How do I use bash for loop to repeat certain task under Linux / UNIX operating system? How do I set infinite loops using for statement? How do I use three-parameter for loop control expression?
A ‘for loop’ is a bash programming language statement which allows code to be repeatedly executed. A for loop is classified as an iteration statement i.e. it is the repetition of a process within a bash script. For example, you can run UNIX command or task 5 times or read and process list of files using a for loop. A for loop can be used at a shell prompt or within a shell script itself.
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for OUTPUT in $(Linux-Or-Unix-Command-Here)
do
command1 on $OUTPUT
command2 on $OUTPUT
commandN
done
Examples
This type of for loop is characterized by counting. The range is specified by a beginning (#1) and ending number (#5). The for loop executes a sequence of commands for each member in a list of items. A representative example in BASH is as follows to display welcome message 5 times with for loop:
#!/bin/bash
for i in 1 2 3 4 5
do
echo “Welcome $i times”
done
Sometimes you may need to set a step value (allowing one to count by two’s or to count backwards for instance). Latest bash version 3.0+ has inbuilt support for setting up ranges:
#!/bin/bash
echo “Bash version ${BASH_VERSION}…”
for i in {0..10..2}
do
echo “Welcome $i times”
done
Sample outputs:
1
2
3
4
5
6
7
Bash version4.0.33(0)-release...
Welcome0times
Welcome2times
Welcome4times
Welcome6times
Welcome8times
Welcome10times
The seq command (outdated)
WARNING! The seq command print a sequence of numbers and it is here due to historical reasons. The following examples is only recommend for older bash version. All users (bash v3.x+) are recommended to use the above syntax.
The seq command can be used as follows. A representative example in seq is as follows:
#!/bin/bash
for i in $(seq 1 2 20)
do
echo “Welcome $i times”
done
There is no good reason to use an external command such as seq to count and increment numbers in the for loop, hence it is recommend that you avoid using seq. The builtin command are fast.
Three-expression bash for loops syntax
This type of for loop share a common heritage with the C programming language. It is characterized by a three-parameter loop control expression; consisting of an initializer (EXP1), a loop-test or condition (EXP2), and a counting expression (EXP3).
#!/bin/bash
for (( ; ; ))
do
echo “infinite loops [ hit CTRL+C to stop]”
done
Conditional exit with break
You can do early exit with break statement inside the for loop. You can exit from within a FOR, WHILE or UNTIL loop using break. General break statement inside the for loop:
for I in 1 2 3 4 5
do
statements1 #Executed for all values of ”I”, up to a disaster-condition if any.
statements2
if (disaster-condition)
then
break #Abandon the loop.
fi
statements3 #While good and, no disaster-condition.
done
Following shell script will go though all files stored in /etc directory. The for loop will be abandon when /etc/resolv.conf file found.
#!/bin/bash
for file in /etc/*
do
if [ “${file}” == “/etc/resolv.conf” ]
then
countNameservers=$(grep -c nameserver /etc/resolv.conf)
echo “Total ${countNameservers} nameservers defined in ${file}”
break
fi
done
Early continuation with continue statement
To resume the next iteration of the enclosing FOR, WHILE or UNTIL loop use continue statement.
<span style="color: #7a0874; font-weight: bold;">continue</span><span style="color: #666666; font-style: italic;">#Go to next iteration of I in the loop and skip statements3</span>
for I in 1 2 3 4 5
do
statements1 #Executed for all values of ”I”, up to a disaster-condition if any.
statements2
if (condition)
then
continue #Go to next iteration of I in the loop and skip statements3
fi
statements3
done
This script make backup of all file names specified on command line. If .bak file exists, it will skip the cp command.
#!/bin/bash
FILES=”[email protected]”
for f in $FILES
do
# if .bak backup file exists, read next file
if [ -f ${f}.bak ]
then
echo “Skiping $f file…”
continue # read next file and skip the cp command
fi
# we are here means no backup file exists, just use cp command to copy file
/bin/cp $f $f.bak
done
Check out related media
This tutorial is also available in a quick video format. The video shows some additional and practical examples such as converting all flac music files to mp3 format, all avi files to mp4 video format, unzipping multiple zip files or tar balls, gathering uptime information from multiple Linux/Unix servers, detecting remote web-server using domain names and much more.
Video 01: 15 Bash For Loop Examples for Linux / Unix / OS X Shell Scripting
Conclusion
You learned how to use the bash for loop with various example. See the following resources for more info.
See all sample for loop shell script in our bash shell directory
Bash for loop syntax and usage page from the Linux shell scripting wiki
WARNING! The seq command print a sequence of numbers and it is here due to historical reasons. The following examples is only recommend for older bash version. All users (bash v3.x+) are recommended to use the above syntax.
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