Bash For Loop Examples
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
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 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
The seq command (outdated)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).
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 $c times..." done
Sample output:
Welcome 1 times Welcome 2 times Welcome 3 times Welcome 4 times Welcome 5 times
How do I use for as infinite loops?
Infinite for loop can be created with empty expressions, such as:
#!/bin/bash for (( ; ; )) do echo "infinite loops [ hit CTRL+C to stop]" done
Conditional exit with break
You can do early exit with break statement inside the for loop. You can exit from within a FOR, WHILE or UNTIL loop using break. General break statement inside the for loop:
for I in 1 2 3 4 5 do statements1 #Executed for all values of ''I'', up to a disaster-condition if any. statements2 if (disaster-condition) then break #Abandon the loop. fi statements3 #While good and, no disaster-condition. done
Following shell script will go though all files stored in /etc directory. The for loop will be abandon when /etc/resolv.conf file found.
#!/bin/bash for file in /etc/* do if [ "${file}" == "/etc/resolv.conf" ] then countNameservers=$(grep -c nameserver /etc/resolv.conf) echo "Total ${countNameservers} nameservers defined in ${file}" break fi done
Early continuation with continue statement
To resume the next iteration of the enclosing FOR, WHILE or UNTIL loop use continue statement.
for I in 1 2 3 4 5 do statements1 #Executed for all values of ''I'', up to a disaster-condition if any. statements2 if (condition) then continue #Go to next iteration of I in the loop and skip statements3 fi statements3 done
This script make backup of all file names specified on command line. If .bak file exists, it will skip the cp command.
#!/bin/bash FILES="$@" 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
Recommended readings:
- See all sample for loop shell script in our bash shell directory.
- man bash
- help for
- help {
- help break
- help continue
Updated for accuracy!
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