The colon (:) is optional; if it’s included, var must be nonnull as well as set. Command substitution means nothing more but to run a shell command and store its output to a variable or display back using echo command. /* But in between those attempted deletions, rm will run on *…so say bye-bye to every file in that directory. And if such commands are processing raw data, it's not unimaginable that the raw data, quite innocently, contains special characters that are destructive to certain Bash programs. Parameter is expanded and the longest match of pattern against its value is replaced with string. This is known as command substitution. The ability to store the output of a command into a variable is called command substitution and it’s by far one of the most amazing features of bash. Ask Question Asked 8 years ago. Let's pretend that IFS has been set to something arbitrary, like a capital Z. or in the special case of a variable representing #!/bin/bash if [ $(whoami) = 'root' ]; then echo "You are root" fi. When Bash expands a variable that happens to contain a Z, the value of that variable will be split into separate words (and the literal Z will disappear): By default, the IFS variable is set to three characters: newline, space, and the tab. The Bash shell, in particular (which is the default on almost every Linux distribution, and available on almost every Unix, too), has some strong string manipulation utilities built-in. To do basic calculations, you can enclose an expression inside $(( )): Check the Bash documentation for the full set of arithmetic operators. Bash command substitution. The most simple example of this behaviour is a referenced variable: Command substitution, in it's most simple form, replaces a command with it's output. But keep in mind the dangers of just pasting in seemingly safe-looking code. With IFS set to just the newline character, rough draft.txt is treated as a single filename. It allows to build very flexible and powerful Bash scripts. Imagine a textfile that contains a bunch of lines of text that, for example, may refer to filenames: When Bash reads each line of the file, the default value of IFS, which includes a space character, will cause Bash to treat the file named rough draft.txt as two files, rough and draft.txt, because the space character is used to split words. This is known as command substitution. 4.1. BoxAdcontent.document.write("