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

Tutorial details
Difficulty	Easy (rss)
Root privileges	No
Requirements	None

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"