Learn Linux Shell Scripting - Fundamentals of Bash 4.4

Command substitution

While not strictly redirection in the Linux sense, command substitution in our eyes is a form of functional redirection: you use the output of a command as an argument to another command. If we needed to use output as input for the next command, we'd use a pipe (as we'll see in a few pages), but sometimes we just need that output at a very specific location in our command.

This is where command substitution is used. We've already seen command substitution in some of our scripts: cd $(dirname $0). Simply put, this does something like cd to the result of dirname $0.

dirname $0 gives back the directory where the script is located (since $0 is the fully-qualified path of the script), so when we use this with scripts, we'll ...