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Bourne Shell Programming

This document is also available in LaTeX2e and gzipped PostScript formats. The slides that accompanied the original talk are also available in LaTeX2e and gzipped PostScript formats.

Note: I've noticed that a few university courses' web pages link to this one. I'm glad that people are finding it useful. Just don't ask me to do your homework.

Justification

Why bother to learn shell programming, when you could be out there rollerblading or trying to get a date?

Because, in a word, it's useful.

Many standard utilities (rdist, make, cron, etc.) allow you to specify a command to run at a certain time. Usually, this command is simply passed to the Bourne shell, which means that you can execute whole scripts, should you choose to do so.

Lastly, Unix runs Bourne shell scripts when it boots. If you want to modify the boot-time behavior of a system, you need to learn to write and modify Bourne shell scripts.

What's it all about?

First of all, what's a shell? Under Unix, a shell is a command interpreter. That is, it reads commands from the keyboard and executes them.

Furthermore, and this is what this tutorial is all about, you can put commands in a file and execute them all at once. This is known as a script. Here's a simple one:

#!/bin/sh
# Rotate procmail log files
cd /homes/arensb/Mail
rm procmail.log.6       # This is redundant
mv procmail.log.5 procmail.log.6
mv procmail.log.4 procmail.log.5
mv procmail.log.3 procmail.log.4
mv procmail.log.2 procmail.log.3
mv procmail.log.1 procmail.log.2
mv procmail.log.0 procmail.log.1
mv procmail.log procmail.log.0

There are several things to note here: first of all, comments begin with a hash (#) and continue to the end of the line (the first line is special, and we'll cover that in just a moment).

Secondly, the script itself is just a series of commands. I use this script to rotate log files, as it says. I could just as easily have typed these commands in by hand, but I'm lazy, and I don't feel like it. Plus, if I did, I might make a typo at the wrong moment and really make a mess.

#!/bin/sh

The first line of any script must begin with #!, followed by the name of the interpreter.

Some versions of Unix allow whitespace between #! and the name of the interpreter. Others don't. Hence, if you want your script to be portable, leave out the blank.

A script, like any file that can be run as a command, needs to be executable: save this script as rotatelog and run

chmod +x rotatelog

to make it executable. You can now run it by running

./rotatelog

Unlike some other operating systems, Unix allows any program to be used as a script interpreter. This is why people talk about ``a Bourne shell script'' or ``an awk script.'' One might even write a more script, or an ls script (though the latter wouldn't be terribly useful). Hence, it is important to let Unix know which program will be interpreting the script.

When Unix tries to execute the script, it sees the first two characters (#!) and knows that it is a script. It then reads the rest of the line to find out which program is to execute the script. For a Bourne shell script, this will be /bin/sh. Hence, the first line of our script must be

#!/bin/sh

After the command interpreter, you can have one, and sometimes more, options. Some flavors of Unix only allow one, though, so don't assume that you can have more.

Once Unix has found out which program will be acting as the interpreter for the script, it runs that program, and passes it the name of the script on the command line. Thus, when you run ./rotatelog, it behaves exactly as if you had run /bin/sh ./rotatelog.

Variables

sh allows you to have variables, just like any programming languages. Variables do not need to be declared. To set a sh variable, use

VAR=value

and to use the value of the variable later, use

$VAR

or

${VAR}

The latter syntax is useful if the variable name is immediately followed by other text:

#!/bin/sh
COLOR=yellow

echo This looks $COLORish
echo This seems ${COLOR}ish

prints

This looks
This seems yellowish

There is only one type of variable in sh: strings. This is somewhat limited, but is sufficient for most purposes.

Local vs. environment variables

A sh variable can be either a local variable or an environment variable. They both work the same way; the only difference lies in what happens when the script runs another program (which, as we saw earlier, it does all the time).

Environment variables are passed to subprocesses. Local variables are not.

By default, variables are local. To turn a local variable into an environment variable, use

export VAR

Here's a simple wrapper for a program:

#!/bin/sh
NETSCAPE_HOME=/usr/imports/libdata

CLASSPATH=$NETSCAPE_HOME/classes
export CLASSPATH

$NETSCAPE_HOME/bin/netscape.bin

Here, NETSCAPE_HOME is a local variable; CLASSPATH is an environment variable. CLASSPATH will be passed to netscape.bin (netscape.bin uses the value of this variable to find Java class files); NETSCAPE_HOME is a convenience variable that is only used by the wrapper script; netscape.bin doesn't need to know about it, so it is kept local.

The only way to unexport a variable is to unset it:

unset VAR

This removes the variable from the shell's symbol table, effectively making as if it had never existed; as a side effect, the variable is also unexported.

Also, if you have a function by the same name as the variable, unset will also delete that function.

Since you may want to use this variable later, it is better not to define it in the first place.

Also, note that if a variable was passed in as part of the environment, it is already an environment variable when your script starts running. If there is a variable that you really don't want to pass to any subprocesses, you should unset it near the top of your script. This is rare, but it might conceivably happen.

If you refer to a variable that hasn't been defined, sh substitutes the empty string.

#!/bin/sh
echo aaa $FOO bbb
echo xxx${FOO}yyy

prints

aaa bbb
xxxyyy

Special variables

sh treats certain variables specially: some are set for you when your script runs, and some affect the way commands are interpreted.

Command-line arguments

The most useful of these variables are the ones referring to the command-line arguments. $1 refers to the first command-line argument (after the name of the script), $2 refers to the second one, and so forth, up to $9.

If you have more than nine command-line arguments, you can use the shift command: this discards the first command-line argument, and bumps the remaining ones up by one position: $2 becomes $1, $8 becomes $7, and so forth.

The variable $0 (zero) contains the name of the script (argv[0] in C programs).

Often, it is useful to just list all of the command-line arguments. For this, sh provides the variables $* (star) and $@ (at). Each of these expands to a string containing all of the command-line arguments, as if you had used $1 $2 $3...

The difference between $* and $@ lies in the way they behave when they occur inside double quotes: $* behaves in the normal way, whereas $@ creates a separate double-quoted string for each command-line argument. That is, "$*" behaves as if you had written "$1 $2 $3", whereas "$@" behaves as if you had written "$1" "$2" "$3".

Finally, $# contains the number of command-line arguments that were given.

Other special variables

$? gives the exit status of the last command that was executed. This should be zero if the command exited normally.

$- lists all of the options with which sh was invoked. See sh(1) for details.

$$ holds the PID of the current process.

$! holds the PID of the last command that was executed in the background.

$IFS (Input Field Separator) determines how sh splits strings into words.

Quasi-variable constructs

The ${VAR} construct is actually a special case of a more general class of constructs:

${VAR:-expression}
Use default value: if VAR is set and non-null, expands to $VAR. Otherwise, expands to expression.
${VAR:=expression}
Set default value: if VAR is set and non-null, expands to $VAR. Otherwise, sets VAR to expression and expands to expression.
${VAR:?[expression]}
If VAR is set and non-null, expands to $VAR. Otherwise, prints expression to standard error and exits with a non-zero exit status.
${VAR:+expression}
If VAR is set and non-null, expands to the empty string. Otherwise, expands to expression.
${#VAR}
Expands to the length of $VAR.

The above patterns test whether VAR is set and non-null. Without the colon, they only test whether VAR is set.

Patterns and Globbing

sh supports a limited form of pattern-matching. The operators are

*
Matches zero or more characters.
?
Matches exactly one character.
[range]
Matches any character in range. range can be either a list of characters that match, or two endpoints separated by a dash: [ak3] matches either a, k, or 3; [a-z] matches any character in the range a through z; [a-mz] matches either a character in the range a through m, or z. If you wish to include a dash as part of the range, it must be the first character, e.g., [-p] will match either a dash or p.

When an expression containing these characters occurs in the middle of a command, sh substitutes the list of all files whose name matches the pattern. This is known as ``globbing.'' Otherwise, these are used mainly in the case construct.

As a special case, when a glob begins with * or ?, it does not match files that begin with a dot. To match these, you need to specify the dot explicitly (e.g., .*, /tmp/.*).

Note to MS-DOS users: under MS-DOS, the pattern *.* matches every file. In sh, it matches every file that contains a dot.

Quoting

If you say something like

echo * MAKE   $$$   FAST *

it won't do what you want: first of all, sh will expand the *s and replace them with a list of all the files in the current directory. Then, since any number of tabs or blanks can separate words, it will compress the three spaces into one. Finally, it will replace the first instance of $$ with the PID of the shell. This is where quoting comes in.

sh supports several types of quotes. Which one you use depends on what you want to do.

Backslash

Just as in C strings, a backslash (``\'') removes any special meaning from the character that follows. If the character after the backslash isn't special to begin with, the backslash has no effect.

The backslash is itself special, so to escape it, just double it: \\.

Single quotes

Single quotes, such as

'foo'

work pretty much the way you'd expect: anything inside them (except a single quote) is quoted. You can say

echo '* MAKE   $$$   FAST *'

and it'll come out the way you want it to.

Note that a backslash inside single quotes also loses its special meaning, so you don't need to double it. There is no way to have a single quote inside single quotes.

Double quotes

Double quotes, such as

"foo"

preserve spaces and most special characters. However, variables and backquoted expressions are expanded and replaced with their value.

Backquotes

If you have an expression within backquotes (also known as backticks), e.g.,

`cmd`

the expression is evaluated as a command, and replaced with whatever the expression prints to its standard output. Thus,

echo You are `whoami`

prints

You are arensb

(if you happen to be me, which I do).

Built-in commands

sh understands several built-in commands, i.e., commands that do not correspond to any program. These commands include:

{ commands ; }, ( commands )

Execute commands in a subshell. That is, run them as if they were a single command. This is useful when I/O redirection is involved, since you can pipe data to or from a mini-script inside a pipeline.

The { commands; } variant is somewhat more efficient, since it doesn't spawn a true subshell. This also means that if you set variables inside of it, the changes will be visible in the rest of the script.

: (colon)
Does nothing. This is generally seen as
: ${VAR:=default}
. filename
The dot command reads in the specified filename, as if it had occurred at that place in the script.
bg [job], fg [job]
bg runs the specified job (or the current job, if none is specified) in the background. fg resumes the specified job (or the current job, if none is specified) in the foreground. Jobs are specified as %number. The jobs command lists jobs.
cd [dir]
Sets the current directory to dir. If dir is not specified, sets the current directory to the home directory.
pwd
Prints the current directory.
echo args
Prints args to standard output.
eval args
Evaluates args as a sh expression. This allows you to construct a string on the fly (e.g., using a variable that holds the name of a variable that you want to set) and execute it.
exec command
Runs the specified command, and replaces the current shell with it. That is, nothing after the exec statement will be executed, unless the exec itself fails.
exit [n]
Exit the current shell with exit code n. This defaults to zero.
kill [-sig] %job
Send signal sig to the specified job. sig can be either numeric or symbolic. kill -l lists all available signals. By default, sig is SIGTERM (15).
read name...
Reads one line from standard input and assigns it to the variable name. If several variables name1, name2, name3 etc. are specified, then the first word of the line read is assigned to name1, the second to name2, and so forth. Any remaining words are assigned to the last variable.
set [+/-flag] [arg]

With no arguments, prints the values of all variables.

set -x turns on the x option to sh; set +x turns it off.

set args... sets the command-line arguments to args.

test expression
Evaluates a boolean expression and exits with an exit code of zero if it is true, or non-zero if it is false. See test for more details.
trap [command sig]...
If signal sig is sent to the shell, execute command. This is useful for exiting cleanly (e.g., removing temporary files etc.) when the script is interrupted.
ulimit
Print or set system limits on resource usage.
umask [nnn]
Sets the umask to nnn (an octal number). With no argument, prints the current umask. This is most useful when you want to create files, but want to restrict who can read or write them.
wait [n]
Wait for the background process whose PID is n to terminate. With no arguments, waits for all of the background processes to terminate.

Bear in mind that the list of builtins varies from one implementation to another, so don't take this list as authoritative.

Flow control

sh supports several flow-control constructs, which add power and flexibility to your scripts.

if

The if statement is a simple conditional. You've seen it in every programming language. Its syntax is

if condition ; then
        commands
[elif condition ; then
        commands]...
[else
        commands]
fi

That is, an if-block, optionally followed by one or more elif-blocks (elif is short for ``else if''), optionally followed by an else-block, and terminated by fi.

The if statement pretty much does what you'd expect: if condition is true, it executes the if-block. Otherwise, it executes the else-block, if there is one. The elif construct is just syntactic sugar, to let you avoid nesting multiple if statements.

#!/bin/sh
myname=`whoami`

if [ $myname = root ]; then
        echo "Welcome to FooSoft 3.0"
else
        echo "You must be root to run this script"
        exit 1
fi

The more observant among you (or those who are math majors) are thinking, ``Hey! You forgot to include the square brackets in the syntax definition!''

Actually, I didn't: [ is actually a command, /bin/[, and is another name for the test command. See below for details.

This is why you shouldn't call a test program test: if you have ``.'' at the end of your path, as you should, executing test will run /bin/test.

The condition can actually be any command. If it returns a zero exit status, the condition is true; otherwise, it is false. Thus, you can write things like

#!/bin/sh
user=arnie
if grep $user /etc/passwd; then
        echo "$user has an account"
else
        echo "$user doesn't have an account"
fi

while

The while statement should also be familiar to you from any number of other programming languages. Its syntax in sh is

while condition; do
        commands
done

As you might expect, the while loop executes commands as long as condition is true. Again, condition can be any command, and is true if the command exits with a zero exit status.

A while loop may contain two special commands: break and continue.

break exits the while loop immediately, jumping to the next statement after the done.

continue skips the rest of the body of the loop, and jumps back to the top, to where condition is evaluated.

for

The for loop iterates over all of the elements in a list. Its syntax is

for var in list; do
        commands
done

list is zero or more words. The for construct will assign the variable var to each word in turn, then execute commands. For example:

#!/bin/sh
for i in foo bar baz "do be do"; do
        echo "$i"
done

will print

foo
bar
baz
do be do

A for loop may also contain break and continue statements. They work the same way as in the while loop.

case

The case construct works like C's switch statement, except that it matches patterns instead of numerical values. Its syntax is

case expression in
        pattern)
                commands
                ;;
        ...
esac

expression is a string; this is generally either a variable or a backquoted command.

pattern is a glob pattern (see globbing).

The patterns are evaluated in the order in which they are seen, and only the first pattern that matches will be executed. Often, you'll want to include a ``none of the above'' clause; to do this, use * as your last pattern.

I/O redirection

A command's input and/or output may be redirected to another command or to a file. By default, every process has three file descriptors: standard input (0), standard output (1) and standard error (2). By default, each of these is connected to the user's terminal.

However, one can do many interesting things by redirecting one or more file descriptor:

< filename
Connect standard input to the file filename. This allows you to have a command read from the file, rather than having to type its input in by hand.
> filename
Connect standard output to the file filename. This allows you to save the output of a command to a file. If the file does not exist, it is created. If it does exist, it is emptied before anything happens.
(Exercise: why doesn't cat * > zzzzzzz work the way you'd expect?)
>> filename
Connects standard output to the file filename. Unlike >, however, the output of the command is appended to filename.
<<word

This construct isn't used nearly as often as it could be. It causes the command's standard input to come from... standard input, but only until word appears on a line by itself. Note that there is no space between << and word.

This can be used as a mini-file within a script, e.g.,

cat > foo.c <<EOT
#include <stdio.h>

main()
{
	printf("Hello, world!\n");
}
EOT

It is also useful for printing multiline messages, e.g.:

line=13
cat <<EOT
An error occurred on line $line.
See page 98 of the manual for details.
EOT

As this example shows, by default, << acts like double quotes (i.e., variables are expanded). If, however, word is quoted, then << acts like single quotes.

<&digit
Use file descriptor digit as standard input.
>&digit
Use file descriptor digit as standard output.
<&-
Close standard input.
>&-
Close standard output.
command1 | command2

Creates a pipeline: the standard output of command1 is connected to the standard input of command2. This is functionally identical to

command1 > /tmp/foo
command2 < /tmp/foo

except that no temporary file is created, and both commands can run at the same time

There is a proverb that says, ``A temporary file is just a pipe with an attitude and a will to live.''

Any number of commands can be pipelined together.

command1 && command2
Execute command1. Then, if it exited with a zero (true) exit status, execute command2.
command1 || command2
Execute command1. Then, if it exited with a non-zero (false) exit status, execute command2.

If any of the redirection constructs is preceded by a digit, then it applies to the file descriptor with that number, rather than the default (0 or 1, as the case may be). For instance,

command 2>&1 > filename

associates file descriptor 2 (standard error) with the same file as file descriptor 1 (standard output), then redirects both of them to filename.

This is also useful for printing error messages:

echo "Danger! Danger Will Robinson!" 1>&2

Note that I/O redirections are parsed in the order they are encountered, from left to right. This allows you to do fairly tricky things, including throwing out standard output, and piping standard output to a command.

Functions

When a group of commands occurs several times in a script, it is useful to define a function. Defining a function is a lot like creating a mini-script within a script.

A function is defined using

name () {
        commands
}

and is invoked like any other command:

name args...

You can redirect a function's I/O, embed it in backquotes, etc., just like any other command.

One way in which functions differ from external scripts is that the shell does not spawn a subshell to execute them. This means that if you set a variable inside a function, the new value will be visible outside of the function.

A function can use return n to terminate with an exit status of n. Obviously, it can also exit n, but that would terminate the entire script.

Function arguments

A function can take command-line arguments, just like any script. Intuitively enough, these are available through $1, $2... $9 just like the main script.

Useful utilities

There are a number of commands that aren't part of sh, but are often used inside sh scripts. These include:

basename

basename pathname prints the last component of pathname:

basename /foo/bar/baz

prints

baz

dirname

The complement of basename: dirname pathname prints all but the last component of pathname, that is the directory part: pathname:

dirname /foo/bar/baz

prints

/foo/bar

[

/bin/[ is another name for /bin/test. It evaluates its arguments as a boolean expression, and exits with an exit code of 0 if it is true, or 1 if it is false.

If test is invoked as [, then it requires a closing bracket ] as its last argument. Otherwise, there must be no closing bracket.

test understands the following expressions, among others:

-e filename
True if filename exists.
-d filename
True if filename exists and is a directory.
-f filename
True if filename exists and is a plain file.
-h filename
True if filename exists and is a symbolic link.
-r filename
True if filename exists and is readable.
-w filename
True if filename exists and is writable.
-n string
True if the length of string is non-zero.
-z string
True if the length of string is zero.
string
True if string is not the empty string.
s1 = s2
True if the strings s1 and s2 are identical.
s1 != s2
True if the strings s1 and s2 are not identical.
n1 -eq n2
True if the numbers n1 and n2 are equal.
n1 -ne n2
True if the numbers n1 and n2 are not equal.
n1 -gt n2
True if the number n1 is greater than n2.
n1 -ge n2
True if the number n1 is greater than or equal to n2.
n1 -lt n2
True if the number n1 is less than n2.
n1 -le n2
True if the number n1 is less than or equal to n2.
! expression
Negates expression, that is, returns true iff expression is false.
expr1 -a expr2
True if both expressions, expr1 and expr2 are true.
expr1 -o expr2
True if either expression, expr1 or expr2 is true.
( expression )
True if expression is true. This allows one to nest expressions.

Note that lazy evaluation does not apply, since all of the arguments to test are evaluated by sh before being passed to test. If you stand to benefit from lazy evaluation, use nested ifs.

echo

echo is a built-in in most implementations of sh, but it also exists as a standalone command.

echo simply prints its arguments to standard output. It can also be told not to append a newline at the end: under BSD-like flavors of Unix, use

echo -n "string"

Under SystemV-ish flavors of Unix, use

echo "string\c"

awk

Awk (and its derivatives, nawk and gawk) is a full-fledged scripting language. Inside sh scripts, it is generally used for its ability to split input lines into fields and print one or more fields. For instance, the following reads /etc/passwd and prints out the name and uid of each user:

awk -F : '{print $1, $3 }' /etc/passwd

The -F : option says that the input records are separated by colons. By default, awk uses whitespace as the field separator.

sed

Sed (stream editor) is also a full-fledged scripting language, albeit a less powerful and more convoluted one than awk. In sh scripts, sed is mainly used to do string substitution: the following script reads standard input, replaces all instances of ``foo'' with ``bar'', and writes the result to standard output:

sed -e 's/foo/bar/g'

The trailing g says to replace all instances of ``foo'' with ``bar'' on a line. Without it, only the first instance would be replaced.

tee

tee [-a] filename reads standard input, copies it to standard output, and saves a copy in the file filename.

By default, tee empties filename before it begins. With the -a option, it appends to filename.

Debugging

Unfortunately, there are no symbolic debuggers such as gdb for sh scripts. When you're debugging a script, you'll have to rely the tried and true method of inserting trace statements, and using some useful options to sh:

The -n option causes sh to read the script but not execute any commands. This is useful for checking syntax.

The -x option causes sh to print each command to standard error before executing it. Since this can generate a lot of output, you may want to turn tracing on just before the section that you want to trace, and turn it off immediately afterward:

set -x
# XXX - What's wrong with this code?
grep $user /etc/passwd 1>&2 > /dev/null
set +x

Style

Here follow a few tips on style, as well as one or two tricks that you may find useful.

Prefer simple, linear application

The advantages of sh are that it is portable (it is found on every flavor of Unix, and is reasonably standard from one implementation to the next), and can do most things that you may want to do with it. However, it is not particularly fast, and there are no good debugging tools for sh scripts.

Therefore, it is best to keep things simple and linear: do A, then do B, then do C, and exit. If you find yourself writing many nested loops, or building awk scripts on the fly, you're probably better off rewriting it in Perl or C.

Put customization variables at the top

If there's any chance that your script will need to be modified in a predictable way, then put a customization variable near the top of the script. For instance, if you need to run gmake, you might be tempted to write

#!/bin/sh
...300 lines further down...
/usr/local/bin/gmake foo

However, someone else might have gmake installed somewhere else, so it is better to write

#!/bin/sh
GMAKE=/usr/local/bin/gmake
...300 lines further down...
$GMAKE foo

Don't go overboard with functions

Functions are neat, but sh is not Pascal or C. In particular, don't try to encapsulate everything inside a function, and avoid having functions call each other. I once had to debug a script where the function calls were six deep at times. It wasn't pretty.

Multi-line strings

Remember that you can put newlines in single- or double-quoted strings. Feel free to use this fact if you need to print out a multi-line error message.

Use : ${VAR:=value} to set defaults

Let's say that your script allows the user to edit a file. It might be tempting to include the line

vi $filename

in your script. But let's say that the user prefers to use Emacs as his editor. In this case, he can set $VISUAL to indicate his preference.

However,

$VISUAL $filename

is no good either, because $VISUAL might not be set.

So use

: ${VISUAL:=vi}
$VISUAL $filename

to set $VISUAL to a reasonable default, if the user hasn't set it.

Paranoia

As with any programming language, it is very easy to write sh scripts that don't do what you want, so a healthy dose of paranoia is a good thing. In particular, scripts that take input from the user must be able to handle any kind of input. CGI-bin scripts will almost certainly be given not only incorrect, but malicious input. Errors in scripts that run as root or bin can cause untold damage as well.

Setuid scripts

DON'T

As we saw above, the way scripts work, Unix opens the file to find out which program will be the file's interpreter. It then invokes the interpeter, and passes it the script's pathname as a command line argument. The interpreter then opens the file, reads it, and executes it.

From the above, you can see that there is a delay between when the OS opens the script, and when the interpreter opens it. This means that there is a race condition that an attacker can exploit: create a symlink that points to the setuid script; then, after the OS has determined the interpeter, but before the interpreter opens the file, replace that symlink with some other script of your choice. Presto! Instant root shell!

This problem is inherent to the way scripts are processed, and therefore cannot easily be fixed.

Compiled programs do not suffer from this problem, since a.out (compiled executable) files are not closed then reopened, but directly loaded into memory. Hence, if you have an application that needs to be setuid, but is most easily written as a script, you can write a wrapper in C that simply exec()s the script. You still need to watch out for the usual problems that involve writing setuid programs, and you have to be paranoid when writing your script, but all of these problems are surmountable. The double-open problem is not.

$IFS

The very first statement in your script should be

IFS=

which resets the input field separator to its default value. Otherwise, you inherit $IFS from the user, who may have set it to some bizarre value in order to make sh parse strings differently from the way you expect, and induce weird behavior.

$PATH

Right after you set $IFS, make sure you set the execution path. Otherwise, you inherit it from the user, who may not have it set to the same value as you do.

In particular, the user might have ``.'' (dot) as the first element of his path, and put a program called ls or grep in the current directory, with disastrous results.

In general, never put ``.'' or any other relative directory on your path.

I like to begin by putting the line

PATH=

at the top of a new script, then add directories to it as necessary (and only add those directories that are necessary).

Quoted variables

Remember that the expansion of a variable might include whitespace or other special characters, whether accidentally or on purpose. To guard against this, make sure you double-quote any variable that should be interpreted as a single word, or which might contain unusual characters (i.e., any user input, and anything derived from that).

I once had a script fail because a user had put a square bracket in his GCOS field in /etc/passwd. You're best off just quoting everything, unless you know for sure that you shouldn't.

Potentially unset variables

Remember that variables may not be set, or may be set to the null string. For instance, you may be tempted to write

if [ $answer = yes ]; then

However, $answer might be set to the empty string, so sh would see if [ = yes ]; then, which would cause an error. Better to write

if [ "$answer" = yes ]; then

The danger here is that $answer might be set to -f, so sh would see if [ -f = yes ]; then, which would also cause an error.

Therefore, write

if [ x"$answer" = xyes ]; then

which avoids both of these problems.

What about the C shell?

The C shell, csh, and its variant tcsh is a fine interactive shell (I use tcsh), but is a lousy shell for writing scripts. See Tom Christiansen's article, Csh Programming Considered Harmful for the gory details.