control operators

In this post we put more than one command on the command line using control operators. We also briefly discuss related parameters ($?) and similar special characters(&).

; semicolon

You can put two or more commands on the same line separated by a semicolon ; . The shell will scan the line until it reaches the semicolon. All the arguments before this semicolon will be considered a separate command from all the arguments after the semicolon. Both series will be executed sequentially with the shell waiting for each command to finish before starting the next one.

echo hello

& ampersand

When a line ends with an ampersand &, the shell will not wait for the command to finish. You will get your shell prompt back, and the command is executed in background. You will get a message when this command has finished executing in background.


$? dollar question mark

The exit code of the previous command is stored in the shell variable $?. Actually $? is a shell parameter and not a variable, since you cannot assign a value to $?.
root@debian5:~/test$ touch file1
root@debian5:~/test$ echo $?
root@debian5:~/test$ rm file1
root@debian5:~/test$ echo $?
root@debian5:~/test$ rm file1
rm: cannot remove `file1′: No such file or directory
root@debian5:~/test$ echo $?

&& double ampersand

The shell will interpret && as a logical AND. When using && the second command is executed only if the first one succeeds (returns a zero exit status).


|| double vertical bar

The || represents a logical OR. The second command is executed only when the first command fails (returns a non-zero exit status).


combining && and ||

You can use this logical AND and logical OR to write an if-then-else structure on the command line. This example uses echo to display whether the rm command was successful.
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
It worked!
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
rm: cannot remove `file1′: No such file or directory
It failed!

# pound sign

Everything written after a pound sign (#) is ignored by the shell. This is useful to write a shell comment, but has no influence on the command execution or shell expansion.
paul@debian4:~$ mkdir test      # we create a directory
paul@debian4:~$ cd test           ##### we enter the directory
paul@debian4:~/test$ ls            # is it empty ?

\ escaping special characters

The backslash \ character enables the use of control characters, but without the shell interpreting it, this is called escaping characters.


end of line backslash

Lines ending in a backslash are continued on the next line. The shell does not interpret the newline character and will wait on shell expansion and execution of the command line until a newline without backslash is encountered.


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