Bash For Loop Examples

What is a Bash for loop?

A bash 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

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

#!/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The seq command to create standard bash for Loop (outdated method) ↑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
# set counter 'c' to 1 and condition 
# c is less than or equal to 5
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
# Count dns name server in the /etc/resolv.conf if found
for file in /etc/*
do
        # check if file exists in bash using the if #  
	if [ "${file}" == "/etc/resolv.conf" ]
	then
		countNameservers=$(grep -c nameserver /etc/resolv.conf)
		echo "Total dns ${countNameservers} nameservers defined in ${file}"
		break
	fi
done

Outputs:

Total dns 2 nameservers defined in /etc/resolv.conf

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:

# Set three aws zones to create RDS instances 
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 to open ssh TCP port 22 using the ufw:

# Defines a variable called _admin_ip that contains a list of IP addresses. 
# The IP addresses are separated by the pipe character (|).
_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

• The for loop example allows Linux sysadmin to open SSH TCP port from the IP addresses in the $_admin_ip variable. This is helpfor for allowing developers, users and administrators to connect to your server from remote locations with specific IP address defined in the list.
• The for loop iterates over the IP addresses in the _admin_ip variable.
• For each IP address, the ufw command command is used to allow SSH connections from that IP address stored in the $e. The ${e%%|*} bash expression extracts the part of the IP address before the pipe character using parameter expansion modifiers.
• The ufw command allows ssh connections from the specified IP address to any port on the local Linux server.

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:

# Install the PHP packages listed in the PKGS variable.
PKGS="php7-openssl-7.3  php7-common-7.3  php7-fpm-7.3  php7-opcache-7.3 php7-7.3"
for p in $PKGS
do
   echo "Installing $p package"
   sudo apk add "$p"
done

The for loop iterates over the PHP package names in the PKGS variable. For each PHP package name, the echo command shows a message stating that the package is being installed on your Alpine Linux machine. The sudo command is used to run the apk command to add installs the specified package stored the $p variable.

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 ##
## Show the name of every PDF file in the /nas directory: 
for f in $(ls /nas/*.pdf)
do
  printf "File is %s\n" "$f"
done

The bash for loop iterates over the output of the ls command. The ls shows and list the contents of the /nas directory on your Linux or Unix machine. The *.pdf pattern matches all files with the .pdf extension. The $(ls /nas/*.pdf) command expands to a list of all PDF files in the /nas directory. The printf command prints each file name to the standard output/screen.

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 accepts Command-line arguments/parameters 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 named patch-debian-vm.sh:

#!/usr/bin/env bash
# Purpose: Update all my Linode servers powered by Debian/Ubuntu Linux
# Name: patch-debian-vm.sh
# 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 during 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 for the file named find-ping-time.sh

#!/bin/bash
# Purpose: Ping all of the IP addresses associated with my domain name www.example.com
# Name: find-ping-time.sh
# Author: Vivek Gite under GPL v2.x+
# -----------------------------------
ips="$(host -t a www.example.com | 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

The host command is used to resolve a domain name to its IP addresses and The awk command is used to extract the IP addresses from the output of the host command. Finally, the bash for loop iterates over the list of IP addresses. For each IP address, the ping command to send 4 requests and print the stats on the screen.

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.

Conclusion

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
• Also read the following manual pages using man command/info command or help command:
  $ man bash
$ info bash
$ help for
$ help {
$ help break
$ help continue