How to read a file line by line

Intro

One of the most common errors of learning scripts bash on GNU / Linux is to read a file line by line, is to use a loop "for" (for line in $ (cat file.txt) do. ..), which in this example leads to an assessment for li not every word of the file.


Sample output with a loop "for":
for line in $ (cat file.txt) do echo "$ line" done
This
is
the
row
No
1
This
is
the
row
No
2
This
[...]

The solution is to use a loop "while" coupled with the internal read.

But it is possible to get me the result with a loop "for" provided to change the value of the variable $ IFS (Internal Field Separator, internal field separator) before starting the loop. This is what we will see more ...
While loop

The loop "while" 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 the No. 1 line
This is line 2
This is the line # 3
This is the line n ° 4
This is the line # 5

The instructions from the command line:
while read line; do echo-e "$ line \ n" done <file.txt

or in a script "bash":
#! / bin / bash

while read line
do
echo-e "$ line \ n"
done <file.txt

The output on the screen (stdout):
This is the No. 1 line

This is line 2

This is the line # 3

This is the line n ° 4

This is the line # 5

Tips

It is entirely possible from a structured file (like an address book or / etc / passwd for example), retrieve the values of each field and assigned to several variables with the command "read" . Be careful to properly assign the variable IFS "good field separator (space by default).

Example:
#! / bin / bash

while IFS =: read user pass full uid gid home shell
do
echo-e "$ full: \ n \
Username: $ 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")
For loop

If the loop "while" is despite the easiest method, it has its side effect, however, that it obliterates the formatting lines including spaces and tabs - \
Moreover the loop "for" coupled with a change of IFS helps keep the structure of the document output.
Syntax

oldIFS = $ IFS # save the field separator
IFS = $ '\ n' # new field separator, the end of line
for line in $ (cat file)
do
command
done
IFS = $ # old_IFS restoration of field separator default