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Pipe vs. Semicolon

CS155 Pipe

Don’t do this

Students occasionally ask me why this doesn’t work:

    % mkdir foo | cd foo
    bash: cd: foo: No such file or directory

because they think that the pipe symbol (|) means “and then”.
It doesn’t!

What does the pipe do?

The pipe symbol, when used like this, means:

    % alpha | beta

• Run the commands alpha and beta at the same time.
• Hook the output of alpha up to the input of beta.

This only makes sense if alpha produces output, and beta reads input, e.g.,

    % ls -l | more

Do this, instead

If you want to execute one command after another, just do so:

    % mkdir foo
    % cd foo

or:

    % mkdir foo; cd foo

Note that using semicolon (;) is no less typing than just pressing the Enter key.