Command substitution (Bash)

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Command substitution involves substituting a command with its output. This can either be directly replaced as variable substitution on the command-line or piped into a virtual file as file substitution. The latter is useful when using command substition with a command that is expecting a filename eg. cat.

Variable Substitution

In variable subtition the output of the command directly replaces the command.

$ echo Most recent file is: $(ls -ltr|tail -1)
$ Most recent file is : -rwxr-xr-x 1 benyg benyg 4820 Sep 13 09:22 test.sh

File substitution

In file substitution the output of the command is inserted in a virtual file and is substituting an argument that requires a filename.

$ echo $(date)
Thu Sep 15 09:10:22 BST 2011
$ cat $(date)
cat: Thu: No such file or directory
cat: Sep: No such file or directory
cat: 15: No such file or directory
cat: 09:10:28: No such file or directory
cat: BST: No such file or directory
cat: 2011: No such file or directory
$ cat <(date)
Thu Sep 15 09:10:37 BST 2011
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