Bash/Prompt customization

Bash has several prompts which can be customized to increase productivity, aesthetic appeal, and nerd cred.

Prompts

Bash has four prompt strings that can be customized:

PS0 is displayed after each command, before any output.
PS1 is the primary prompt which is displayed before each command, thus it is the one most people customize.
PS2 is the secondary prompt displayed when a command needs more input (e.g. a multi-line command).
PS3 is not very commonly used. It is the prompt displayed for Bash's select built-in which displays interactive menus. Unlike the other prompts, it does not expand Bash escape sequences. Usually you would customize it in the script where the select is used rather than in your .bashrc.
is also not commonly used. It is displayed when debugging bash scripts to indicate levels of indirection. The first character is repeated to indicate deeper levels.

All of the prompts are customized by setting the corresponding variable to the desired string (usually in ), for example

PS2='> '

Techniques

While one can simply set their prompt to a plain string, there are a variety of techniques for making the prompt more dynamic and useful.

Bash escape sequences

When printing the prompt string, Bash looks for certain backslash-escaped characters and will expand them into special strings. For example, is expanded into the current username and is expanded to the current time. So a PS1 of '\A \u $ ' would be printed like 17:35 username $ .

See the man page or the Bash reference manual for a complete list of escape sequences.

Terminfo escape sequences

Aside from the escape characters recognized by Bash, most terminals recognize special escape sequences that affect the terminal itself rather than printing characters. For example they might change the color of subsequent printed characters, move the cursor to an arbitrary location, or clear the screen. These escape sequences can be somewhat illegible and can vary from terminal to terminal, so they are documented in the terminfo database. To see what capabilities your terminal supports, run

$ infocmp

The capability names (the part before the =) can be looked up in for a description of what they do. For example, sets the foreground color of whatever text is printed after it. To get the escape code for a capability, you can use the command. For example

$ tput setaf 2

Prints the escape sequence to set the foreground color to green.

To practically incorporate these capabilities into your prompt, you can use Bash's command substitution and string interpolation. For example

my prompt> 

ANSI escape sequences

Unfortunately, valid ANSI escape sequences may be missing from your terminal's terminfo database. This is especially common with escape sequences for newer features such as 256 color support. In that case you cannot use tput, you must input the escape sequence manually.

See Wikipedia:ANSI escape code for examples of escape sequences. Every escape sequence starts with a literal escape character, which you can input using the Bash escape sequence . So for example,\e[48;5;209m sets the background to a peachy color (if you have 256 color support) and \e[2;2H moves the cursor near the top-left corner of the screen.

In cases where Bash escape sequences are not supported (such as PS3) you can get a literal escape character using Bash's printf builtin:

ESC=$(printf "\e")
PEACH="$ESC[48;5;209m"

Embedding commands

If you want to add the output of some command to your prompt, you might be tempted to use command substitution. For example, to add the amount of free memory to your prompt you might try:

But this does not work; the amount of memory shown is the same every time! This is because the command is run once, when PS1 is first set, and never again. The trick is to simply prevent the substitution either by escaping the or by defining it in single quotes — either way it will be substituted when the prompt is actually displayed:

PS1="\$(awk '/MemFree/{print \$2}' /proc/meminfo) prompt > "
# or
PS1='$(awk "/MemFree/{print \$2}" /proc/meminfo) prompt > '

To prevent long commands from making your PS1 huge, you can define functions:

PROMPT_COMMAND

If the variable is set, it will be evaluated right before PS1 is displayed. This can be used to achieve quite powerful effects. For example it can reassign PS1 based on some condition, or perform some operation on your Bash history every time you run a command.

Escapes between command input and output

You can affect your input text in Bash by not resetting the text properties at the end of your PS1. For example, inserting at the end of your PS1 will make your typed commands blink. However this effect will also continue through the command's output since the text properties are not reset when you hit Enter.

In order to insert escape sequences after you type a command but before the output is displayed, you can set PS0. Alternatively, you can trap Bash's DEBUG signal, which is sent right before each command is executed:

$ trap 'tput sgr0' DEBUG

Customizing root prompts

To ensure that you know when you are running as root, you can customize your root prompt to make it clearly stand out (perhaps blinking red?). This is done by customizing the Bash prompt as usual but in root's home directory, . Start off by copying the skeleton files /etc/skel/.bash_profile and to , then edit as desired.

Examples

Colors

To see the full range of colors your terminal supports, you can use a simple loop with tput (change to for text foregrounds):

If that does not work (and you cannot fix it by setting the correct TERM value), you can test the different manual escape sequences:

To change the manual escapes from background to foreground, the standard color range is , the high intensity range is , and the 48 should be changed to 38 for 256 colors.

Common capabilities

The following terminfo capabilities are useful for prompt customization and are supported by many terminals. #1 and #2 are placeholders for numeric arguments.

Capability	Escape sequence	Description
Text attributes
blink	\e[5m	blinking text on
bold	\e[1m	bold text on
dim	\e[2m	dim text on
rev	\e[7m	reverse video on (switch text/background colors)
sitm	\e[3m	italic text on
ritm	\e[23m	italic text off
smso	\e[7m	highlighted text on
rmso	\e[27m	highlighted text off
smul	\e[4m	underlined text on
rmul	\e[24m	underlined text off
setab #1	\e[4#1m	set background color #1 (0-7)
setaf #1	\e[3#1m	set text color #1 (0-7)
sgr0	\e(B\e[m	reset text attributes
Cursor movement
sc	\e7	save cursor position
rc	\e8	restore saved cursor position
clear	\e[H\e[2J	clear screen and move cursor to top left
cuu #1	\e[#1A	move cursor up #1 rows
cud #1	\e[#1B	move cursor down #1 rows
cuf #1	\e[#1C	move cursor right #1 columns
cub #1	\e[#1D	move cursor left #1 columns
home	\e[H	move cursor to top left
hpa #1	\e[#1G	move cursor to column #1
vpa #1	\e[#1d	move cursor to row #1, first column
cup #1 #2	\e[#1;#2H	move cursor to row #1, column #2
Removing characters
dch #1	\e#1P	remove #1 characters (like backspacing)
dl #1	\e#1M	remove #1 lines
ech #1	\e#1X	clear #1 characters (without moving cursor)
ed	\eE[J	clear to bottom of screen
el	\e[K	clear to end of line
el1	\e[1K	clear to beginning of line

Visualizing exit codes

Using the same trick from embedding commands you can delay the interpolation of special Bash variables like . So the following prompt shows the exit code of the previous command:

PS1="\$? > "
# or
PS1='$? > '

> true
> false
>

This can be made more interesting using conditionals and functions:

exitstatus()
{
    if [[ $? == 0 ]]; then
        echo ':)'
    else
        echo 'D:'
    fi
}
PS1='$(exitstatus) > '

:) > true
:) > false
D: >

Positioning the cursor

It is possible to move the cursor around the screen inside of PS1 to make different parts of the prompt appear in different locations. However, to ensure that Bash positions the cursor and output in the right position, you must move the cursor back to the original position after you are done printing elsewhere. This can be done using the tput capabilities and to save and restore the cursor position. The general pattern for a prompt that moves the cursor is

PS1="\[$(tput sc; cursor-moving code) positioned prompt stuff $(tput rc)\] normal prompt stuff"

where the entire block of repositioned prompt is wrapped in to prevent Bash from counting it as part of the regular prompt.

Right-justified text

The simplest way to print text on the right side of the screen is to use printf

left prompt > right prompt

This creates a right-justified variable-sized field and sets its size to the current number of columns of the terminal $COLUMNS.

Arbitrary positioning

The capability moves the cursor to a specific position on the screen, for example moves the cursor to line 20, column 5 (where 0 0 is the top left corner). , cud, , and (up, down, forward, back) move the cursor relative to its current position. For example moves the cursor 10 characters to the right. You can use the and variables in the arguments to move the cursor relative to the bottom and right edges. For example, to move 10 lines and 5 columns away from the bottom right corner:

$ tput cup $((LINES - 11)) $((COLUMNS - 6))

Customizing the terminal window title

The terminal window title can be customized in much the same way as the prompt: by printing escape sequences in the shell. Thus it is common for users to include window title customizations in their prompt. Although this is technically a feature of xterm, many modern terminals support it. The escape sequence to use is where and are the escape and bell characters. Using #Bash escape sequences, changing the title in your prompt looks like

PS1='\[\e]2;new title\a\]prompt > '

Of course your window title string can include output from embedding commands or variables such as $PWD, so that the title changes with each command.

gollark: (ethically)

gollark: I might convert you into muons.

gollark: No.

gollark: Same issue.