How to read a file line by line?

1.Example 1.Tips

2.Bonus

2.Initiate a Loop

3.See also: While read LINE

This article introduces the concept of playing a file line by line in Linux with the help of examples and tips along with a guided tour of initiating a loop. The article discusses the errors committed while reading a file line by line on the Linux platform.
With samples and illustrations, it shows how the 'for loop' and 'while loop' differ in their respective outputs. It also provides tips on how to use the while loop and depicts its syntax. It concludes with the process behind initiating a loop along with the
side effects the while loops can exhibit.

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. ..), which in this example leads to an assessment for each line and not every word of 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.

Sample output with a for loop:

for line in $ (cat file.txt) 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 possible to get the result with a for loop provided to change the value of the variable $ IFS (Internal Field Separator, internal field separator) before starting the loop.

While loop

The while loop remains the most appropriate and easiest way to read a file line by line.

Syntax

while read line

do

command

done <file

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\n"; done < file.txt

or in a "bash" script:

#! / bin / bash

while read line

do

echo-e "$ line \ n"

done <file.txt

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

It is entirely possible from a structured file (like an address book or /etc/passwd, for example), 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 :\n\

Pseudo : $user\n\

UID :\t $uid\n\

GID :\t $gid\n\

Home :\t $home\n\

Shell :\t $shell\n\n"

done < /etc/passwd

Bonus

while read i; do echo -e "Parameter : $i"; done < <(echo -e "a\nab\nc")

Initiate a Loop

Although the while loop is the easiest method, it has its side effects. It obliterates the formatting of lines including spaces and tabs. •Moreover, 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=$'\n' # new field separator, the end of line

for line in $(cat fichier)

do

command

done

IFS=$old_IFS # restore default field separator