When you want to run multiple commands you can use ;, && and |
Like this: killall Finder; killall SystemUIServer, cd ~/Desktop/ && rm Caches Or: man grep | man cat for example.
But, is there a difference between |, ; and &&? If so, what is the difference?
;: commands separated by a ; are executed sequentially. The shell waits for each command to terminate in turn.
&&: command after && is executed if, and only if, command before && returns an exit status of zero. You can think of it as AND operator.
|: a pipe. In expression command1 | command2 The standard output of command1 is connected via a pipe to the standard input of command2.
There are more similar control operators, worth to mention:
||: command after || is executed if, and only if, command before || returns a non-zero exit status. You can think of it as OR operator. Please note, that | and || are completely different animals.
&: the shell executes the command terminated by & in the background, does not wait for the command to finish and immediately returns exit code 0. Once again, & has nothing to do with &&.
|&: a shorthand for 2>&1 | i.e. both standard output and standard error of command1 are connected to command2's standard input through the pipe.
Additionally if you use zsh then you can also start command with &| or &!. In this case job is immediately disowned, after startup it does not have a place in the job table.
Consider two commands A and B. When you write
A | B
A and B are executed in parallel, and the standard output of A is sent as the standard input of B.
When you write
A; B
A is executed, then B is executed (unless set -e has been used and A terminates with a non-zero exit status, in which case the shell terminates just after A).
When you write
A && B
A is executed, then B is executed only if A has terminated successfully (zero exit status). This form can be safer than the previous one, e.g. with
cd some_dir && rm file*
The && makes sure that the right files are removed, i.e. files are removed only if the cd has succeeded.
The other answers already explain |, && and ;, so I'll add something additional you haven't explicitly asked for: ||.
A || B
A is executed, and then B is only executed if A failed. This is useful for reacting on error conditions:
martin@martin ~ % rm doesnotexist 2>/dev/null || echo "failed to delete file"
failed to delete file
(The 2>/dev/null is necessary to hide the error message of rm itself.)
All three are rather different from one-another, especially |.
|: First of all it's called a 'pipe', and can be used to redirect the output of one program into the other. Your example won't actually do anything meaningful tho because man doesn't use standard input for anything. A more meaningful example would be man grep | grep -- -v which will find '-v' in the output from man grep
;: The semicolon can be used instead of a newline, to make things take up less space.
&&: The two ampersand operator is a boolean AND operator which means that the command after the && will be run if, and only if, the command on the left side returns true, which on UNIX systems means that it returned with the exit code 0.
No one seems to be explaining why you would use each one, given your actual examples. I'll take a stab at it:
killall Finder; killall SystemUIServer
A command like killall can take some time to execute. By using the semicolon, you can give both commands at once and then do something else while you wait. That way you don't have to wait twice (once for each command).
cd ~/Desktop/ && rm Caches
You want to remove the directory ~/Desktop/Caches and end up in the directory ~/Desktop. What you do not want to do is to remove the directory Caches from the current directory (before you change it). So you use && to check that the directory change was successful before you do your remove command.
man grep | man cat
This makes no sense to me. The man command does not process input, so piping things to it won't do anything. Perhaps you meant
man grep || man cat
That would try to find the man page for grep and then if it couldn't, would show the man page for cat. Or possibly you meant
man grep | cat
That would use cat to display the man page rather than the pager (normally less). The | redirects the output from man to the command cat (which just dumps everything to screen). This can be useful in a GUI where you can use the scroll bar to go back and forth in the file rather than using the keyboard commands in less.
Note: if you want to know what ; && | || do in the more general cases, then I'd suggest looking at jimmij's answer. I'm just trying to explain what's happening in the examples from the question.
