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.
& 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 $?
0
root@debian5:~/test$ rm file1
root@debian5:~/test$ echo $?
0
root@debian5:~/test$ rm file1
rm: cannot remove `file1′: No such file or directory
root@debian5:~/test$ echo $?
1
root@debian5:~/test$
&& 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!
paul@laika:~/test$
# 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 ?
paul@debian4:~/test$
\ 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.
