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 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 }
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The seq command to create standard bash for Loop (outdated method)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
}
Ref: https://www.cyberciti.biz/faq/bash-for-loop/
