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 , bourne again shell, 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 >
do
<perform some action on variable name to holding the values >
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
1. 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
2. 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
fidone | 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
fidone | 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.
More bash and related readings: