HowTo: Bash For While Loop Through File Contents Script

Posted on in Categories , , last updated December 1, 2012

How can I iterate through each line of a text file with Bash for or while loop under Linux or Unix like operating systems?

The best way to read a text file line by line is to use the bash while loop. The while loop syntax is as follows:

while COMMANDS
do
COMMANDS
done


OR

while read line
do
echo "$line"
done < /path/to/input.txt

In this example, read /etc/passwd file line by line using while loop:

#!/bin/bash
while IFS= read -r line
do 
  echo "$line"
done < "/etc/passwd"

Another example:

#!/bin/bash
## script.sh ##
_file="${1:-/dev/null}"   #fail safe 
while IFS= read -r line
do 
  # Do something on $line
  echo "$line"
done < "$_file"

Run it as follows:
chmod +x script.sh
./script.sh /path/to/text/file.name

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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