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HowTo: Bash For While Loop Through File Contents Script

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

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