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.
Numeric ranges for syntax is as follows:
for VARIABLE in 1 2 3 4 5 .. N do command1 command2 commandN done
OR
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:
#!/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The seq command to create standard bash for Loop (outdated method)
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/step (EXP3).
for (( EXP1; EXP2; EXP3 )) do command1 command2 command3 done ## The C-style Bash for loop ## for (( initializer; condition; step )) do shell_COMMANDS 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 the cp command fi # we are here means no backup file exists, just use cp command to copy file /bin/cp $f $f.bak done
For loop with array elements
In this example, we use the for loop to iterate over an array of elements defined as follows:
DB_AWS_ZONE=('us-east-2a' 'us-west-1a' 'eu-central-1a') for zone in "${DB_AWS_ZONE[@]}" do echo "Creating rds (DB) server in $zone, please wait ..." aws rds create-db-instance \ --availability-zone "$zone" --allocated-storage 20 --db-instance-class db.m1.small \ --db-instance-identifier test-instance \ --engine mariadb \ --master-username my_user_name \ --master-user-password my_password_here done
Loop with a shell variable
Sometimes we store important data in the shell variable, and we can use for a loop as follows to read the data:
_admin_ip="202.54.1.33|MUM_VPN_GATEWAY 23.1.2.3|DEL_VPN_GATEWAY 13.1.2.3|SG_VPN_GATEWAY" for e in $_admin_ip do ufw allow from "${e%%|*}" to any port 22 proto tcp comment 'Open SSH port for ${e##*|}' done
Loop with a number
We can specify a range in loops as follows:
for i in {START..END} do commands done ## step value ## for i in {START..END..STEP} do commands done ## example: ping cbz01, cbz02, cbz03, and cbz04 using a loop ## for i in 0{1..4} do h="cbz${i}" ping -c 1 -q "$h" &>/dev/null if [ $? -eq 0 ] then echo "server $h alive" else echo "server $h dead or can not ping." fi done
Loop with strings
Say we have a variable named PKGS, and we need to loop through a list of strings to install those packages:
PKGS="php7-openssl-7.3.19-r0 php7-common-7.3.19-r0 php7-fpm-7.3.19-r0 php7-opcache-7.3.19-r0 php7-7.3.19-r0" for p in $PKGS do echo "Installing $p package" sudo apk add "$p" done
Command substitution
Command substitution means run a shell command and store its output to a variable. For example:
up=$(uptime) echo "Server uptime is $up"
The for Loop argument list also workes command substitution as follows:
for var in $(command) do print "$var" done ## example ## for f in $(ls /nas/*.pdf) do print "File $f" done
Command-line arguments
A command line argument is nothing but an argument sent to a program being called. A program can take any number of command line arguments. For example, we are going to use the grep command to search for user names in the /etc/passwd file:$ grep 'vivek' /etc/passwd
grep is the name of an actual command and shell executed this command when you type command at shell prompt. The first word on the command line is:
- grep – name of the command to be executed.
- Everything else on command line is taken as arguments to this command.
The for Loop argument list also accpents Command-line arguments/paramenters as follows:
## $@ expands to the positional parameters, starting from one. ## for i in $@ do echo "Script arg is $i" done
You run it as follows:./script one foo bar
Putting all together
Bash for loop is useful for automating repetitive tasks in IT. Let us see how to run a simple command (such as uptime) on multiple Linux or Unix servers:
for s in server1 server2 server3 do ssh vivek@${s} "uptime" done
OR combine the echo command along with command substitution as follows:
for s in server1 server2 server3 do echo "Server ${s}: $(ssh vivek@${s} uptime)" done
Sample outputs:
Server server1: 09:34:46 up 12 days, 21:57, 0 users, load average: 0.08, 0.09, 0.09 Server server2: 09:34:50 up 17 days, 2:30, 0 users, load average: 0.03, 0.03, 0.00 Server server3: 09:34:53 up 17 days, 2:31, 0 users, load average: 0.04, 0.04, 0.00
In this standard bash for loop example we are going to update all CentOS/RHEL based servers using the yum command or apt command/apt-get command in case we have a Debian/Ubuntu based servers:
## CENTOS/RHEL example (for fedora replace yum with dnf) ## for s in server0{1..8} do echo "*** Patching and updating ${s} ***" ssh root@${s} -- "yum -y update" done
Here is simple but useful shell script example:
#!/usr/bin/env bash # Purpose: Update all my Linode servers powered by Debian/Ubuntu Linux # Author: Vivek Gite under GPL v2.x+ # ---------------------------------------- log="/tmp/apt-get.log" >"${log}" for s in ln.cbz0{1..5} do echo "Updating and patching $s, please wait..." | tee -a "${log}" ssh root@${s} -- apt-get -q -y update >/dev/null ssh root@${s} -- DEBIAN_FRONTEND=noninteractive apt-get -y -q upgrade >>"${log}" done echo "Check $log file for details."
See why we used DEBIAN_FRONTEND apt-get variable to avoid any prompts duding updates. It would be best if you set up ssh keys for automation purposes or running scripts from Linux/Unix cron jobs.
Finding ping time for multiple IP addresses
Here is my sample code:
#!/bin/bash ips="$(host -t a www.cyberciti.biz | awk '{ print $4}')" for i in $ips; do ping -q -c 4 "$i"; done
This will produce average stats as follows, indicating that multiple load balancer ICMP ECHO_REQUEST average time was 19-20 milliseconds
PING 104.22.10.214 (104.22.10.214) 56(84) bytes of data. --- 104.22.10.214 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3006ms rtt min/avg/max/mdev = 20.612/21.255/22.054/0.624 ms PING 172.67.7.239 (172.67.7.239) 56(84) bytes of data. --- 172.67.7.239 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3005ms rtt min/avg/max/mdev = 19.199/20.279/21.433/0.862 ms PING 104.22.11.214 (104.22.11.214) 56(84) bytes of data. --- 104.22.11.214 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time 3005ms rtt min/avg/max/mdev = 20.232/20.710/21.500/0.479 ms
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.
You learned how to use the bash for loop with various example. For loops can save time and help you with automation for tiny tasks. However, for complicated IT automation tasks, you should use tools like Ansible, Salt, Chef, pssh and others. 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
- man bash
- $ help for
- # help {
- $ help break
- $ help continue
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