Bash

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for

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. for loop syntax

Numeric ranges for syntax is as follows:

for VARIABLE in 1 2 3 4 5 .. N
do
	command1
	command2
	commandN
done

for VARIABLE in file1 file2 file3 do command1 on $VARIABLE command2 commandN done OR

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:

1. !/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:

1. !/bin/bash

for i in {1..5} do

  echo "Welcome $i times"

done Bash v4.0+ has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax:

1. !/bin/bash

echo "Bash version ${BASH_VERSION}..." for i in {0..10..2}

 do
    echo "Welcome $i times"
done

Sample outputs:

Bash version 4.0.33(0)-release... Welcome 0 times Welcome 2 times Welcome 4 times Welcome 6 times Welcome 8 times Welcome 10 times The seq command (outdated)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:

1. !/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).

for (( EXP1; EXP2; EXP3 )) do command1 command2 command3 done A representative three-expression example in bash as follows:

1. !/bin/bash

for (( c=1; c<=5; c++ )) do

  echo "Welcome $c times"

done

Sample output:

Welcome 1 times Welcome 2 times Welcome 3 times Welcome 4 times Welcome 5 times How do I use for as infinite loops?

Infinite for loop can be created with empty expressions, such as:

1. !/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.

1. !/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.

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.

1. !/bin/bash

FILES="$@" 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 cp command fi

       # we are hear means no backup file exists, just use cp command to copy file

/bin/cp $f $f.bak done