unexpand command in Linux with Examples

To convert the leading spaces and tabs into tabs, there exists a command line utility called unexpand command. The unexpand command by default converts each space into tabs writing the produced output to the standard output.

Syntax 

$unexpand [OPTION]... [FILE]...

where,

• OPTION: Represents the various options or flags available for modifying the behavior of unexpand.
• FILE: Refers to the name of the file on which the command operates.

Using unexpand command

To convert all the space characters into tab characters in the file kt.txt, use unexpand as: 

$cat -vet kt.txt
have    a    nice    day$
always    try    harder$
to    achieve    better$


/* In the below output $ refers to 
   the new line feed and ^I refers 
   to the tab */
$unexpand kt.txt
have^Ia^Inice^Iday$
always^Itry^Iharder$
to^Iachieve^Ibetter$

In order to save the produced output by unexpand command in another file let's say dv.txt use the below syntax: 

/* Saving output in file, dv.txt */
$unexpand kt.txt>dv.txt

$cat -vet dv.txt
have^Ia^Inice^Iday$
always^Itry^Iharder$
to^Iachieve^Ibetter$

Key Options for the unexpand command

1. -a, - -all option

This option is used to convert all blanks, instead of just initial blanks (which is by default). 

/* This converts all the blanks 
   also into tabs */
$unexpand -a kt.txt>dv.txt

2. --first-only option

This is used to convert only leading sequences of blanks (overrides -a option). 

/* This converts only the leading
   sequences of blanks */
$unexpand --first-only kt.txt>dv.txt

3. -t, - -tabs=N option

This set tabs N characters apart instead of the default of 8 (enables -a option). 

/* the -t option with numerical value 
   2 forces to change the spaces
   into tabs of only 2 characters */
$unexpand -t2 kt.txt>dv.txt

Other available Options

Also see: expand command  

Conclusion

The unexpand command in Linux is a simple yet powerful tool for converting spaces to tabs in text files. With its various options, such as -a for converting all spaces, -t for custom tab widths, and --first-only for restricting changes to leading spaces, it offers flexibility in managing text file formatting.