This article will introduce the concept of playing a file line by line in Linux with the help of examples and best user tips. We'll walk you through some of the most common errors made when reading a file on the Linux platform, and show you examples of how the for loop and while loop outputs differ.
We'll also provide you with some tips and examples on how to initiate a loop
, and how to use the while loop
How To Read a File Line by Line
Common Errors with For Loops
One of the most common errors when using scripts bash on GNU/Linux is to read a file line by line by using a for loop
(for line in $ (cat file.txt) do. ..). In this example, the for loop leads to an assessment for each line, rather than as assessment of every word in the file.
It is possible to change the value of the variable $ IFS (Internal Field Separator, internal field separator) with a for loop before starting the loop.
Here is a sample output with a for loop
for line in $ (cat file.txt) do echo "$ line" done
The solution is to use a while loop coupled with the internal read.
It is also possible to get the result with a for loop
, provided you change the value of the variable $ IFS (Internal Field Separator, internal field separator) before starting the loop.
While Loop Example
The while loop
remains the most appropriate and easiest way to read a file line by line.
while read line
For Loop Example
The starting file:
This is line 1
The instructions in the command line:
- This is line 2 *This is line 3 *This is line 4 *This is line 5
while read line; do echo -e "$line\n"; done < file.txt
or in a "bash" script:
#! / bin / bash
while read line
echo-e "$ line \ n"
The output on the screen (stdout):
This is line 1
This is line 2
This is line 3
This is line 4
This is line 5
Tips for For Loops
From a structured file (such as an address book or /etc/passwd
), it is entirely possible to retrieve the values of each field and assign them to several variables with the command 'read'. Be careful to properly assign the IFS variable with good field separators (space by default).
while IFS=: read user pass uid gid full home shell
echo -e "$full :\n\
Pseudo : $user\n\
UID :\t $uid\n\
GID :\t $gid\n\
Home :\t $home\n\
Shell :\t $shell\n\n"
done < /etc/passwd
while read i; do echo -e "Parameter : $i"; done < <(echo -e "a\nab\nc")
How To Initiate a Loop
Although the while loop
remains the easiest method for reading a file line by line, it does have its side effects. The while loop will obliterate the formatting of lines, including spaces and tabs. Furthermore, the for loop
coupled with a change of IFS
helps keep the structure of the document output.
old_IFS=$IFS # save the field separator
Image: © Jaroslav Machacek - Shutterstock.com
IFS=$'\n' # new field separator, the end of line
for line in $(cat fichier)
IFS=$old_IFS # restore default field separator
This document, titled « How To Read a Linux File Line by Line », is available under the Creative Commons
license. Any copy, reuse, or modification of the content should be sufficiently credited to CCM