bash: loop

Posted 2021-05-02 云服务圈

ow 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 .. Ndo command1 command2 commandNdone

OR

for VARIABLE in file1 file2 file3do command1 on $VARIABLE command2 commandNdone

OR

for OUTPUT in $(Linux-Or-Unix-Command-Here)do command1 on $OUTPUT command2 on $OUTPUT commandNdone

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/bashfor 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/bashfor 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/bashecho "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The seq command (outdated)

 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/bashfor 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/bashfor (( 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/bashfor (( ; ; )) 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 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