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.


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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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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/


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For example:




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Alternatively you may see:


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For example:


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Conditional Execution

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Alternatively you may see:


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While/Until Iteration

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Alternatively you may see:

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Variables

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Variable Assignment

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or

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Exporting Variables

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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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!"#$ %  % !"#







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.

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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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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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Special Variables

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

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The exit status of the last command.

 











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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

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Back Quotes

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and

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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

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+      ' 
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. command

+         /


F 

 
 












 

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.

  

  

  

  

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Test

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and (note the matching bracket argument)


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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.?

 

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Are strings the same or different?

 

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

 


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

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String matching

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Of course getopt would work much better.

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SysV vs BSD echo

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On SysV systems you'd say:


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




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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?

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Creating Input

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alternatively, redirection from a file:


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You can also construct files on the fly.


 
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Note: that variables are expanded in the input.

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String Manipulations

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








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Debugging

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