TITLE: Constructing a Useful Bash Prompt
DATE: 2017-08-14
AUTHOR: John L. Godlee
====================================================================


This is what my bash prompt looks like at the moment:

  ![Current bash 
prompt](https://johngodlee.xyz/img_full/bash_prompt/prompt_current_c
rop.png)

Here is what the various parts refer to:

  ![Annotated bash 
prompt](https://johngodlee.xyz/img_full/bash_prompt/prompt_current_a
nnot_crop.png)

And here is a link to a .bashrc,

What is a bash prompt?

The bash prompt is a piece of text placed at start of a command 
line interface using the bash shell. The primary function of the 
prompt is to let the user know that the computer is ready for the 
next command, the secondary function is to provide the user with 
some information about the status of the current session.

The default bash prompt looks like this:

  ![Default bash 
prompt](https://johngodlee.xyz/img_full/bash_prompt/prompt_default_c
rop.png)

so fire up your terminal of choice and see that your bash prompt 
looks similar. The default prompt shows the currently logged in 
username, the hostname (i.e. the name of th computer), the current 
directory path relative to ~ and finally a $, which marks the end 
of the prompt and the start of the area that you can type commands.

Customising the bash prompt

The .bashrc, among other things related to how bash functions, is 
where you can define a custom bash prompt. .bashrc is normally 
found in the root (~) directory. Check if you have a ~/.bashrc 
using:

    cat ~/.bashrc

If bash says that there is no such file, create one using:

    touch ~/.bashrc

Finally, start editing .bashrc using your favourite text editor, 
e.g. vim:

    vim ~/.bashrc

Super basic

To start with, let's replace the default bash prompt with something 
really simple like this:

  ![Dollar sign minimal 
prompt](https://johngodlee.xyz/img_full/bash_prompt/prompt_dollar_cr
op.png)

To achieve this, type the following into your .bashrc:

    PS1='\$'

Then exit the text editor and enter the following into the 
terminal, which forces bash to reload .bashrc:

    source ~/.bashrc

PS1 refers to only one of five bash prompts, with PS1 being the 
default prompt that is shown most of the time; PS2,PS3,PS4 and 
PROMPT_COMMAND are used under special conditions. This article has 
a good description of what they're used for.

  [This article]: 
http://www.thegeekstuff.com/2008/09/bash-shell-take-control-of-ps1-p
s2-ps3-ps4-and-prompt_command/

Incorporating variables

The simplest way to add a variable (i.e. something which changes 
depending on environment conditions, like the current working 
directory) is to use one that bash understands by default, a full 
list of which can be found here.

  [a full list of which can be found here]: 
http://www.gnu.org/software/bash/manual/bashref.html#Controlling-the
-Prompt

Start by adding the time to our simple $ prompt:

    PS1='\T \$'

  ![Bash prompt with 
time](https://johngodlee.xyz/img_full/bash_prompt/prompt_time_crop.p
ng)

Note the space between \T and \$. Adding spaces between variables 
can help everything to look neater.

Read through the list of available variables and add a few to your 
own prompt.

Colours

To change the colours parts of the bash prompt, wrap the variable 
in ANSI escape sequences, like so:

    PS1='\[\e[31m\]\w\[\e[m\] \T \$'

  ![bash prompt with 
colours](https://johngodlee.xyz/img_full/bash_prompt/prompt_col_simp
le_crop.png)

This makes the directory path appear in red text. 31m is the 
section of that sequence that actually defines the colour red.

ANSI escape sequences can also be used to change the background 
colour of the text, add underlines, make the text bold, or high 
contrast. I often refer to both this wikipedia page on ANSI colour 
codes and this blog post on jafrog.com when choosing colour codes.

  [this wikipedia page on ANSI colour codes]: 
http://en.wikipedia.org/wiki/ANSI_escape_code#colors
  [this blog post on jafrog.com]: 
http://www.jafrog.com/2013/11/23/colors-in-terminal

A more elaborate example which makes the current directory 
light-cyan coloured, with bold font:

    PS1='\[\e[96;1m\]\w\[\e[m\] \T \$'

  ![Bash prompt with complex 
colours](https://johngodlee.xyz/img_full/bash_prompt/prompt_col_comp
lex_crop.png)

The ;1 is the part which makes the text bold. Note how [\e[m\] 
always needs to be placed at the end of the coloured part of the 
bash prompt, to return the colours to normal.

Try colouring the time (\T) so it is underlined and magenta.

Unicode characters

Another way escape sequences can be used is to add unicode 
characters to the prompt, whether this is a cool lightning bolt, a 
tick or a cross, or any of the other 136,000 characters available 
in Unicode 10.0.

Wikipedia has an excellent resource for browsing unicode characters

Browse through the wikipedia page and pick your chosen character, 
then note down its code, e.g. u2602 for the umbrella symol. The 
syntax for adding this to your bash prompt is as follows:

    PS1=$'\u2602'

Note that inserting unicode can also by accomplished just by 
copying and pasting the unicode character itself into the .bashrc.

  ![Bash prompt with 
unicode](https://johngodlee.xyz/img_full/bash_prompt/prompt_umbrella
_crop.png)

Note how the $ isn't escaped this time, as we don't actually want 
to see the $ in our prompt

Formatting

Brackets, spaces, hyphens, colons can all be used to great effect 
in your bash prompt to help separate the different parts. As an 
example, try setting this in your .bashrc and source ~/.bashrc to 
see the results:

    PS1='[\T] {\u:\h} -- \w'

  ![Bash prompt with brackets for 
formatting](https://johngodlee.xyz/img_full/bash_prompt/prompt_forma
tting_crop.png)

Building PS1 incrementally

After a while, if your bash prompt becomes long and complicated, 
your PS1 code may start to look busy and hard to read, but it's 
easy to restructure the PS1 code to increase readability. In the 
example below, a long and complicated prompt is rewritten so each 
part is on a separate line and spaces are given their own lines to 
make them easier to see. Additionally, this allows each part to 
have its own comment, further increasing readability:

    # Messy one liner
    PS1='┏[\T] \u@\h \[\e[31m\]\w\[\e[m\]\n┗$ '

    # Tidy on multiple lines
    PS1=$'\u250F'       # Elbow
    PS1+='[\T]' # Time
    PS1+=' '    # Space 
    PS1+='\u@\h'    # User@hostname
    PS1+=' '    # Space
    PS1+='\[\e[31m\]\w\[\e[m\]' # current dir
    PS1+='\n'   # New line
    PS1+=$'\u2517'  # Elbow
    PS1+='\$'   # $
    PS1+=' '    # Space

  ![Multi-line bash 
prompt](https://johngodlee.xyz/img_full/bash_prompt/prompt_multiline
_crop.png)

Conditional statements and functions

From here onwards I am writing about stuff I don't fully understand 
and have just gathered from other websites and tutorials.

If you want to expand the number of variables you can use in your 
bash prompt you can take create functions in your .bashrc, which 
you then source in your PS1 code. The example below (which I 
adapted from here) shows how to present a unicode tick if the 
previous command was successful, and a unicode cross if it was 
unsuccessful, add the following to your .bashrc:

  [which I adapted from here]: 
https://www.reddit.com/r/linux/comments/2uf5uu/this_is_my_bash_promp
t_which_is_your_favorite/co7ygus/?utm_content=permalink&utm_medium=f
ront&utm_source=reddit&utm_name=linux

    SUCCESS='[✓]'
    FAIL='[✗]'

    error_test () {
        if [ $? == 0 ]; then
            echo -e $SUCCESS
        else
            echo -e $FAIL
        fi
    }

    ## Bash prompt
    PS1="\$(error_test)"

  ![Bash prompt with conditional 
status](https://johngodlee.xyz/img_full/bash_prompt/prompt_tick_crop
.png)

Note the __stat. As far as I know the __ aren't necessary, the 
function could just be called stat, but it seems to be a common 
convention in bash prompt functions.

Sourcing external scripts

I use the git-prompt.sh shell script, which is part of the 
contributed materials in the git github repo, to get the current 
git branch displayed in my bash prompt. Download and save the 
script in the link as ~/.git-prompt.sh. Then add the following to 
your .bashrc:

  [git-prompt.sh shell script]: 
http://www.github.com/git/git/tree/master/contrib/completion/git-pro
mpt.sh

    source ~/.git-prompt.sh

    PS1='$(__git_ps1 "[%s]")'

  ![Bash prompt with git 
repository](https://johngodlee.xyz/img_full/bash_prompt/prompt_git_c
rop.png)

git-prompt.sh also has some variables which can be set from within 
.bashrc. Details can be found by reading the preamble of the script 
in a text editor.

Now that you've read through the tutorial, try to construct your 
own bash prompt, adding the bits that you find useful. Refer back 
to the linked .bashrc for some inspiration.

  [linked .bashrc]: 
https://github.com/johngodlee/bash_prompt_tut/tree/master/files