Bash Environment
****************

Environment Variables
=====================

Local Variables
---------------

Not available to child processes by default.

.. code-block:: bash

    # setting a local variable
    a=apples

    # setting multiple local variables in one statement
    b=berries c=carrots
    d=dogs; e=elephants

    echo $a $b $c $d $e

    # None of the variables above are inherited by child processes
    bash -c 'echo $a $b $c $d $e'

Setting, Unsetting and Testing
------------------------------

Variables can be set and unset within a shell session, and tested for conditional logic

.. code-block:: bash

    # From a new shell
    test ! $a && echo 'variable a does not exit'

    # Setting and testing the variable
    a=apples
    test $a && echo 'variable "a" now exists'

    # unsetting and testing the variable, we don't include thr $
    unset a
    test ! $a && echo 'Once again variable "a" does not exit'


Exporting Variables to child processes
---------------------------------------

To make a variable or a set of variables available to subshells (or sub processes),
they need to be exported.

.. code-block:: bash

    # before exporting
    a=apricot
    bash -c 'test ! $a && echo variable "a" not set in the subprocess'

    # after exporting
    export a
    bash -c 'echo $a'

    # the variable needs to be exported once, changing the value doesn't require re-exporting
    a=apples
    bash -c 'echo $a'

    # to un-export the variable
    export -n a
    bash -c 'test ! $a && echo variable "a" not set in the subprocess'


Exporting Variables Automatically
---------------------------------

To export variables using the ``export`` command explicitly, the ``set -a`` option can be used

.. code-block:: bash

    set -a
    # these two variables are exported automatically
    a=apples
    b=berries

    set +a
    # these two variables are NOT exported automatically
    c=carrots
    d=durian

    bash -c 'echo $a $b $c $d'


Exporting Variables From a file
-------------------------------

``set -a`` useful when souring environment variables from a .env file. This is the best
way to export variables from a file, especially when some lines in the file are # commented,
or contains other shell logic

.. code-block:: bash

    # creating a file with 4 variable assignments
    echo 'a=apples' >> fruits.env
    echo 'b=berries' >> fruits.env
    echo 'c=carrots' >> fruits.env
    echo 'd=durian' >> fruits.env

    set -a
    # load the variables from the .env file
    source fruits.env
    rm fruits.env
    set +a

    bash -c 'echo $a $b $c $d'


Passing Variables to Subprocess without exporting
-------------------------------------------------

If it is desirable to pass variables that shouldn't be set in the parent, variables
can be set (without exporting) before executing the subprocess.

.. code-block:: bash

    # Variables Set and passed to subprocess
    a=apples b=berries bash -c 'echo $a $b'

    # Variables are not set in the parent (current) Shell
    echo $a $b



Shell Expansions
================

https://www.gnu.org/software/bash/manual/html_node/Shell-Expansions.html

Brace Expansion
---------------
Brace expansion is a mechanism by which arbitrary strings may be generated

.. code-block:: bash

    # sequence expression over ASCII
    echo ``letter_{a..z}``

    # sequence expression over integers
    echo ``number_{1..24}``

    # sequence expression with an increment setting
    echo ``number_{1..24,2}``

    echo ``software_{alpha,beta,gamma}``



Variable Expansions
-------------------

Many more expansion forms on. Mostly to match on variable names.
https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html

``${parameter:-word}``
    If parameter is unset or null, the expansion of word is substituted. Otherwise,
    the value of parameter is substituted.


.. code-block:: bash

    echo ${PING:-SHELL} # PONG

    echo ${PING2:-SHELL} # SHELL

    echo ${PING2:-$SHELL} # /bin/bash


``${parameter:=word}``
    If parameter is unset or null, the expansion of word is assigned to parameter.
    The value of parameter is then substituted. Positional parameters and special parameters
    may not be assigned to in this way.


``${parameter:?word}``
    If parameter is null or unset, the expansion of word
    (or a message to that effect if word is not present)
    is written to the standard error and the shell, if it is not interactive, exits.
    Otherwise, the value of parameter is substituted.


``${parameter:+word}``
    If parameter is null or unset, nothing is substituted,
    otherwise the expansion of word is substituted.


``${parameter:offset}`` and ``${parameter:offset:length}``
    Substring of the parameter




.. code-block:: bash

    # We gotta use the braces in this context
    prefix=super
    echo ${prefix}man and ${prefix}woman

    # Treat unset variables as an error
    set -u

    echo $prefix2

    echo "last exit status: $?"

    name=postgresql
    # cut the last two characters
    echo ${name:0:-2}



Command Substitution
--------------------

``$()`` string substitution of the std out of a command.
is nestable in "" and within other expansions


.. code-block:: bash

    cd $(pwd)
    echo "pulse" > file_$(date +%s).log

    # latest Distro Kernel
    latest_kernel=$(ls /lib/modules -At | head -n 1)


Arithmatic Substitution
-----------------------

.. code-block:: bash

    # arithmatic in the shell
    v=$(( 4 - 3 )); echo $v

    # inside a double-quote string
    echo "4 - 3 is $(( 4 - 3 ))"

    # inside a command expansion
    echo $(echo $(( 4 - 3 )))

    # inside all sort of Expansion and substitution expressions
    array=( a b c d )
    echo "last element is ${array[$(( ${#array[@]} - 1 ))]}"