply picking the columns from a text file with a predictable format and printing the values that you want to use to populate an `\/etc\/hosts` file.\n\nHere the assumption is that the file has columns delimited by spaces or tabs and that there are *no* spaces in the content of the columns. That could shift the content of the fields and not give you what you needed.\n\nNotice that you're just doing a simple operation to extract and manipulate information and not concerned about the command's reusability. I would classify this as one of those \"quick and dirty tricks.\"\n\nOf course, if this was something that you would repeatedly do, you should run it from a script, use proper names for the variables, and all those good practices (including transforming the filename in an argument and defining where to send the output, but today, the topic is `while` loops).\n```\n#!\/bin\/bash\n\ncat servers.txt | grep -v CPU | while read servername cpu ram ip\ndo\n   echo $ip $servername\ndone\n```\n***\\[ Learn the basics of using Kubernetes in this [free cheat sheet](https:\/\/developers.redhat.com\/promotions\/kubernetes-cheatsheet?intcmp=701f20000012ngPAAQ). \\]***\n\n## Wrap up\n`while` loops are useful in scripts that depend on a certain condition to be true or false, but you can also use them interactively to monitor some condition.\n\nThe important things to remember are:\n\nAlways be clear about what is the condition to *end* the loop. If that is something that you expect to do yourself with a **CTRL-C**, then keep an eye on your command. Think about *how fast* you want each loop cycle to be repeated. If you do any operation that involves I\/O inside the loop, that becomes a critical aspect of planning.\n\nWith that in mind, have fun `while` you can.","meta_canonical":null}

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Boilerpipe Text	Shell scripting, specifically Bash scripting, is a great way to automate repetitive or tedious tasks on your systems. Why type when you can schedule and run scripts in a hands-free manner? One of the many scripting constructs is the loop. A loop is a section of code that picks up data or generates data and then performs some operation on the data and then begins the process over again until some condition is met, the script is disrupted, or when input data is exhausted. When you use Bash scripting , sometimes it is useful to perform actions using some form of loops. [ Readers also liked: Five ways to use redirect operators in bash ] The basic loop commands in Bash scripts are for and while . for loops are typically used when you have a known, finite list, like a series of numbers, a list of items, or counters. while loops can be used with lists but are also useful for conditions that do not have a known limit. You can use it to run commands while a condition is true (or invert the logic and run it while the condition is false). So, while (pun intended) there are different ways to perform loops, in this article, I focus on examples using while loops. 1. A simple while loop Imagine that you're installing a huge application, and you're concerned that it may eventually use too much space on the file system. Instead of running df in a separate terminal, you can run a simple command to monitor the disk utilization every second. This allows you to interrupt the installation process if you see that it will hit that limit. while true do df -k \| grep home sleep 1 done In this case, you're running the loop with a true condition, which means it will run forever or until you hit CTRL-C. Therefore, you need to keep an eye on it (otherwise, it will remain using the system's resources). Note : If you use a loop like this, you need to include a command like sleep to give the system some time to breathe between executions. Running anything non-stop could become a performance issue, especially if the commands inside the loop involve I/O operations. 2. Waiting for a condition to become true There are variations of this scenario. For example, you know that at some point, the process will create a directory, and you are just waiting for that moment to perform other validations. You can have a while loop to keep checking for that directory's existence and only write a message while the directory does not exist. If you want to do something more elaborate, you could create a script and show a more clear indication that the loop condition became true: #!/bin/bash while [ ! -d directory_expected ] do echo "`date` - Still waiting" sleep 1 done echo "DIRECTORY IS THERE!!!" 3. Using a while loop to manipulate a file Another useful application of a while loop is to combine it with the read command to have access to columns (or fields) quickly from a text file and perform some actions on them. In the following example, you are simply picking the columns from a text file with a predictable format and printing the values that you want to use to populate an /etc/hosts file. Here the assumption is that the file has columns delimited by spaces or tabs and that there are no spaces in the content of the columns. That could shift the content of the fields and not give you what you needed. Notice that you're just doing a simple operation to extract and manipulate information and not concerned about the command's reusability. I would classify this as one of those "quick and dirty tricks." Of course, if this was something that you would repeatedly do, you should run it from a script, use proper names for the variables, and all those good practices (including transforming the filename in an argument and defining where to send the output, but today, the topic is while loops). #!/bin/bash cat servers.txt \| grep -v CPU \| while read servername cpu ram ip do echo $ip $servername done [ Learn the basics of using Kubernetes in this free cheat sheet . ] Wrap up while loops are useful in scripts that depend on a certain condition to be true or false, but you can also use them interactively to monitor some condition. The important things to remember are: Always be clear about what is the condition to end the loop. If that is something that you expect to do yourself with a CTRL-C , then keep an eye on your command. Think about how fast you want each loop cycle to be repeated. If you do any operation that involves I/O inside the loop, that becomes a critical aspect of planning. With that in mind, have fun while you can.
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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") # Linux scripting: 3 how-tos for while loops in Bash March 11, 2021[Roberto Nozaki](https://www.redhat.com/en/authors/roberto-nozaki "See more by Roberto Nozaki")3\-minute read [Linux](https://www.redhat.com/en/blog?f[0]=taxonomy_topic_tid:27061#rhdc-search-listing) [Automation and management](https://www.redhat.com/en/blog?f[0]=taxonomy_topic_tid:27011#rhdc-search-listing) Share Subscribe to RSS - [Back to all posts](https://www.redhat.com/en/blog) * Shell scripting, specifically Bash scripting, is a great way to automate repetitive or tedious tasks on your systems. Why type when you can schedule and run scripts in a hands-free manner? One of the many scripting constructs is the loop. A loop is a section of code that picks up data or generates data and then performs some operation on the data and then begins the process over again until some condition is met, the script is disrupted, or when input data is exhausted. When you use [Bash scripting](https://www.redhat.com/sysadmin/using-bash-automation), sometimes it is useful to perform actions using some form of loops. \[ Readers also liked: [Five ways to use redirect operators in bash](https://www.redhat.com/sysadmin/redirect-operators-bash) \]** The basic loop commands in Bash scripts are `for` and `while`. - `for` loops are typically used when you have a known, finite list, like a series of numbers, a list of items, or counters. - `while` loops can be used with lists but are also useful for conditions that do not have a known limit. You can use it to run commands while a condition is true (or invert the logic and run it while the condition is false). So, while (pun intended) there are different ways to perform loops, in this article, I focus on examples using `while` loops. ## 1\. A simple while loop Imagine that you're installing a huge application, and you're concerned that it may eventually use too much space on the file system. Instead of running `df` in a separate terminal, you can run a simple command to monitor the disk utilization every second. This allows you to interrupt the installation process if you see that it will hit that limit. ``` while true do df -k \| grep home sleep 1 done ``` In this case, you're running the loop with a true condition, which means it will run forever or until you hit CTRL-C. Therefore, you need to keep an eye on it (otherwise, it will remain using the system's resources). Note: If you use a loop like this, you need to include a command like `sleep` to give the system some time to breathe between executions. Running anything non-stop could become a performance issue, especially if the commands inside the loop involve I/O operations. ## 2\. Waiting for a condition to become true There are variations of this scenario. For example, you know that at some point, the process will create a directory, and you are just waiting for that moment to perform other validations. You can have a `while` loop to keep checking for that directory's existence and only write a message while the directory does not exist. If you want to do something more elaborate, you could create a script and show a more clear indication that the loop condition became true: ``` #!/bin/bash while [ ! -d directory_expected ] do echo "`date` - Still waiting" sleep 1 done echo "DIRECTORY IS THERE!!!" ``` ## 3\. Using a while loop to manipulate a file Another useful application of a `while` loop is to combine it with the `read` command to have access to columns (or fields) quickly from a text file and perform some actions on them. In the following example, you are simply picking the columns from a text file with a predictable format and printing the values that you want to use to populate an `/etc/hosts` file. Here the assumption is that the file has columns delimited by spaces or tabs and that there are no spaces in the content of the columns. That could shift the content of the fields and not give you what you needed. Notice that you're just doing a simple operation to extract and manipulate information and not concerned about the command's reusability. I would classify this as one of those "quick and dirty tricks." Of course, if this was something that you would repeatedly do, you should run it from a script, use proper names for the variables, and all those good practices (including transforming the filename in an argument and defining where to send the output, but today, the topic is `while` loops). ``` #!/bin/bash cat servers.txt \| grep -v CPU \| while read servername cpu ram ip do echo $ip $servername done ``` *\[ Learn the basics of using Kubernetes in this [free cheat sheet](https://developers.redhat.com/promotions/kubernetes-cheatsheet?intcmp=701f20000012ngPAAQ). \]* ## Wrap up `while` loops are useful in scripts that depend on a certain condition to be true or false, but you can also use them interactively to monitor some condition. The important things to remember are: Always be clear about what is the condition to end the loop. If that is something that you expect to do yourself with a CTRL-C, then keep an eye on your command. Think about how fast you want each loop cycle to be repeated. If you do any operation that involves I/O inside the loop, that becomes a critical aspect of planning. With that in mind, have fun `while` you can. *** ### About the author [![Roberto Nozaki](https://www.redhat.com/rhdc/managed-files/styles/media_thumbnail/private/sysadmin/pictures/2022-11/RoberoNozaki.jpeg?itok=WBbw9DVe)](https://www.redhat.com/en/authors/roberto-nozaki) [Roberto Nozaki](https://www.redhat.com/en/authors/roberto-nozaki) Roberto Nozaki (RHCSA/RHCE/RHCA) is an Automation Principal Consultant at Red Hat Canada where he specializes in IT automation with Ansible. He has experience in the financial, retail, and telecommunications sectors, having performed different roles in his career, from programming in mainframe environments to delivering IBM/Tivoli and Netcool products as a pre-sales and post-sales consultant. Roberto has been a computer and software programming enthusiast for over 35 years. He is currently interested in hacking what he considers to be the ultimate hardware and software: our bodies and our minds. Roberto lives in Toronto, and when he is not studying and working with Linux and Ansible, he likes to meditate, play the electric guitar, and research neuroscience, altered states of consciousness, biohacking, and spirituality. 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Readable Markdown	Shell scripting, specifically Bash scripting, is a great way to automate repetitive or tedious tasks on your systems. Why type when you can schedule and run scripts in a hands-free manner? One of the many scripting constructs is the loop. A loop is a section of code that picks up data or generates data and then performs some operation on the data and then begins the process over again until some condition is met, the script is disrupted, or when input data is exhausted. When you use [Bash scripting](https://www.redhat.com/sysadmin/using-bash-automation), sometimes it is useful to perform actions using some form of loops. *\[ Readers also liked: [Five ways to use redirect operators in bash](https://www.redhat.com/sysadmin/redirect-operators-bash) \]* The basic loop commands in Bash scripts are `for` and `while`. - `for` loops are typically used when you have a known, finite list, like a series of numbers, a list of items, or counters. - `while` loops can be used with lists but are also useful for conditions that do not have a known limit. You can use it to run commands while a condition is true (or invert the logic and run it while the condition is false). So, while (pun intended) there are different ways to perform loops, in this article, I focus on examples using `while` loops. ## 1\. A simple while loop Imagine that you're installing a huge application, and you're concerned that it may eventually use too much space on the file system. Instead of running `df` in a separate terminal, you can run a simple command to monitor the disk utilization every second. This allows you to interrupt the installation process if you see that it will hit that limit. ``` while true do df -k \| grep home sleep 1 done ``` In this case, you're running the loop with a true condition, which means it will run forever or until you hit CTRL-C. Therefore, you need to keep an eye on it (otherwise, it will remain using the system's resources). Note: If you use a loop like this, you need to include a command like `sleep` to give the system some time to breathe between executions. Running anything non-stop could become a performance issue, especially if the commands inside the loop involve I/O operations. ## 2\. Waiting for a condition to become true There are variations of this scenario. For example, you know that at some point, the process will create a directory, and you are just waiting for that moment to perform other validations. You can have a `while` loop to keep checking for that directory's existence and only write a message while the directory does not exist. If you want to do something more elaborate, you could create a script and show a more clear indication that the loop condition became true: ``` #!/bin/bash while [ ! -d directory_expected ] do echo "`date` - Still waiting" sleep 1 done echo "DIRECTORY IS THERE!!!" ``` ## 3\. Using a while loop to manipulate a file Another useful application of a `while` loop is to combine it with the `read` command to have access to columns (or fields) quickly from a text file and perform some actions on them. In the following example, you are simply picking the columns from a text file with a predictable format and printing the values that you want to use to populate an `/etc/hosts` file. Here the assumption is that the file has columns delimited by spaces or tabs and that there are no spaces in the content of the columns. That could shift the content of the fields and not give you what you needed. Notice that you're just doing a simple operation to extract and manipulate information and not concerned about the command's reusability. I would classify this as one of those "quick and dirty tricks." Of course, if this was something that you would repeatedly do, you should run it from a script, use proper names for the variables, and all those good practices (including transforming the filename in an argument and defining where to send the output, but today, the topic is `while` loops). ``` #!/bin/bash cat servers.txt \| grep -v CPU \| while read servername cpu ram ip do echo $ip $servername done ``` *\[ Learn the basics of using Kubernetes in this [free cheat sheet](https://developers.redhat.com/promotions/kubernetes-cheatsheet?intcmp=701f20000012ngPAAQ). \]* ## Wrap up `while` loops are useful in scripts that depend on a certain condition to be true or false, but you can also use them interactively to monitor some condition. The important things to remember are: Always be clear about what is the condition to end the loop. If that is something that you expect to do yourself with a CTRL-C, then keep an eye on your command. Think about how fast you want each loop cycle to be repeated. If you do any operation that involves I/O inside the loop, that becomes a critical aspect of planning. With that in mind, have fun `while` you can.
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