Pipe vs. Semicolon

CS155 Pipe

Don’t do this

Students occasionally ask me why this doesn’t work:

% mkdir foo | cd foo
/tmp/pmwiki-bash-scriptAFBCkU: line 10: cd: foo: No such file or directory

because they think that the pipe symbol (|) means “and then”.

It doesn’t!

The pipe symbol means:

Run the two commands at the same time.
Hook the output of the first command to the input of the second command.
This only makes sense if the first command produces output, and the second command reads input:

% echo -e 'my\ndog\nspot'
my
dog
spot
% echo -e 'my\ndog\nspot' | sort
dog
my
spot

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.