Question: 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?

Answer: 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

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). It can be done easily with seq command. A representative example in bash as follows:

#!/bin/bash
for i in $(seq 1 2 20)
do
echo “Welcome $i times”
done

Latest bash version 3.0+ has inbuilt support for setting up ranges:

#!/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:

#!/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

4.0.33(0)-release

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:

#!/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:

#!/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
done
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.

for I in 1 2 3 4 5
done
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=“$@”
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

Further readings:

  • man bash
  • help for
  • help {
  • help break
  • help continue

Updated for accuracy!