Linux - Bash Loops
Bash shell can repeat particular instruction again and again, until particular condition satisfies. A group of instruction that is executed repeatedly is called a loop. Bash supports:
The for loop
The while loop
Each and every loop must:
- First, the variable used in loop condition must be initialized, then execution of the loop begins.
- A test (condition) is made at the beginning of each iteration.
- The body of loop ends with a statement that modifies the value of the test (condition) variable.
- Repeatedly execute a block of statements.
The for loop syntax
The for loop syntax is as follows:
for var in item1 item2 ... itemN
do
command1
command2
....
...
commandN
done
The for loop numerical explicit list syntax:
for var in list-of-values
do
command1
command2
....
...
commandN
done
The for loop explicit file list syntax:
for var in file1 file2 file3 fileN
do
command1
command2
....
...
commandN
done
The for loop variable's contents syntax:
for var in $fileNames
do
command1
command2
....
...
commandN
done
The for loop command substitution syntax:
for var in $(Linux-command-name)
do
command1
command2
....
...
commandN
done
The for loop explicit file list using bash array syntax:
# define an array
ArrayName=(~/.config/*.conf)
for var in "${ArrayName[@]}"
do
command1 on $var
command2
....
...
commandN
done
The for loop three-expression syntax ( this type of for loop share a common heritage with the C programming language ):
for (( EXP1; EXP2; EXP3 ))
do
command1
command2
command3
done
The above syntax 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).
More about the for loop
The for loop execute a command line once for every new value assigned to a var (variable) in specified list (item1...itemN) i.e. repeat all statement between do and done till condition is not satisfied. The lists or values are normally:
- Strings
- Numbers
- Command line arguments
- File names
- Linux command output
Example
Create a shell script called testforloop.sh:
#!/bin/bash
for i in 1 2 3 4 5
do
echo "Welcome $i times."
done
Save and close the file. Run it as follows:
chmod +x testforloop.sh
./testforloop.sh
The for loop first creates i variable and assigned a number to i from the list of number from 1 to 5. The shell execute echo statement for each assignment of i. This is known as iteration. This process will continue until all the items in the list were not finished. See bash for loop examples page for more information.
The For Loop Using Strings
Create a shell script called forcars.sh
#!/bin/bash
# A simple shell script to print list of cars
for car in bmw ford toyota nissan
do
echo "Value of car is: $car"
done
Another example, create a shell script called forcmds.sh:
#!/bin/bash
# A simple shell script to run commands
for command in date pwd df
do
echo
echo "*** The output of $command command >"
#run command
$command
echo
done
Save and close the file. Run it as follows:
chmod +x forcmds.sh
./forcmds.sh
Sample outputs:
*** The output of date command > Sun Sep 6 14:32:41 IST 2009 *** The output of pwd command > /1.5/share/data/songs *** The output of df command > Filesystem 1K-blocks Used Available Use% Mounted on /dev/sdb2 96116904 27589760 63644592 31% / tmpfs 4149972 0 4149972 0% /lib/init/rw varrun 4149972 272 4149700 1% /var/run varlock 4149972 0 4149972 0% /var/lock udev 4149972 2808 4147164 1% /dev tmpfs 4149972 356 4149616 1% /dev/shm /dev/sdb5 286374908 274733944 11640964 96% /share /dev/sdc2 240402848 159452732 68738308 70% /disk1p2 /dev/sda5 1341352436 412128756 861086932 33% /1.5 /dev/sdd1 1442145212 26365188 1342523224 2% /media/backup
The For Loop Using Variable's Contents
Create a shell script called forfilenames.sh
#!/bin/bash
# A shell script to verify user password database
files="/etc/passwd /etc/group /etc/shadow /etc/gshdow"
for f in $files
do
[ -f $f ] && echo "$f file found" || echo "*** Error - $f file missing."
done
The For Loop Using Command-line Arguments
Create a shell script called forcmdargs.sh:
#!/bin/bash
# A simple shell script to display a file on screen passed as command line argument
[ $# -eq 0 ] && { echo "Usage: $0 file1 file2 fileN"; exit 1; }
# read all command line arguments via the for loop
for f in $*
do
echo
echo "< $f >"
[ -f $f ] && cat $f || echo "$f not file."
echo "------------------------------------------------"
done
Save and close the file. Run it as follows:
chmod +x forcmdargs.sh
./forcmdargs.sh /etc/resolv.conf /etc/hostname
Sample outputs:
< /etc/resolv.conf > nameserver 127.0.0.1 nameserver 4.2.2.1 nameserver 4.2.2.2 ------------------------------------------------ < /etc/hostname > vivek-desktop ------------------------------------------------
The for loop using command substitution
Command substitution is nothing but a shell command output stored in into a string or a variable. The command is a shell command and must be enclosed between grave accents or $(..). The syntax is as follows:
$(command-name)
`command-name`
var=$(command-name)
NOW=$(date)
echo $NOW
Create a shell script called forcmdsub.sh:
#!/bin/bash
echo "Printing file names in /tmp directory:"
for f in $(ls /tmp/*)
do
echo $f
done
The for loop using ranges or counting
The for loop can be set using the numerical range. The range is specified by a beginning and ending number. 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 multiplication table with for loop (multiplication.sh):
#!/bin/bash
n=$1
# make sure command line arguments are passed to the script
if [ $# -eq 0 ]
then
echo "A shell script to print multiplication table."
echo "Usage : $0 number"
exit 1
fi
# Use for loop
for i in {1..10}
do
echo "$n * $i = $(( $i * $n))"
done
Save and close the file. Run it as follows:
chmod +x multiplication.sh
./multiplication.sh
./multiplication.sh 13
Sample outputs:
13 * 1 = 13 13 * 2 = 26 13 * 3 = 39 13 * 4 = 52 13 * 5 = 65 13 * 6 = 78 13 * 7 = 91 13 * 8 = 104 13 * 9 = 117 13 * 10 = 130
Source: https://bash.cyberciti.biz/guide/For_loop
