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command substitution
In computing, command substitution is a facility originally introduced in the Unix shells that allows a command
to be run and its output to be pasted back on the command line as
arguments to another command. Shells typically do this by creating a child process to run the first command with its standard output piped back to the shell, which reads that output, parsing it into words separated by whitespace.
Because the shell can’t know it has all the output from the child until
the pipe closes or the child dies, it waits until then before it starts
another child process to run the second command.