Small, Sharp Software Tools

Using Command Substitution

You can use the output of one command when invoking another command. This is handy when a program’s argument needs to be generated from another program’s output. You can do this with a feature called command substitution. You’ve already seen variable expansion with curly braces, like ${variable}, where the variable’s value is expanded. Command substitution works in a similar way. With command substitution, the output of the command replaces the command itself. To define a command substitution, you wrap the command in parentheses instead of curly braces.

You saw an example of command substitution in ​Building Your Prompt​, when you used the tput command to emit color codes:

$ echo -e "$(tput setaf 6)Hello ...