A shell is a command line interpretor. It takes commands and executes them. As such, it
implements a programming language. The Bourne shell is used to create shell scripts -- ie.
programs that are interpreted/executed by the shell. You can write shell scripts with the C-
shell; however, this is not covered here.

Suppose you often type the command

and you'd rather type a simple command, say

Create a shell script

This quick example is far from adequate but some observations:

1. Shell scripts are simple text files created with an editor.
2. Shell scripts are marked as executeable

4. Should be located in your search path and ~/bin should be in your search path.
5. You likely need to rehash if you're a Csh (tcsh) user (but not again when you login).
6. Arguments are passed from the command line and referenced. For example, as $1.

All Bourne Shell scripts should begin with the sequence

From the man page for exec(2):



!"
#$%&'(
)*&

Comments are any text beginning with the pound (#) sign. A comment can start anywhere on
a line and continue until the end of the line.

All shell scripts should include a search path specifica- tion:

!"#$%%& !"#

A PATH specification is recommended -- often times a script will fail for some people
because they have a different or incomplete search path.
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A good shell script should verify that the arguments sup- plied (if any) are correct.

'(&
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This script requires three arguments and gripes accordingly.

All Unix utilities should return an exit status.

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A non-zero exit status indicates an error condition of some sort while a zero exit status
indicates things worked as expected.
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Exit codes are important for those who use your code. Many constructs test on the exit status
of a command.
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For example,

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Your code should be written with the expectation that others will use it. Making sure you
return a meaningful exit status will help.

Standard input, output, and error are file descriptors 0, 1, and 2. Each has a particular role and
should be used accordingly:

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Error messages should appear on stderr not on stdout! Output should appear on stdout. As for
input/output dialogue:

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F@$55

F@$55

Note: this code behaves differently if there's a user to communicate with (ie. if the standard
input is a tty rather than a pipe, or file, or etc. See tty(1)).

For loop iteration
',,0/

<9

For example:

>G/HI>

<"/;!3
<
JJK

Alternatively you may see:

<9&&

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Case

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For example:

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&&
'-.('-.('-.('-.(?
1$>1.->
&&
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&&

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Conditional Execution

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For example:

'(&
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Alternatively you may see:

&&'&(

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While/Until Iteration

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For example:

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Alternatively you may see:

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Variables

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Variable Assignment

",, ,7,/

!"#$%%& !"#

or

"/;!3$>O4>4>&?>

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Exporting Variables

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'5;QI G!35$55(&
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5;QI G!3O>>%--?245&
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'5;QI G!35$55(&
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Likewise, for variables like the PRINTER which you want hon- ored by lpr(1).
From a user's .profile:

FQD"8F$ I& FQD"8F

Note: that the Cshell exports all environment variables.

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Referencing Variables

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!"#$#/@8% !"#

!"#$%% !"#

The braces are required for concatenation constructs.

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The value of the variable "p_01".

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The value of the variable "p" with "_01" pasted onto the end.

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Conditional Reference

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If the variable has been set, use it's value, else use word.

/I"ISFQ "$L /I"ISFQ " IM&
/I"ISFQ "

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If the variable has been set and is not null, use it's value, else use word.

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0

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If variable is set use it's value, else print out word and exit. Useful for bailing
out.

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Arguments

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The command and arguments. With $0 the command and the rest the
arguments.

The number of arguments.

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All the arguments as a blank separated string. Watch out for "$*" vs. "$@".
And, some commands:

Shift the postional variables down one and decrement number of arguments.

Set the positional variables to the argument list.

)/

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A use of the set command:

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>G/HI>

<"/;!3
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Special Variables

o

Current process id. This is very useful for constructing temporary files.

$-
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2

The exit status of the last command.

F

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Quotes/Special Characters

'/

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These are for command sequences, background jobs, etc. To quote any of these use a
backslash (\) or bracket with quote marks ("" or '').

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Double Quotes

>3 ,,%
(%?@3(+

'5LM5(&
$LMLMLM

Back Quotes

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'5>>5$55(&
$55
$545

$55
$55

and

"/;!3$>O4>4>&?>

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Functions

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For example:

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Within a function the positional parmeters $0, $1, etc. are the arguments to the
function (not the arguments to the script).

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Sourcing commands

"/

+'
/

. command

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What are the virtues of each? What's the difference? The second form is useful for
configuration files where environment variable are set for the script. For example:

#/I"#/I"I&

2

'LW!SX, #/@8ML#/I"M(&
LW!SX, #/@8ML#/I"M

Using configuration files in this manner makes it possible to write scripts that are
automatically tailored for differ- ent situations.

•

Test

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and (note the matching bracket argument)

'(&

On System V machines this is a builtin (check out the com- mand /bin/test).

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is file writeable, readable, executeable, empty, etc?

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are numbers equal, not equal, greater than, etc.?

L$6$M+

Are strings the same or different?

L6M+

Binary or; binary and; use ! for unary negation.

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Learn this command inside out! It does a lot for you.

•

String matching

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B?$>C>&&
?&$&&
B?$>C>&&
?&$&&
B?,&&&
B?7$B&7&&

Of course getopt would work much better.

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SysV vs BSD echo

&'- ! /

/72&

On SysV systems you'd say:

/724&

In an effort to produce portable code we've been using:

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$55&$545

$55&$55

/72&

•

Is there a person?

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The tradition also extends to output.

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<$

Beware: just because stdin is a tty that doesn't mean that stdout is too. User prompts
should be directed to the user terminal.

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Have you ever had a program stop waiting for keyboard input when the output is
directed elsewhere?

•

Creating Input

>!/

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alternatively, redirection from a file:

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'55$55(&
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N

You can also construct files on the fly.

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%VT99)
%
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8/E

Note: that variables are expanded in the input.

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String Manipulations

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"Q@8$>C+.>

"Q@8$>CRBBB4OB4?BB4R>

"Q@8$>C97RLKMR>

"Q@8$>4>4>&K>

"Q@8$>CO<91Z&?>

With some care, redefining the input field separators can help.

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'(&
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$>>

7VV8/EC55CRB$R
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Debugging

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