4 Prompting Yourself

with a Custom

Technique Prompt

Save Time By

Keeping useful information handy

Colorizing your prompt to convey useful information

Saving your prompt preferences

Warning yourself when you hold potentially dangerous privileges

Your prompt is your connection to the Linux world when you’re working in the shell. If you haven’t already modified it, your prompt displays your machine name and current directory. But

why settle for less information than you could really use?

Customize your prompt to keep information that you need in plain sight when you’re working at the command line. You can add information such as the time, date, number of users, and more. In addition to displaying system information, your prompt can change colors. If you use multiple terminal windows connected to multiple machines, use a different-colored prompt on each machine to give you a quick clue about your location, all without taking up screen space.

The prompt also reflects your status as a superuser (or as a mere mortal). Keep an eye on your privilege level to prevent damage from the accidental use of privileges. We’ve included code in this technique to make the prompt change color when you hold elevated (and thus potentially dangerous) privileges.

In this technique, we show you how to manipulate your prompt to display the information that lets you get the job done quickly. Information is power, and power definitely saves time.

Long prompts can take up a lot of screen real estate and also consume a lot of space on the printed page. In this technique, we show you complete prompts that enable you to see useful info quickly and easily. However, you don’t want to work with these prompts all the time. In other techniques, we shorten the prompt to $ or # to save space.

Making Basic Prompt Transformations

In the bash shell, the prompt is controlled by a set of environment variables, the most important of which is $PS1. Change $PS1, and you change your prompt. The $PS1 variable is displayed when bash is waiting for a command from you. The $PS2 variable is also worth mentioning — it’s displayed when bash needs more input to complete a current task.

24 Technique 4: Prompting Yourself with a Custom Prompt

If $PS1 contains a simple text string (such as “Hi, I’m the prompt”), that string is displayed whenever a command completes and the shell is waiting for the next command. Modifying the prompt is easy: Just enter strings that you want to test and hit Enter, and the results are displayed instantly. Saving your changes takes a bit of maneuvering, but we cover that in the next section. Here’s a quick example of how to change the prompt:

[freddie@bastille] PS1=”Hi, I’m the prompt “

Hi, I’m the prompt

Notice how the prompt changed from

[freddie@bastille] to “Hi, I’m the prompt”.

Adding Dynamically Updated Data to Your Prompt

Static prompts, such as the example in the preceding section, are kind of boring, so bash lets you include special character sequences (we’ll call them macros) that represent changing data. Each macro starts with a backslash and is followed by a single character that tells bash which chunk of data you want to display. For example, if you want to display the current date and time whenever the prompt is displayed, use the \d (date) and \t (time) macros like this:

[freddie@bastille] PS1=”\d \t “ Thu Dec 18 03:37:48

TABLE 4-1: HANDY MACROS FOR YOUR PROMPT

You can mix dynamic macros and static text in the same prompt. To enclose the date and time in brackets, just include the brackets in $PS1:

[freddie@bastille] PS1=”[\d \t] “ [Thu Dec 18 03:37:50 ]

It’s usually a good idea to end each prompt with static text (a character like ], -, or >) and a space to make the prompt easier to read.

If you press Enter a few times with this prompt, you see that the macros in $PS1 are evaluated each time the prompt is displayed:

[freddie@bastille] PS1=”[\d \t] “ [Thu Dec 18 03:37:51 ]

[Thu Dec 18 03:37:51 ]

[Thu Dec 18 03:37:58 ]

[Thu Dec 18 03:38:01 ]

You can include as many macros as you want, in any order that you want, in $PS1. For example, to display the current date and time, your user name, your host name, and the current working directory (in that order), try this:

[freddie@bastille] PS1=”[\d \t \u@\h:\w] “ [Thu Dec 18 03:40:20

freddie@bastille:/home/freddie]

Spacing is important. Be sure to leave some white space between macros to make the info easier to read. Table 4-1 lists some of the most useful macros that you can include in a bash prompt.

Macro	What It Does/Displays	Timesaving Bonus Info
\a	Speaker beep	To keep users on their toes, code the $PS2 variable to beep when
		the user needs to input additional information. Just enter
		$PS2=”\a >”, and the computer beeps when it needs attention!

\d Weekday (Sun–Sat), month name, and date (“Thu Dec 18”, for example)

Handy when you’re pulling all-nighters and you need to know when Saturday morning rolls around.

	Adding Dynamically Updated Data to Your Prompt		25
Macro	What It Does/Displays	Timesaving Bonus Info
\D{}	Date and/or time in a format of your choosing	The \D macro must be followed by a format string enclosed in
		braces. bash interprets the format string by using the same rules
		as the strftime library function (see man strftime for more
		details). If the format string is empty, the braces are still required,
		but bash chooses a display format appropriate to your locale.
\e	Escape character; used for complex strings	Escape characters introduce complex, unfriendly terminal
		command sequences. We show you a better way later in this
		technique.

\h Host name up to the first . (dot)

If you work on a number of different hosts from the same workstation, \h can help you remember which one you’re currently connected to.

\H	Entire host name	Similar to \h, but takes up too much screen real estate for our
		taste.
\n	Newline	Use a new line to create a multiline prompt.
\s	Shell name — such as bash or csh	We’ve never found a particularly good use for this one because
		we always stick to bash.
\t	Current time in 24-hour (HH:MM:SS) format
\T	Current time in 12-hour (HH:MM:SS) format
\@	Current time in 12-hour (am/pm) format	Is it 5:00 yet?
\A	Current time in 24-hour (HH:MM) format
\u	Current user name	Include \u if you need to do work on someone else’s behalf (in
		other words, if you’re an administrator). That way you won’t for-
		get who you are and send flaming e-mail using someone else’s
		name!
\W	Trailing component of your current	This is probably the most useful macro you could include in a
	working directory	custom prompt — sort of a “You Are Here” sign.
\w	Entire current working directory	Similar to \W, but takes up a lot of room on your command line.

\\Backslash character

\!	History number	Every command that you execute is stored in a history log, and
		you can refer to a specific command in the log by its history num-
		ber. Include the \! macro in your prompt, and you’ll see the his-
		tory number assigned to each command. (We talk more about
		history processing in Technique 9.)
\$	If the effective UID is 0, a #; otherwise a $	The \$ macro displays a pound sign (#) if you hold superuser
		privileges or a dollar sign ($) if you don’t. You can use the \$
		macro to help you remember when you have enough privileges to
		seriously damage your system, but we show you a better way in
		the section “Seeing a Red Alert When You Have Superuser
		Privileges,” later in this technique.

26 Technique 4: Prompting Yourself with a Custom Prompt

Colorizing Your Prompt

Changing the color of your prompt may not save you tons of time, but it can make the prompt more readable and convey extra information without taking up screen real estate.

What kind of information can you encode with colorized prompts? Just about anything. Turn your prompt green when you’re logged into one host and blue when you’re logged into another. Display your prompt in green when the system load is low, yellow as it increases, and red when you’re running into resource bottlenecks. Or, just change the color of your prompt to a fixed color so that it stands out on the screen.

You can colorize your prompt two ways. The most common (but not the most timesaving) way is to include special “escape” characters (characters that your terminal window understands, but humans don’t) in your prompt. For example, the following string turns your prompt blue:

[freddie@bastille] PS1=”\[\033[0;34m\] [\u@\h]\[\033[0m\] “

[freddie@bastille]

Of course, because this is a black-and-white book, you can’t see the color here, but if you try this example, you’ll see that the prompt turns blue. This method works, but it has two drawbacks. First, the syntax is hard to read (and hard to get right in the first place). Second, this method works only if your terminal emulator supports ANSI escape sequences — many terminal emulators (and many terminals) don’t. Fortunately, you can fix both problems at once by using tput.

When changing the color of your prompt, using tput makes your prompts portable. That is, if you move to another terminal emulator, you don’t have to change prompts. tput also

knows the right escape sequences, so you don’t have to spend time looking them up.

The blue prompt in the preceding example looks like this when you use tput:

[freddie@bastille] BLUE=$(tput setaf 4) [freddie@bastille] BLACK=$(tput setaf 0) [freddie@bastille] PS1=”\[$BLUE\]\u@\h]\

[$BLACK\] “ [freddie@bastille]

The first line uses tput to find the character sequence that changes the foreground color to blue. The second line finds the character sequence that changes the foreground color to black. Notice that you don’t need to know the magic escape sequences; tput keeps a database of terminal descriptions and consults that database to find the sequence that corresponds to the terminal (or terminal emulator) you’re using. The third line patches the $BLUE and $BLACK sequences into the $PS1 prompt string.

The $PS1 string, however, is still more complicated than it needs to be — it’s got a few extra \[ and \] sequences. Those extra characters are required so that bash knows which prompt characters take up screen real estate and which ones don’t (the invisible characters must appear between a \[ and \] pair).

When you use tput, you can clean up extra characters a bit more by including those extra characters in the $BLUE and $BLACK variables:

[freddie@bastille] BLUE=”\[$(tput setaf 4)\]”

[freddie@bastille] BLACK=”\[$(tput setaf 0)\]”

[freddie@bastille] PS1=”$BLUE\u@\h]$BLACK “ [freddie@bastille]

tput can do much more than just change the foreground color of the prompt. Table 4-2 shows a few of the more useful tput sequences. (For a complete list, see man tput and man terminfo.)