11. Prompt Code Snippets

This section shows how to put various pieces of information into the Bash prompt. There are an infinite number of things that could be put in your prompt. Feel free to send me examples, I will try to include what I think will be most widely used. If you have an alternate way to retrieve a piece of information here, and feel your method is more efficient, please contact me. It's easy to write bad code, I do it often, but it's great to write elegant code, and a pleasure to read it. I manage it every once in a while, and would love to have more of it to put in here.

To incorporate shell code in prompts, it has to be escaped. Usually, this will mean putting it inside \$(<command>) so that the output of command is substituted each time the prompt is generated.

11.1 Built-in Escape Sequences

See Bash Prompt Escape Sequences for a complete list of built-in escape sequences. This list is taken directly from the Bash man page, so you can also look there.

11.2 Date and Time

If you don't like the built-ins for date and time, extracting the same information from the date command is relatively easy. Examples already seen in this HOWTO include date +%H%M, which will put in the hour in 24 hour format, and the minute. date "+%A, %d %B %Y" will give something like "Sunday, 06 June 1999". For a full list of the interpreted sequences, type date --help or man date.

11.3 Counting Files in the Current Directory

To determine how many files there are in the current directory, put in ls -l | wc -l. This uses wc wordcount to do a count of the number of lines (-l) in the output of ls -l. It doesn't count dotfiles. If you want to count only files and NOT include symbolic links (just an example of what else you could do), you could use ls -l | grep -v ^l | wc -l. Here, grep checks for any line beginning with "l" (indicating a link), and discards that line (-v).

11.4 Total Bytes in the Current Directory

If you want to know how much space the contents of the current directory take up, you can use something like the following:

# The sed command replaces all the spaces with only one space.
# cut -d" " -f5 : -d determines a delimiter, which means that (in 
# this case) a space begins a new column.
# -f says to take out a certain column, in this case the fifth one

let TotalBytes=0

for Bytes in $(ls -l | grep "^-" | sed -e "s/ \+/ /g" | cut -d" " -f5)
do
   let TotalBytes=$TotalBytes+$Bytes
done

# The if...fi's give a more specific output in byte, kilobyte, megabyte, 
# and gigabyte

if [ $TotalBytes -lt 1024 ]; then
   TotalSize=$(echo -e "scale=3 \n$TotalBytes \nquit" | bc)
else if [ $TotalBytes -lt 1048576 ]; then
   TotalSize=$(echo -e "scale=3 \n$TotalBytes/1024 \nquit" | bc)
else if [ $TotalBytes -lt 1073741824 ]; then
   TotalSize=$(echo -e "scale=3 \n$TotalBytes/1048576 \nquit" | bc)
else
   TotalSize=$(echo -e "scale=3 \n$TotalBytes/1073741824 \nquit" | bc)
fi
fi
fi

Code courtesy of Sam Schmit (id@pt.lu) and his uncle Jean-Paul, who ironed out a fairly major bug in my original code, and just generally cleaned it up.

11.5 Checking the Current TTY

The tty command returns the filename of the terminal connected to standard input. This comes in two formats on the Linux systems I have used, either "/dev/tty4" or "/dev/pts/2". I have taken to using a more general solution to this: tty | sed -e "s:/dev/::", which removes the leading "/dev/". Older systems (in my experience, RedHat through 5.2) returned only filenames in the "/dev/tty4" format, so I used tty | sed -e "s/.*tty\(.*\)/\1/".

An alternative method: ps aux | grep $$ | awk '{ print $7 }'.

11.6 Suspended Job Count

To find out how many suspended jobs you have, use jobs | wc -l | awk '{print $1}'. awk is used to trim the output, which would otherwise include blank spaces that waste space in a prompt. If you start netscape from an xterm, this will also be counted. If you want to avoid that, and only count stopped jobs, use jobs -s instead. Type help jobs for more info on jobs. jobs will always return nothing to a pipe in version 2.02 of Bash: this problem is not present in any other version.

11.7 Uptime and Load

Current load is taken from the uptime command. What I use at the moment is uptime | sed -e "s/.*load average: \(.*\...\), .*\..., .*\.../\1/" -e "s/ //g" which is clunky in the extreme, but works. Replacements welcome. uptime can also be used in a very similar manner to find out how long the machine has been up (obviously) or how many users are logged in, and the data could be massaged with sed to look the way you want it to.

11.8 Number of Processes

ps ax | wc -l | tr -d " " OR ps ax | wc -l | awk '{print $1}' OR ps ax | wc -l | sed -e "s: ::g". In each case, tr or awk or sed is used to remove the undesirable whitespace.

11.9 Controlling the Width of $PWD

Unix allows long file names, which can lead to the value of $PWD being very long. Some people (notably the default RedHat prompt) choose to use the basename of the current working directory (ie. "giles" if $PWD="/home/giles"). I like more info than that, but it's often desirable to limit the length of the directory name, and it makes the most sense to truncate on the left.

#   How many characters of the $PWD should be kept
local pwd_length=30
if [ $(echo -n $PWD | wc -c | tr -d " ") -gt $pwd_length ]
then
   newPWD="...$(echo -n $PWD | sed -e "s/.*\(.\{$pwd_length\}\)/\1/")"
else
   newPWD="$(echo -n $PWD)"
fi

The above code can be executed as part of PROMPT_COMMAND, and the environment variable generated (newPWD) can then be included in the prompt.

11.10 Laptop Power

Again, this isn't elegant, but it works (most of the time). If you have a laptop with APM installed, try power=$(apm | sed -e "s/.*: \([1-9][0-9]*\)%/\1/" | tr -d " ") executed from PROMPT_COMMAND to create an environment variable you can add to your prompt. This will indicate percentage power remaining.

11.11 Having the Prompt Ignored on Cut and Paste

This one is weird but cool. Rory Toma (rory@corp.webtv.net) wrote to suggest a prompt like this: : rory@demon ; . How is this useful? Well, if you type a command after the prompt (odd idea, that), you can triple click on that line (in Linux, anyway) to highlight the whole line, then paste that line in front of another prompt, and the stuff between the ":" and the """ is ignored, like so:

: rory@demon ; uptime
  5:15pm  up 6 days, 23:04,  2 users,  load average: 0.00, 0.00, 0.00
: rory@demon ; : rory@demon ; uptime
  5:15pm  up 6 days, 23:04,  2 users,  load average: 0.00, 0.00, 0.00

The prompt is a no-op, and if your PS2 is set to a space, multiple lines can be cut and pasted as well.

11.12 Setting the Window Title and Icon Title Separately

A suggestion from Charles Lepple (clepple@negativezero.org) on setting the window title of the Xterm and the title of the corresponding icon separately (first check out the earlier section Xterm Title Bar Manipulations). He uses this under WindowMaker because the title that's appropriate for an Xterm is usually too long for a 64x64 icon. "\[\e]1;icon-title\007\e]2;main-title\007\]". He says to set this in the prompt command because "I tried putting the string in PS1, but it causes flickering under some window managers because it results in setting the prompt multiple times when you are editing a multi-line command (at least under bash 1.4.x -- and I was too lazy to fully explore the reasons behind it)." I had no trouble with it in the PS1 string, but didn't use any multi-line commands. He also points out that it works under xterm, xwsh, and dtterm, but not gnome-terminal (which uses only the main title). I also found it to work with rxvt, but not kterm.