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:
Here is what the various parts refer to:
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:
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:
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 \$'
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 \$'
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 \$'
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.
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'
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='ā[\T] \u@\h \[\e[31m\]\w\[\e[m\]\nā$ '
# 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
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='[ā]'
FAIL='[ā]'
error_test () {
if [ $? == 0 ]; then
echo -e $SUCCESS
else
echo -e $FAIL
fi
}
## Bash prompt
PS1="\$(error_test)"
Note the __stat. As far as I know the __ aren't necessary, the
function could just be called stat, but it seems to be a common
convention in bash prompt functions.
Sourcing external scripts
I use the git-prompt.sh shell script, which is part of the
contributed materials in the git github repo, to get the current
git branch displayed in my bash prompt. Download and save the
script in the link as ~/.git-prompt.sh. Then add the following to
your .bashrc:
[git-prompt.sh shell script]:
http://www.github.com/git/git/tree/master/contrib/completion/git-pro
mpt.sh
source ~/.git-prompt.sh
PS1='$(__git_ps1 "[%s]")'
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
