Command Chaining with && and ||
We can chain command using the logical AND (&&) operator or the logical OR (||) operator.They work like this.
&&-operator
The command on the right side of && will only be executed if the command on the left side of && sets and exit code of 0.||-operator
The command on the right side of || will only be executed if the command on the left side of ||sets an exit code other than 0, i.e. signals an error condition.Examples
The examples to follow use another method of chaining command, which is the “;” operator. This one simply chains commands disregarding their respective exit code. I use “;” to isolate the workings of && and ||.Here are some straight forward examples.
Example 1
$ # example 1 $ # simple use of && $ mkdir /test && echo "$?: directory created"; echo "$?: this is the second command" mkdir: cannot create directory ‘/test’: Permission denied 1: this is the second command
Due to the
error, the command to the right of && was not executed. The exit code
was still 1 when the next echo was executed.
Example 2:
Now the same check with an initial command that
works:
$ mkdir /vagrant/test && echo "$?: directory created"; echo "$?: this is the second command" 0: directory created 0: this is the second command
This time
the initial command sets exit code 0 which cases the first echo to be executed.
The second echo is then executed unconditionally.
Example 3
The
combination of && and || is often used to simulate an if-else on one
line.
$ # example 3 $ # if-else simulation $ # $ # initial command failes $ mkdir /test && echo "$?: directory created" || echo "$?: directory creation failed" mkdir: cannot create directory ‘/test’: Permission denied 1: directory creation failed $ # $ # initila command succeeds $ mkdir /vagrant/test && echo "$?: directory created" || echo "$?: directory creation failed" 0: directory created
It really looks like it works if-else like.
But don’t be fooled, it’s only a simulation that breaks down easily, as the next example shows:
Example 4:
In this example, we let the second command, the one that represents the if-case, fail.
$ # example 4 $ # not really if-else $ mkdir /vagrant/test && mkdir /test || echo "$?: /vagrant/test could not be created" mkdir: cannot create directory ‘/test’: Permission denied 1: /vagrant/test could not be created
This is
what happened:
- The initial command succeeded, causing the second command to be executed
- The second command fails, causing the third command (representing the else-case) to be executed
This was
probably not what was intended.
But it
demonstrates that && and || are simply chaining the commands in a row and
execute them based on the exit code of the previous command.
The last
example supports this.
Example 5
Here we turned the sequence of && and ||
around and again let the initial command fail.
$ # example 5 $ mkdir /test || echo "$?: directory creation failed" && echo "$?: directory created" mkdir: cannot create directory ‘/test’: Permission denied 1: directory creation failed 0: directory created
This is
what happened:
- The initial command failed, causing an exit status of 1, causing the first echo to be executed
- The first echo completed successfully, causing an exit status of 0, causing the second echo to be executed
Takeaway: &&
and || are simply chaining commands on condition.
