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.  


'



(&
 











)*+
, %
-
.
/
.











+0







This script requires three arguments and gripes accordingly.  

   

   

   

   

All Unix utilities should return an exit status.  




1 




2








'
1 

.-



1 

+-..
(&
 


)*+
3 
451  45














+0






















!
 6

7








-

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,  


1
&
 











8
 

9
4:;







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 




2








'
1 

.-



1 

+-..
(&
 











)*+
3 
451  45


1












+0
















76
1
<

 

1

=
6









> 
1

0>






-?











@&&





?











" &&









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


<

 9



A








1

&
 











"
9
<

 

B














/7

   2
&


















55
















BCDB?






E
 
 &






-


&&





















F@$5
 5
















F@$5
 5







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 
&

 &


• 

Case  

'     


9 






'
 
'
C
 

(
?










 
&&
(








For example:  


51  5








'-.('-.(?













1  $.L1  M













1  $> 
1 

.->













&&





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













1  $> 
1 

.->













&&





B?














)*+
3 
451  45


















+0













&&







• 

Conditional Execution  

+    


 





 










 





'


 
(







For example:  


'



(&
 














)*+
, %
-
.
/
.














+0







Alternatively you may see:  


 &
 
 &
'

 &
(


• 

While/Until Iteration  

10      

L9
C
M
 
















 







For example:  




  
 6
 

  1







9
'



(&














N 






















Alternatively you may see:  

9
 &

 &


• 

Variables  

2  3          
    +   ,   
 4444

5 , % 046(    ,   
   

o 

Variable Assignment  

" , ,  , 7, /

!"#$ % %&
 
!"#

or  

"/;!3$>O 
4> 4>&

?>

o 

Exporting Variables  

2           0


P
@,I"
<

;QI G!3
<  
< 








'
5;QI G!35
$
55
(&
 














1

&
 







5;QI G!3
O> >%--?2
45&







;QI G!3




























'
5;QI G!35
$
55
(&
 






;QI G!3$> >%--



























 
;QI G!3







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.  

o 

Referencing Variables  

84, %    49, :(   ,


@
 R
<



 
9


'
5,I8F5
$
5 5
(&
 













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



















!"#$ %  % !"#







The braces are required for concatenation constructs.  

N-

The value of the variable "p_01".  

LMN-

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

o 

Conditional Reference  

o 

L<  9 M

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

 /I"ISFQ "$L /I"ISFQ " I M&

 
/I"ISFQ "



L<  %9 M

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

+        ,  ;
    , ,  )  
 0 %6(  <1;5+=1, ,   
 0    

L<  %29 M

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

o 

Arguments  

)         , /

-6
6


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



The number of arguments.  

B6
T

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.  

)     /


 
  








9
'



(&



























  
  



































A use of the set command:  




9
1









"/;!3$>O 
4> 4>&

?>








I //G










>
G/HI>















<

"/;!3














 <














 
JJK








o 

Special Variables  

o 



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

 $ -










 
5

 
 
 +5










 


+

U










  
) 



2

The exit status of the last command.  

 











F
  



 















'
2
 A
-
(










 
















• 

Quotes/Special Characters  

'       /

&
*
O
?
C
:
V
)
 9



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

 

>   3    3   0+
   

 
%LM%
  
C
97
E%
RL 
MR

Double Quotes  

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


'

5L M5
(&
 












 $L  ML ML M












Back Quotes  

&03     


'
5> 
>5
$
55
(&
 


 

$55


 

$545













 

$55


 

$55












and  

"/;!3$>O 
4> 4>&

?>

• 

Functions  

      !  '  


O?





L










 





M

For example:  


 

  1







N  O?





L
















 


  1
















'



(&
 







%
D

  1
)*+






 
































M

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

>        

            
  /


 
9

 6

 











N O?





L













9
  N





















M
















7
 

O

  
 ?











'
55
$
55
(&
 















C
N



















N



• 

Sourcing commands  

      "   /



 

+      ' 
 /


  

. 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.  

  

  

  

  

• 

Test  

+     %6(


 
 &
 














and (note the matching bracket argument)  


'
 
(&
 














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

&'- % 0'(      
  A

8   /

 
L
96
 6
6
6

M
 

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

 

L
 A6
 6
6

M
+

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

 

L
$6
$
M
+

Are strings the same or different?  

 

L
6

M
+

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

 


'
1 

.-



1 

+-..
(&
 











)*+
3 
451  45














+0







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

• 

String matching  

+  ,     "   
 0     


 
  








9
'



(&






























B?



 $> 

C

>&&













?




&

 $
&&













B?



 $> 

C

>&&













?




&

 $
&&













B?





, &


&&













B?





7$B&






 7


&&







































Of course getopt would work much better.  

• 

SysV vs BSD echo  

&'-    ! /



/7

   2&




On SysV systems you'd say:  


/7

   2
4&



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




9
7












'
5> 
>5
$
55
(&
 













$55&

$545



















$55&




$55

















/7

   2
&



• 

Is there a person?  

+8        3    
=      $ 

8   ! 3   !  


Q
 R

 

 6
 











1
&
 











8
 

9
4:;







The tradition also extends to output.  


Q





 6

< 








1

V*&
 










<  $ 
















<  $







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


Q
 R

 

 6
 











1
&
 











8
 

9
4:;
)*-







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

• 

Creating Input  

>!          /


7
 

O

  
 ?











'
55
$
55
(&
 















C
N



















N







alternatively, redirection from a file:  


7
 

O

  
 ?











'
55
$
55
(&
 













N
V




















N







You can also construct files on the fly.  


 
V






%











  %
VT 99)





%






IY %
I
+







 
+
,  
E 

,P/











A





8/E

Note: that variables are expanded in the input.  

• 

String Manipulations  

      !     '  0

"Q@8$>
C

+.>







"Q@8$>
C
 
RB
B
B
4OB4?
B
B4R>

"Q@8$>
C
97
RL 
KMR>







"Q@8$> 
4> 4>&

K>







"Q@8$>
C
O 

<
9

1
Z&

?>

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








< 
Q
 

  








 O?





L



 
>QEI$55& 
>











K+  





M








'



(&
 











)*+
, %
1
Q  











+0













$>
>







7
V
V
8/E
C
 
55
C
 
RB$
R





 
1 $1











8/E

• 

Debugging  

+  0   /




 

1  ! ;=0 




 

-     ! ;   
  !


,