Bash/Prompt customization

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

Prompts

Bash has four prompts that can be customized:

  • 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.
  • PS4 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 ~/.bashrc), for example

export 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, \u is expanded into the current username and \A is expanded to the current time. So a PS1 of '\A \u $ ' would be printed like 17:35 username $ .

Check the "PROMPTING" section of the Bash man page 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 the terminfo man page for a description of what they do. For example, setaf sets the foreground color of whatever text is printed after it. To get the escape code for a capability, you can use the tput command. For example

$ tput setaf 2

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

Note: If tput commands are failing for you, ensure that you have set the correct TERM value for your shell. For example, if you have set xterm instead of xterm-256color, tput setaf will only work with color numbers 0-7.

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

~/.bashrc
GREEN="\[$(tput setaf 2)\]"
RESET="\[$(tput sgr0)\]"

export PS1="${GREEN}my prompt${RESET}> "
my prompt>
Note: Wrapping the tput output in \[ \] is recommended by the Bash man page. This helps Bash ignore non-printable characters so that it correctly calculates the size of the 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 \e. 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:

export PS1="$(awk '/MemFree/{print $2}' /proc/meminfo) prompt > "
53718 prompt >
53718 prompt >
53718 prompt >

But this doesn't 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:

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

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

free_mem()
{
    awk '/MemFree/{print $2}' /proc/meminfo
}

export PS1='$(free_mem) prompt > '
Note: You can use terminfo/ANSI escape sequences inside substituted functions but not Bash escapes. In particular \[ \] won't work for surrounding non-printable characters. Instead you can use the octal escapes \001 and \002 (e.g. using printf or echo -e).

PROMPT_COMMAND

If the PROMPT_COMMAND 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.

Warning: PROMPT_COMMAND generally should not be used to print characters directly to the prompt. Characters printed outside of PS1 are not counted by Bash, which will cause it to incorrectly place the cursor and clear characters. Either use PROMPT_COMMAND to set PS1 or look at embedding commands.

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 tput blink 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 trap Bash's DEBUG signal, which is sent right before each command is executed:

~/.bashrc
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 customize the Bash prompt as usual but in root's home directory, /root. Start off by copying the skeleton files /etc/skel/.bash_profile and /etc/skel/.bashrc to /root, then edit /root/.bashrc as desired.

Examples

Colors

Tip: infocmp shows the number of colors tput works with, for example colors#8.

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

for C in {0..255}; do
    tput setab $C
    echo -n "$C "
done
tput sgr0
echo

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

# standard colors
for C in {40..47}; do
    echo -en "\e[${C}m$C "
done
# high intensity colors
for C in {100..107}; do
    echo -en "\e[${C}m$C "
done
# 256 colors
for C in {16..255}; do
    echo -en "\e[48;5;${C}m$C "
done
echo -e "\e(B\e[m"

To change the manual escapes from background to foreground, the standard color range is 30..37, the high intensity range is 90..97, 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 \E[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:

export PS1="\$? > "
# or
export PS1='$? > '
0 > true
0 > false
1 >

This can be made more interesting using conditionals and functions:

exitstatus()
{
    if [[ $? == 0 ]]; then
        echo ':)'
    else
        echo 'D:'
    fi
}
export PS1='$(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 sc and rc to save and restore the cursor position. The general pattern for a prompt that moves the cursor is

export 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

rightprompt()
{
    printf "%*s" $COLUMNS "right prompt"
}

export PS1='\[$(tput sc; rightprompt; tput rc)\]left prompt > '
left prompt > right prompt

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

Arbitrary positioning

The cup capability moves the cursor to a specific position on the screen, for example tput cup 20 5 moves the cursor to line 20, column 5 (where 0 0 is the top left corner). cuu, cud, cuf, and cub (up, down, forward, back) move the cursor relative to its current position. For example tput cuf 10 moves the cursor 10 characters to the right. You can use the LINES and COLUMNS 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 ESC]2;new titleBEL where ESC and BEL are the escape and bell characters. Using #Bash escape sequences, changing the title in your prompt looks like

export 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.

See also