Bash

bash is the abbreviation of Bourne Again Shell.

It is a commonly-used shell in many Linux distributions. Bash is a command interpreter.  It is a command-line–only interface containing a handful of built-in commands; it has the ability to launch other programs and to control programs that have been launched from it (job control).

Each time you log in into a Linux session a file called .bashrc is run.  This file is located in your home directory and is installed when installing the Linux distro.

There's quite a lot of stuff inside the default .bashrc file which you better not touch, unless you know what you're doing (like in so many cases...).

However, you can add stuff that benefits your use of Linux.  One of them is to add calls to other files that can contain different things like:

• aliases

• path extensions

• exports

• ...

To do this, follow the steps below:

1. Open the file ~/.bashrc using vi (or another editor)

2. Somewhere at the end of the file, add the following 3 commands (YMMV):
   [ -f ~/.bash_aliases ] && . ~/.bash_aliases
   [ -f ~/.bash_paths ] && . ~/.bash_paths
   [ -f ~/.bash_exports ] && . ~/.bash_exports

3. Save the file

What do those 3 commands do exactly?

They first check if the file given is existing.  Only then, the command following the && command is executed.  That command is then just sourcing what is in it.  That's it, nothing complicated.

An alias file can contain shortcuts (or "aliases") to more elaborated commands.  Mine currently looks as follows:

alias lla='ls -als'
alias ltr='ls -ltr'
alias temp='/opt/vc/bin/vcgencmd measure_temp'

So, each time when I type lla on the command line, it's actually as if I'm typing/executing the command ls -als.  Same for the other two aliases.

As you can see, the longer the real command, the more convenient the corresponding alias is.  See the temp alias for an example.

Extending the paths can also be done in a separate file.  This way, you're avoiding to clutter the original .bashrc file.  An example could be like this:

PATH=/usr/sbin:$PATH

This command will extend the existing path with /usr/sbin.  It also adds it in front of the existing path so that files are first searched in the current directory (always like that in Linux) then in the directory /usr/sbin and only if not found, the search will continue in the paths defined in the original PATH variable.

The content of the original PATH variable is retrieved by using the $ token.

Same mechanism/principle can be applied for exports and other stuff you want to "predefine" in files that should be called whenever .bashrc is called.

In the next section there's more detailed information given about the above approach (some repetition of what's mentioned here might be in).

As mentioned in the previous section you can add "extension" to bash.  One of them is adding a couple of files that does the following:

• Add aliases

• Extend/change the default $PATH variable

• Add extra EXPORT commands

Add aliasses

To add extra aliases, create a file ~/.bash_aliases.  The content of it could be, for example:

alias lla='ls -als'

alias ltr='ls -ltr'

alias grepi='grep -i'

alias gitl="git log --graph --pretty=format:'%Cred%h%Creset%n'"

alias gs='git status'

alias gd='git diff'

alias grepi='grep -i'

alias grepw='grep -w'

Apart from the last two aliases, the other ones are self-explaining.

grepi means search case-insensitive while grepw means search whole words.

Add extra paths

To add extra paths, create a file ~/.bash_paths and define the content like so:

PATH=<paths_you_want_to_add_before_the_official_paths>:$PATH

Example:

PATH=/home/pi/mystuff/libgpiod/gvcresult/bin:/usr/sbin:/home/pi/mystuff/azul_java/java_11/zulu11.60.19-ca-jdk11.0.17-linux_aarch32hf/bin:$PATH

Add extra exports

To add extra exports, create a file ~/.bash_exports and add, for instance, the following content:

export CC=/usr/bin/gcc

From now onwards, each time you refer to ${CC} in makefiles or so the GNU C compiler will be invoked.

Note: you could also shorten the path and just mention export CC=gcc but it's never a bad idea to always give the complete path.  There's no ambiguous situation then.

Adapt .bashrc

To have all the above up and running, those files must be called of course.  Therefor, the .bashrc file has to be adapted.

Add the following 3 lines to the .bashrc file:

[ -f ~/.bash_aliases ] && . ~/.bash_aliases

[ -f ~/.bash_paths ] && . ~/.bash_paths

[ -f ~/.bash_exports ] && . ~/.bash_exports

Sourcing the file using the command . ~/.bashrc will immediately put those files at work!

If you want to log in into a remote server (e.g. Raspberry Pi running an SSH server) and you want to execute immediately a command you can do so using the -t option of SSH.

The example below does the following:

• Moving to a certain directory on the RPi

• Activating a Python virtual environment

• Starting up a Python application

ssh -X pi@192.168.1.40 -t "cd /home/pi/mystuff/flask/gfg; . venv/bin/activate; python app.py;" 

Note that this command also creates an X11 connection to see graphical output on the client, if relevant.

Putting this command on the client machine in a bash script allows you to only run that bash script and it will automatically launch - in this case - a Flask web server.  Less typing required from the user, which is good!