bash for loop Explained with Examples

bash for loop is one of the most popular used function for the data processing . Learn more about bash for loop basic construction , how to process a range of data, define increments, using command outputs as inputs to for loop and

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ell, provides shell scripting features in Linux and Unix /Unix like environments. bash off lots of functions and commands and bash for loop is one of the most popular used function for the data processing . Learn more about bash for loop basic construction , how to process a range of data, define increments, using command outputs as inputs to for loop and more.

bash for loop construction

a bash for loop in its basic construction takes a input list of items, which can be numbers, letter or list derived from the output of a command.

Basic structure of a bash for loop is

for <variable name to hold the value > in < list of items to be processed >

done

The loop progresses by taking one item at a time from the list, perform some action and then pick up the next item, until all items in the list are processed.

List of items can be a static list or items

list of numbers - 1 2 3 4 as in,

for i in 1 2 3 4 5

List of alphabetic characters - a b c d as in,

for i in a b c d

Output of a command

like - ls *.log ,as in,

for i in $(ls *.log)

Content of a file

like cat <filename> as in,

for i in $(cat /logs/ipaddress)

A variable name

like a=“abc def ghi jkl mno ” as in

for i in $a

bash for loop examples

To process the list of five numbers and print their values.

for i in 1 2 3 4 5 do echo “Value is $i” done

Value is 1 Value is 2 Value is 3 Value is 4 Value is 5

In this example:

i is used as variable name, a short name for index value, most commonly used in bash loops. 1 2 3 4 5 is the list of items echo command prints the list item.

Same way for list of non numeric values.

for i in AA BB CC DD EE do echo “Value is $i” done

Value is AA Value is BB Value is CC Value is DD Value is EE

bash for loop one liner

for loop can be written on a single line as by using ; as line breaks separating for and do lines.

for i in AA BB CC DD EE ; do echo “Value is $i” ; done

bash for loop - specify range

Specifying a range

Specify and range from to 5 .

for i in ( 1..5 ) do echo “Value is $i” done

Value is 1 Value is 2 Value is 3 Value is 4 Value is 5

Specify range start, end & incremental value

Specify start value as one , increment by one and final value as 5.

for i in ( c=1,c++,c=5 ) do echo “Value is $i” done

Value is 1 Value is 2 Value is 3 Value is 4 Value is 5

Using a command as input list

Use ls as input list of file, you can perform any operation on these files based on name, size, permission etc.

in the following example the for loop just prints the file name.

for i in $(ls) do echo “Name is $i ” done

To find and kill all the httpd processes

for i in $(ps -ef | grep httpd |awk ’{ print $2 }’ do kill -9 $i done

Processing for loop output at once

if you have a big file with number of ip addresses and you want to filter and count the select pattern;

for i in $(cat /tmp/ipaddresses ) do

 if [[ $i =~ 192 ]]
then 
echo $i 
fi

done | sort -u ### This will print a unique list of matching ips

and to get the count of matches

for i in $(cat /tmp/ipaddresses ) do

 if [[ $i =~ 192 ]]
then 
echo $i 
fi

done | sort -u | wc -l ### This will print a total number of unique matching ips

Break and continue, for loop operation

For example if you want to scan a huge number of file and find the first empty file, you can use following bash for loop.

for i in $( ls /tmp/log*) do echo “Name is $i ”

if [ -s $i ] ## file not empty then continue else ## file is empty echo “Empty file found $i” break

done

Bash versions

Current GNU bash version is bash 5.0 , latest and older versions also can be downloaded from GNU ftp site

<http://gnu.mirrors.pair.com/bash/>

$bash —version

tells the bash version installed, if you have older bash version it may not support newer features or newer may not support deprecated functions.