How to write a shell script.pdf

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.

+&',,

A good shell script should verify that the arguments sup- plied (if any) are correct.

'(&

)*+,%-./.

This script requires three arguments and gripes accordingly.

All Unix utilities should return an exit status.

'1.-1+-..(&

)*+345145

!67

A non-zero exit status indicates an error condition of some sort while a zero exit status

indicates things worked as expected.

&'- !.

Exit codes are important for those who use your code. Many constructs test on the exit status

of a command.

For example,

894:;

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:

'1.-1+-..(&

)*+3451451

761<1=6

>10>

@&&

"&&

Error messages should appear on stderr not on stdout! Output should appear on stdout. As for

input/output dialogue:

<9A

"9<B

/72&

BCDB?

-&&

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/

For example:

>G/HI>

<"/;!3

JJK

Alternatively you may see:

<9&&

Case

''C(?

&&(

For example:

515

'-.('-.(?

1$.L1M

1$>1.->

'-.('-.('-.('-.(?

1$>1.->

)*+345145

Conditional Execution

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