3. Bash Programming and Shell Scripts

3.1 Variables

I'm not going to try to explain all the details of Bash scripting in a section of this HOWTO, just the details pertaining to prompts. If you want to know more about shell programming and Bash in general, I highly recommend Learning the Bash Shell by Cameron Newham and Bill Rosenblatt (O'Reilly, 1998). Oddly, my copy of this book is quite frayed. Again, I'm going to assume that you know a fair bit about Bash already. You can skip this section if you're only looking for the basics, but remember it and refer back if you proceed much farther.

Variables in Bash are assigned much as they are in any programming language:

testvar=5
foo=zen
bar="bash prompt"

Quotes are only needed in an assignment if a space (or special character, discussed shortly) is a part of the variable.

Variables are referenced slightly differently than they are assigned:

> echo $testvar
5
> echo $foo
zen
> echo ${bar}
bash prompt
> echo $NotAssigned

> 

A variable can be referred to as $bar or ${bar}. The braces are useful when it is unclear what is being referenced: if I write $barley do I mean ${bar}ley or ${barley}? Note also that referencing a value that hasn't been assigned doesn't generate an error, instead returning nothing.

3.2 Quotes and Special Characters

If you wish to include a special character in a variable, you will have to quote it differently:

> newvar=$testvar
> echo $newvar
5
> newvar="$testvar"
> echo $newvar
5
> newvar='$testvar'
> echo $newvar
$testvar
> newvar=\$testvar
> echo $newvar
$testvar
>

The dollar sign isn't the only character that's special to the Bash shell, but it's a simple example. An interesting step we can take to make use of assigning a variable name to another variable name is to use eval to dereference the stored variable name:

> echo $testvar
5
> echo $newvar
$testvar
> eval echo $newvar
5
> 

Normally, the shell does only one round of substitutions on the expression it is evaluating: if you say echo $newvar the shell will only go so far as to determine that $newvar is equal to the text string $testvar, it won't evaluate what $testvar is equal to. eval forces that evaluation.

3.3 Command Substitution

In almost all cases in this document, I use the $(<command>) convention for command substitution: that is,

$(date +%H%M)

means "substitute the output from the date +%H%M command here." This works in Bash 2.0+. In some older versions of Bash, prior to 1.14.7, you may need to use backquotes (`date +%H%M`). Backquotes can be used in Bash 2.0+, but are being phased out in favor of $(), which nests better. If you're using an earlier version of Bash, you can usually substitute backquotes where you see $(). If the command substitution is escaped (ie. \$(command) ), then use backslashes to escape BOTH your backquotes (ie. \'command\' ).

3.4 Non-Printing Characters in Prompts

Many of the changes that can be made to Bash prompts that are discussed in this HOWTO use non-printing characters. Changing the colour of the prompt text, changing an Xterm title bar, and moving the cursor position all require non-printing characters.

If I want a very simple prompt consisting of a greater-than sign and a space:

[giles@nikola giles]$ PS1='> '
> 

This is just a two character prompt. If I modify it so that it's a bright yellow greater-than sign (colours are discussed in their own section):

> PS1='\033[1;33m>\033[0m '
> 

This works fine - until you type in a large command line. Because the prompt still only consists of two printing characters (a greater-than sign and a space) but the shell thinks that this prompt is eleven characters long (I think it counts '\033' , '[1' and '[0' as one character each). You can see this by typing a really long command line - you will find that the shell wraps the text before it gets to the edge of the terminal, and in most cases wraps it badly. This is because it's confused about the actual length of the prompt.

So use this instead:

> PS1='\[\033[1;33m\]>\[\033[0m\] '

This is more complex, but it works. Command lines wrap properly. What's been done is to enclose the '\033[1;33m' that starts the yellow colour in '\[' and '\]' which tells the shell "everything between these escaped square brackets, including the brackets themselves, is a non-printing character." The same is done with the '\033[0m' that ends the colour.

3.5 Sourcing a File

When a file is sourced (by typing either source filename or . filename at the command line), the lines of code in the file are executed as if they were printed at the command line. This is particularly useful with complex prompts, to allow them to be stored in files and called up by sourcing the file they are in.

In examples, you will find that I often include #!/bin/bash at the beginning of files including functions. This is not necessary if you are sourcing a file, just as it isn't necessary to chmod +x a file that is going to be sourced. I do this because it makes Vim (my editor of choice, no flames please - you use what you like) think I'm editing a shell script and turn on colour syntax highlighting.

3.6 Functions, Aliases, and the Environment

As mentioned earlier, PS1, PS2, PS3, PS4, and PROMPT_COMMAND are all stored in the Bash environment. For those of us coming from a DOS background, the idea of tossing big hunks of code into the environment is horrifying, because that DOS environment was small, and didn't exactly grow well. There are probably practical limits to what you can and should put in the environment, but I don't know what they are, and we're probably talking a couple of orders of magnitude larger than what DOS users are used to. As Dan put it:

"In my interactive shell I have 62 aliases and 25 functions. My rule of thumb is that if I need something solely for interactive use and can handily write it in bash I make it a shell function (assuming it can't be easily expressed as an alias). If these people are worried about memory they don't need to be using bash. Bash is one of the largest programs I run on my linux box (outside of Oracle). Run top sometime and press 'M' to sort by memory - see how close bash is to the top of the list. Heck, it's bigger than sendmail! Tell 'em to go get ash or something."

I guess he was using console only the day he tried that: running X and X apps, I have a lot of stuff larger than Bash. But the idea is the same: the environment is something to be used, and don't worry about overfilling it.

I risk censure by Unix gurus when I say this (for the crime of over-simplification), but functions are basically small shell scripts that are loaded into the environment for the purpose of efficiency. Quoting Dan again: "Shell functions are about as efficient as they can be. It is the approximate equivalent of sourcing a bash/bourne shell script save that no file I/O need be done as the function is already in memory. The shell functions are typically loaded from [.bashrc or .bash_profile] depending on whether you want them only in the initial shell or in subshells as well. Contrast this with running a shell script: Your shell forks, the child does an exec, potentially the path is searched, the kernel opens the file and examines enough bytes to determine how to run the file, in the case of a shell script a shell must be started with the name of the script as its argument, the shell then opens the file, reads it and executes the statements. Compared to a shell function, everything other than executing the statements can be considered unnecessary overhead."

Aliases are simple to create:

alias d="ls --color=tty --classify"
alias v="d --format=long"
alias rm="rm -i"

Any arguments you pass to the alias are passed to the command line of the aliased command (ls in the first two cases). Note that aliases can be nested, and they can be used to make a normal unix command behave in a different way. (I agree with the argument that you shouldn't use the latter kind of aliases - if you get in the habit of relying on "rm *" to ask you if you're sure, you may lose important files on a system that doesn't use your alias.)

Functions are used for more complex program structures. As a general rule, use an alias for anything that can be done in one line. Functions differ from shell scripts in that they are loaded into the environment so that they work more quickly. As a general rule again, you would want to keep functions relatively small, and any shell script that gets relatively large should remain a shell script rather than turning it into a function. Your decision to load something as a function is also going to depend on how often you use it. If you use a small shell script infrequently, leave it as a shell script. If you use it often, turn it into a function.

To modify the behaviour of ls, you could do something like the following:

function lf
{ 
    ls --color=tty --classify $*
    echo "$(ls -l $* | wc -l) files"
}

This could readily be set as an alias, but for the sake of example, we'll make it a function. If you type the text shown into a text file and then source that file, the function will be in your environment, and be immediately available at the command line without the overhead of a shell script mentioned previously. The usefulness of this becomes more obvious if you consider adding more functionality to the above function, such as using an if statement to execute some special code when links are found in the listing.