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 output.

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 filet) 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 filet) do echo "$ line" done

This

is

row

No

1

This

is

row

No

2

This

[...]

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.

Syntax:

while read line

do

command

done

For Loop Example

The starting file:

This is line 1

This is line 2 *This is line 3 *This is line 4 *This is line 5

The instructions in the command line:

while read line; do echo -e "$line

"; done < filet

or in a "bash" script:

#! / bin / bash

while read line

do

echo-e "$ line n"

done

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).

Example:

#! /bin/bash

while IFS=: read user pass uid gid full home shell

do

echo -e "$full :

Pseudo : $user

UID :t $uid

GID :t $gid

Home :t $home

Shell :t $shell

"

done < /etc/passwd

Bonus

while read i; do echo -e "Parameter : $i"; done <

ab

c")

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.

Syntax :

old_IFS=$IFS # save the field separator

IFS=$'

' # new field separator, the end of line

for line in $(cat fichier)

do

command

done

IFS=$old_IFS # restore default field separator