Convert file's whitespace characters to tabs
unexpand command is used to convert blank characters (space) in a given file to tab characters (TAB), and display the conversion results on the standard output device (display terminal).
unexpand(option)(parameter)
-a or --all: Convert all whitespace characters in the file;
--first-only: Convert only the leading whitespace characters;
-t<N>: Specify the number of N (N is an integer) characters represented by TAB. The default N value is 8.
File: Specify a list of files to convert blanks to TAB.