.. title: Shell Scripting Cookbook
.. slug: shell-scripting-cookbook
.. date: 2016-11-10 19:36:14 UTC
.. updated: 2016-11-10 19:36:14 UTC
.. tags: shell
.. category:
.. link:
.. description:
.. type: text

Tips I have learned on how to do things in shell scripts.

.. TEASER_END: Read more

Quit or Exit the Script if a Failure Occurs
-------------------------------------------

Set the shell's *e* option.

::

    #!/bin/sh
    set -e
    true
    false

Alternative way to set the *e* option.

::

    #!/bin/sh -e
    true
    false

To disable the option use ``+``.

::

    #!/bin/sh
    set -e
    true
    set +e
    false

Print Command Before Executing
------------------------------

Set the shell's *x* option.

::

    #!/bin/sh
    set -x
    true
    false

Alternative way to set the *e* option.

::

    #!/bin/sh -x
    true
    false

There's a difference between *x* and *v*. In *v* a variable is printed as a
variable and in *x* the variable's value is printed.

::

    #!/bin/sh
    a='Hello World!'
    set -v
    printf "${a}"
    set +v
    set -x
    printf "${a}"
    set +x

To disable the option use ``+`` as shown in the above example.

Compare Version Numbers
-----------------------

Comparing two version numbers in dot separated format (e.g. 1.2.3) can be done
in two ways.

Using BSD or GNU ``sort`` to find older version.

::

    #!/bin/sh
    a=1.2.0
    b=1.2.1
    printf "${a}\n${b}\n" | sort -t. -k 1,1n -k 2,2n -k 3,3n | head -n1

Using BSD or GNU ``sort`` to find newer version.

::

    #!/bin/sh
    a=1.2.0
    b=1.2.1
    printf "${a}\n${b}\n" | sort -t. -k 1,1nr -k 2,2nr -k 3,3nr | head -n1

Using GNU ``sort`` to find older version.

::

    #!/bin/sh
    a=1.2.0
    b=1.2.1
    printf "${a}\n${b}\n" | sort -V | head -n1

Using GNU ``sort`` to find newer version.

::

    #!/bin/sh
    a=1.2.0
    b=1.2.1
    printf "${a}\n${b}\n" | sort -Vr | head -n1

Assert
------

The ``test`` command is a rough equivalent to assert in other languages. Read
its man page (``man test``) for more information.

Recover from Error Return Code
------------------------------

For commands that can return a non-zero exit code and possibly stop the script,
use OR (``||``) operator with an alternative command that is sure to succeed.
The ``true`` boolean value is one such command.

::

    #!/bin/sh -e
    git clone https://example.com/example.git /path/already/exists || true

Another example is to create and checkout a git branch that already exists.
::

    #!/bin/sh -e
    git checkout -b mybranch origin/mybranch || git checkout mybranch

The shell builtin ``:`` (used without arguments here) returns a zero exit code.
In this case ``true`` and ``:`` are functionally equivalent.

::

    #!/bin/sh -e
    git clone https://example.com/example.git /path/already/exists || :


Source a File
-------------

To source a file means to execute it as if it was entered by a user manually.
It is not run in a separate shell. So e.g. any ``export``-ed variables become
available in the current script as environment variables.

In Bourne shell (``/bin/sh``) command ``.`` is used to source a file. Command
``source`` does not work in ``/bin/sh``.

::

    #!/bin/sh
    . file_to_source.sh