🕷️ Crawler Inspector

Cookie Consent by [Free Privacy Policy Generator](https://www.freeprivacypolicy.com/) [Update cookies preferences](https://www.shellscript.sh/loops.html)  A Bourne Shell Programming / Scripting Tutorial for learning about using the Unix shell. Learn Linux / Unix shell scripting by example along with the theory. I'll have you mastering Unix shell scripting in no time. - [Home](https://www.shellscript.sh/) - [Philosophy](https://www.shellscript.sh/philosophy.html) - [A First Script](https://www.shellscript.sh/first.html) - [Variables (Part 1)](https://www.shellscript.sh/variables1.html) - [Wildcards](https://www.shellscript.sh/wildcards.html) - [Escape Characters](https://www.shellscript.sh/escape.html) - [Loops](https://www.shellscript.sh/loops.html#home) - [Test](https://www.shellscript.sh/test.html) - [Case](https://www.shellscript.sh/case.html) - [Variables (Part 2)](https://www.shellscript.sh/variables2.html) - [Variables (Part 3)](https://www.shellscript.sh/variables3.html) - [External Programs](https://www.shellscript.sh/external.html) - [Functions](https://www.shellscript.sh/functions.html) - [Hints and Tips](https://www.shellscript.sh/hints.html) - [Quick Reference](https://www.shellscript.sh/quickref.html) - [Interactive Shell](https://www.shellscript.sh/interactive.html) - [Exercises](https://www.shellscript.sh/exercises.html) - [Tips and Examples](https://www.shellscript.sh/examples/) - [Shell Scripting: Expert Recipes](https://amzn.to/3J1JYSx) [Buy the tutorial as a PDF for only \$5](https://gum.co/shellscript) The Shell Scripting Tutorial ## The Shell Scripting Tutorial *** ## Loops Most languages have the concept of loops: If we want to repeat a task twenty times, we don't want to have to type in the code twenty times, with maybe a slight change each time. As a result, we have `for` and `while` loops in the Bourne shell. This is somewhat fewer features than other languages, but nobody claimed that shell programming has the power of C. ## For Loops `for` loops iterate through a set of values until the list is exhausted: *** [for.sh](https://www.shellscript.sh/eg/for.sh.txt) ``` #!/bin/sh for i in 1 2 3 4 5 do echo "Looping ... number $i" done ``` *** Try this code and see what it does. Note that the values can be anything at all: *** [for2.sh](https://www.shellscript.sh/eg/for2.sh.txt) ``` #!/bin/sh for i in hello 1 * 2 goodbye do echo "Looping ... i is set to $i" done ``` *** This is well worth trying. Make sure that you understand what is happening here. Try it without the `*` and grasp the idea, then re-read the [Wildcards](https://www.shellscript.sh/wildcards.html) section and try it again with the `*` in place. Try it also in different directories, and with the `*` surrounded by double quotes, and try it preceded by a backslash (`\*`) In case you don't have access to a shell at the moment (it is very useful to have a shell to hand whilst reading this tutorial), the results of the above two scripts are: ``` Looping .... number 1 Looping .... number 2 Looping .... number 3 Looping .... number 4 Looping .... number 5 ``` and, for the second example: ``` Looping ... i is set to hello Looping ... i is set to 1 Looping ... i is set to (name of first file in current directory) ... etc ... Looping ... i is set to (name of last file in current directory) Looping ... i is set to 2 Looping ... i is set to goodbye ``` So, as you can see, `for` simply loops through whatever input it is given, until it runs out of input. ## While Loops `while` loops can be much more fun! (depending on your idea of fun, and how often you get out of the house... ) *** [while.sh](https://www.shellscript.sh/eg/while.sh.txt) ``` #!/bin/sh INPUT_STRING=hello while [ "$INPUT_STRING" != "bye" ] do echo "Please type something in (bye to quit)" read INPUT_STRING echo "You typed: $INPUT_STRING" done ``` *** What happens here, is that the echo and read statements will run indefinitely until you type "bye" when prompted. Review [Variables - Part I](https://www.shellscript.sh/variables1.html) to see why we set `INPUT_STRING=hello` before testing it. This makes it a repeat loop, not a traditional while loop. The colon (`:`) always evaluates to true; whilst using this can be necessary sometimes, it is often preferable to use a real exit condition. Compare quitting the above loop with the one below; see which is the more elegant. Also think of some situations in which each one would be more useful than the other: *** [while2.sh](https://www.shellscript.sh/eg/while2.sh.txt) ``` #!/bin/sh while : do echo "Please type something in (^C to quit)" read INPUT_STRING echo "You typed: $INPUT_STRING" done ``` *** Another useful trick is the `while read` loop. This example uses the [case](https://www.shellscript.sh/case.html) statement, which we'll cover later. It reads from the file `myfile.txt`, and for each line, tells you what language it thinks is being used. (note: Each line must end with a LF (newline) - if `cat myfile.txt` doesn't end with a blank line, that final line will not be processed.) This reads the file "`myfile.txt`", one line at a time, into the variable "`$input_text`". The [case](https://www.shellscript.sh/case.html) statement then checks the value of `$input_text`. If the word that was read from `myfile.txt` was "hello" then it `echo`es the word "English". If it was "gday" then it will `echo Australian`. If the word (or words) read from a line of `myfile.txt` don't match any of the provided patterns, then the catch-all "\*" default will display the message "Unknown Language: \$input\_text" - where of course "\$input\_text" is the value of the line that it read in from `myfile.txt`. *** [while3.sh](https://www.shellscript.sh/eg/while3.sh.txt) ``` #!/bin/sh while read input_text do case $input_text in hello) echo English ;; howdy) echo American ;; gday) echo Australian ;; bonjour) echo French ;; "guten tag") echo German ;; *) echo Unknown Language: $input_text ;; esac done < myfile.txt ``` Let's say our `myfile.txt` file contains the following five lines: ``` this file is called myfile.txt and we are using it as an example input. hello gday bonjour hola ``` A sample run of this script would go like this: ``` $ ./while3.sh Unknown Language: this file is called myfile.txt and we are using it as an example input. English Australian French Unknown Language: hola ``` *** A handy Bash (but not Bourne Shell) tip I learned recently from the [Linux From Scratch](http://www.linuxfromscratch.org/) project is: ``` mkdir rc{0,1,2,3,4,5,6,S}.d ``` instead of the more cumbersome: ``` for runlevel in 0 1 2 3 4 5 6 S do mkdir rc${runlevel}.d done ``` And this can be done recursively, too: ``` $ cd / $ ls -ld {,usr,usr/local}/{bin,sbin,lib} drwxr-xr-x 2 root root 4096 Oct 26 01:00 /bin drwxr-xr-x 6 root root 4096 Jan 16 17:09 /lib drwxr-xr-x 2 root root 4096 Oct 27 00:02 /sbin drwxr-xr-x 2 root root 40960 Jan 16 19:35 usr/bin drwxr-xr-x 83 root root 49152 Jan 16 17:23 usr/lib drwxr-xr-x 2 root root 4096 Jan 16 22:22 usr/local/bin drwxr-xr-x 3 root root 4096 Jan 16 19:17 usr/local/lib drwxr-xr-x 2 root root 4096 Dec 28 00:44 usr/local/sbin drwxr-xr-x 2 root root 8192 Dec 27 02:10 usr/sbin ``` *** We will use while loops further in the [Test](https://www.shellscript.sh/test.html) and [Case](https://www.shellscript.sh/case.html) sections. [Get this tutorial as a PDF for only \$5](https://gum.co/shellscript) *** [Previous: Escape Characters](https://www.shellscript.sh/escape.html) *** ## My Paperbacks and eBooks My Shell Scripting books, available in Paperback and eBook formats. This [tutorial](http://amzn.to/2mPj2tK) is more of a general introduction to Shell Scripting, the longer [Shell Scripting: Expert Recipes for Linux, Bash and more](http://amzn.to/2mPhTlK) book covers every aspect of Bash in detail. | | | |---|---| | [ Shell Scripting Tutorial](http://amzn.to/2mPj2tK) is this tutorial, in 88-page Paperback and eBook formats. Convenient to read on the go, and in paperback format good to keep by your desk as an ever-present companion. Also available in PDF form from Gumroad:[Get this tutorial as a PDF](https://gum.co/shellscript) | [ Shell Scripting: Expert Recipes for Linux, Bash and more](http://amzn.to/2mPhTlK) is my 564-page book on Shell Scripting. The first half covers all of the features of the shell in every detail; the second half has real-world shell scripts, organised by topic, along with detailed discussion of each script. | Copyright © 2000 - 2026 Steve Parker

Most languages have the concept of loops: If we want to repeat a task twenty times, we don't want to have to type in the code twenty times, with maybe a slight change each time. As a result, we have `for` and `while` loops in the Bourne shell. This is somewhat fewer features than other languages, but nobody claimed that shell programming has the power of C. ## For Loops `for` loops iterate through a set of values until the list is exhausted: *** [for.sh](https://www.shellscript.sh/eg/for.sh.txt) ``` #!/bin/sh for i in 1 2 3 4 5 do echo "Looping ... number $i" done ``` *** Try this code and see what it does. Note that the values can be anything at all: *** [for2.sh](https://www.shellscript.sh/eg/for2.sh.txt) ``` #!/bin/sh for i in hello 1 * 2 goodbye do echo "Looping ... i is set to $i" done ``` *** This is well worth trying. Make sure that you understand what is happening here. Try it without the `*` and grasp the idea, then re-read the [Wildcards](https://www.shellscript.sh/wildcards.html) section and try it again with the `*` in place. Try it also in different directories, and with the `*` surrounded by double quotes, and try it preceded by a backslash (`\*`) In case you don't have access to a shell at the moment (it is very useful to have a shell to hand whilst reading this tutorial), the results of the above two scripts are: ``` Looping .... number 1 Looping .... number 2 Looping .... number 3 Looping .... number 4 Looping .... number 5 ``` and, for the second example: ``` Looping ... i is set to hello Looping ... i is set to 1 Looping ... i is set to (name of first file in current directory) ... etc ... Looping ... i is set to (name of last file in current directory) Looping ... i is set to 2 Looping ... i is set to goodbye ``` So, as you can see, `for` simply loops through whatever input it is given, until it runs out of input. ## While Loops `while` loops can be much more fun! (depending on your idea of fun, and how often you get out of the house... ) *** [while.sh](https://www.shellscript.sh/eg/while.sh.txt) ``` #!/bin/sh INPUT_STRING=hello while [ "$INPUT_STRING" != "bye" ] do echo "Please type something in (bye to quit)" read INPUT_STRING echo "You typed: $INPUT_STRING" done ``` *** What happens here, is that the echo and read statements will run indefinitely until you type "bye" when prompted. Review [Variables - Part I](https://www.shellscript.sh/variables1.html) to see why we set `INPUT_STRING=hello` before testing it. This makes it a repeat loop, not a traditional while loop. The colon (`:`) always evaluates to true; whilst using this can be necessary sometimes, it is often preferable to use a real exit condition. Compare quitting the above loop with the one below; see which is the more elegant. Also think of some situations in which each one would be more useful than the other: *** [while2.sh](https://www.shellscript.sh/eg/while2.sh.txt) ``` #!/bin/sh while : do echo "Please type something in (^C to quit)" read INPUT_STRING echo "You typed: $INPUT_STRING" done ``` *** Another useful trick is the `while read` loop. This example uses the [case](https://www.shellscript.sh/case.html) statement, which we'll cover later. It reads from the file `myfile.txt`, and for each line, tells you what language it thinks is being used. (note: Each line must end with a LF (newline) - if `cat myfile.txt` doesn't end with a blank line, that final line will not be processed.) This reads the file "`myfile.txt`", one line at a time, into the variable "`$input_text`". The [case](https://www.shellscript.sh/case.html) statement then checks the value of `$input_text`. If the word that was read from `myfile.txt` was "hello" then it `echo`es the word "English". If it was "gday" then it will `echo Australian`. If the word (or words) read from a line of `myfile.txt` don't match any of the provided patterns, then the catch-all "\*" default will display the message "Unknown Language: \$input\_text" - where of course "\$input\_text" is the value of the line that it read in from `myfile.txt`. *** [while3.sh](https://www.shellscript.sh/eg/while3.sh.txt) ``` #!/bin/sh while read input_text do case $input_text in hello) echo English ;; howdy) echo American ;; gday) echo Australian ;; bonjour) echo French ;; "guten tag") echo German ;; *) echo Unknown Language: $input_text ;; esac done < myfile.txt ``` Let's say our `myfile.txt` file contains the following five lines: ``` this file is called myfile.txt and we are using it as an example input. hello gday bonjour hola ``` A sample run of this script would go like this: ``` $ ./while3.sh Unknown Language: this file is called myfile.txt and we are using it as an example input. English Australian French Unknown Language: hola ``` *** A handy Bash (but not Bourne Shell) tip I learned recently from the [Linux From Scratch](http://www.linuxfromscratch.org/) project is: ``` mkdir rc{0,1,2,3,4,5,6,S}.d ``` instead of the more cumbersome: ``` for runlevel in 0 1 2 3 4 5 6 S do mkdir rc${runlevel}.d done ``` And this can be done recursively, too: ``` $ cd / $ ls -ld {,usr,usr/local}/{bin,sbin,lib} drwxr-xr-x 2 root root 4096 Oct 26 01:00 /bin drwxr-xr-x 6 root root 4096 Jan 16 17:09 /lib drwxr-xr-x 2 root root 4096 Oct 27 00:02 /sbin drwxr-xr-x 2 root root 40960 Jan 16 19:35 usr/bin drwxr-xr-x 83 root root 49152 Jan 16 17:23 usr/lib drwxr-xr-x 2 root root 4096 Jan 16 22:22 usr/local/bin drwxr-xr-x 3 root root 4096 Jan 16 19:17 usr/local/lib drwxr-xr-x 2 root root 4096 Dec 28 00:44 usr/local/sbin drwxr-xr-x 2 root root 8192 Dec 27 02:10 usr/sbin ``` *** We will use while loops further in the [Test](https://www.shellscript.sh/test.html) and [Case](https://www.shellscript.sh/case.html) sections.