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 .. NThis 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:
do
command1
command2
commandN
done
#!/bin/bashSometimes 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:
for i in 1 2 3 4 5
do
echo "Welcome $i times"
done
#!/bin/bashBash v4.0+ has inbuilt support for setting up a step value using {START..END..INCREMENT} syntax:
for i in {1..5}
do
echo "Welcome $i times"
done
#!/bin/bashSample outputs:
echo "Bash version ${BASH_VERSION}..."
for i in {0..10..2}
do
echo "Welcome $i times"
done
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The seq command (outdated)
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/bashThere 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.
for i in $(seq 1 2 20)
do
echo "Welcome $i times"
done
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 ))A representative three-expression example in bash as follows:
do
command1
command2
command3
done
#!/bin/bashSample output:
for (( c=1; c<=5; c++ ))
do
echo "Welcome $c times..."
done
Welcome 1 times
Welcome 2 times
Welcome 3 times
Welcome 4 times
Welcome 5 times
How do I use for as infinite loops?
Infinite for loop can be created with empty expressions, such as:#!/bin/bash
for (( ; ; ))
do
echo "infinite loops [ hit CTRL+C to stop]"
done
Conditional exit with break
You can do early exit with break statement inside the for loop. You can exit from within a FOR, WHILE or UNTIL loop using break. General break statement inside the for loop:for I in 1 2 3 4 5Following shell script will go though all files stored in /etc directory. The for loop will be abandon when /etc/resolv.conf file found.
do
statements1 #Executed for all values of ''I'', up to a disaster-condition if any.
statements2
if (disaster-condition)
then
break #Abandon the loop.
fi
statements3 #While good and, no disaster-condition.
done
#!/bin/bash
for file in /etc/*
do
if [ "${file}" == "/etc/resolv.conf" ]
then
countNameservers=$(grep -c nameserver /etc/resolv.conf)
echo "Total ${countNameservers} nameservers defined in ${file}"
break
fi
done
Early continuation with continue statement
To resume the next iteration of the enclosing FOR, WHILE or UNTIL loop use continue statement.for I in 1 2 3 4 5This script make backup of all file names specified on command line. If .bak file exists, it will skip the cp command.
do
statements1 #Executed for all values of ''I'', up to a disaster-condition if any.
statements2
if (condition)
then
continue #Go to next iteration of I in the loop and skip statements3
fi
statements3
done
#!/bin/bash
FILES="$@"
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 cp command
fi
# we are hear means no backup file exists, just use cp command to copy file
/bin/cp $f $f.bak
done
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