Unix bourne Shell Scripting (an Introduction)

---

 UNIX Bourne Shell Scripting (an Introduction) 

Purpose 

The purpose of this lab is to familiarise you with the basics of Bourne shell scripting. There are 

various scripts included in this material which illustrate the points being made, and which you can 

copy into a file and run to see what they do. At the end of this file are some exercises which test 

your knowledge of shell scripting. 

 

In shell scripting the Bourne shell (sh) and its extension the Bourne again shell (Bash) are very 

similar.  On robin, type  

 

sh or /bin/sh 

 to invoke the Bourne shell. (To invoke Bash, type bash) 

References  



 

Bourne Shell Programming: http://www.ooblick.com/text/sh/ 



 

Steve Bourne's original shell tutorial: http://partmaps.org/era/unix/shell.html 

Calling external programs / Exit status and Return code 



 

Chaining. First, note that by convention C programs return a 0 to indicate successful exit 

status, e.g.,  

PROGRAM: code.c 

#include  

int main(int argc, char *argv[]) {        

  printf("Hello World!\n"); 

  return 0; 

} 

This C hello program can be tested by running: 

gcc -o out  code.c 

and then run with: 

./out 

 

after calling out use this command to get the exit code: 

 

 

echo $? 

 

Try changing the return value in the C program to 10, recompile, run, and finally output the 

 

return code 

 

Hello World!  

Add the following to hello.sh.  

 

#!/bin/sh 

 

echo Hello World! 

You must have execute permission on this script in order to run it.  Make the file executable by 

typing the following at the command prompt and hitting return: 

 

$ chmod u+x hello.sh  

You can then run the file as follows: 

---

 

$./hello.sh 

 

Hello World! 

Command-line arguments and variables  

The Command-line parameters provide scripts with a way to access data from the shell. These 

reserved parameter variables start with the name of the script, $0, and are assigned by position for 

each argument $i. The number of parameters is set in $#. The script below simply prints its first 

two arguments: 

 

#!/bin/sh 

 

echo $1 

 

echo $2 

The example below shows how the programmer can define variables. The variable $string is 

assigned the value '' argument 1: '' and then concatenated to the command-line parameter $1. Copy 

the text into a file and call it hello2.sh: 

 

#!/bin/sh 

 

string="The argument 1 is: " 

 

echo $string$1                 #concatenate $string with $1 

The script prints out its first command-line argument.  

 

$ ./hello2.sh "Hello World!" 

 

The argument 1 is: Hello World! 

With regard to user-defined variables (like $string in the above example), NOTE the following: 



 

Variables can simply be used without any explicit introduction; 



 

Changes to variables within a script are not visible outside the script, e.g.: 

$ string="original string" 

$ ./hello2.sh “Hello World!” 

argument 1: Hello World! 

$ echo $string 

original string 



 

The export command makes variables visible to subscripts, but not outer scripts, e.g. 

suppose script s1.sh uses the variable $STR: 

$ export STR="original string" 

$ ./s1.sh         # s1.sh will see STR="original string" 



 

 There are various built-in variables set by the shell, for example, PATH, HOME, etc.  

  

---

Input/Output 

 

Bash has several interactive commands. echo command allow users to print a string of characters. 

read command could be used to read a string from the keyboard and assigns it to one or more 

variables. While printf command prints out a formatted string of characters.  

 

Type in the follow fragment into a text file and save it as read.sh 

 

#!/bin/sh  

echo -n "Please type in your first and last name: " 

read firstname lastname  

echo "Hello ${firstname}(fn) ${lastname}(ln)“ 

 

Now execute the above script, it will prompt you to type in your first name and last name. Type in 

your first name and last them and then press carriage return. What do you get? What output do you 

get when you type in your first name only? What do you get when you type in your first name, 

middle name, and last name? 

 

Now edit the script and remove –n option in the first echo command, did you notice any 

differences?   

 

printf is a powerful string formatting command. Copy the following script in to a file and 

execute it. You may also try other format options that are listed in the lecture slides.  

 

#!/bin/sh  

avgmark=65.765 

fname=“Ruibin” 

lname=“Bai” 

printf “Hello %10s, %-10s, average=%7.2f\n” $fname $lname $avgmark 

 

 

 

---

 

Conditionals  

There are several forms for conditionals, the simplest of which uses if statements. This script tests 

to see if there is more than one command-line argument:  

#!/bin/sh 

MIN_ARGS=1 

if [ "$#" -gt "$MIN_ARGS" ]; then 

  echo more than one command line argument is present 

else 

  echo Fail 

fi 

The following script tests to see if FILE exists and if it is a “regular file” (i.e. one that is neither a 

directory nor device file): 

#!/bin/sh 

FILE="fifo" 

if [ -f $FILE ] 

then 

  echo FILE fifo is a regular file 

fi 

The appendix at the end of this lab gives some further test conditions that can be used in if 

statements. 

---

 

Looping  

Loops are useful in shell scripting and have several forms. This script prints three strings: 

#!/bin/sh 

 

for num in one two three 

do 

  echo $num 

done 

When run, it outputs:  

one 

two 

three 

This example prints out a directory listing: 

 

#!/bin/sh 

 

for val in $( ls ) 

     do 

   

 

echo “item: $val” 

 

done 

Try copying this script into a file and running it. What do you get? 

 

C-style loops are also available.  

 

#!/bin/sh 

 

UB=3 

 

for (( i=1; i

     do 

   

 

echo item: $i 

 

done 

When run it outputs:  

item: 1 

item: 2 

The while loop is also available: 

 

#!/bin/sh 

 

i=1 

 

LIM=3 

 

while [ "$i" -lt "$LIM" ] 

     do 

   

 

echo item: $i 

  

 

 (( i++ )) 

 

done

 

---

When run this outputs:  

item: 1 

item: 2 

Arithmetic operations  

Performing arithmetic in a shell script does not work as expected. E.g., if you did the following: 

 

$ x=1+2 

 

$ echo $x 

 

you would get the result: 

 

1+2 

Instead, use the double parenthesis to perform arithmetic: 

 

 $ ii=1 

 

 $ (( jj=$ii+1 )) 

 

 $ echo $jj 

 

2 

 

Appendix 

This section shows different test operations that can be done within IF statements in shell scripts.  

“num1”, “num2”, “string” and “FILE” are numbers strings and files respectively: 

 

string1 = string2       string 1 is equal to string 2 

string1 !=string2       string 1 is not equal to string 2 

-n string               string is not zero length 

-z string               string is zero length 

-d FILE                 FILE is a directory 

-e FILE                 FILE exists 

-f FILE                 FILE exists and is a regular file 

-r FILE                 FILE exists and is readable 

-s FILE                 FILE exists and has length greater than zero 

-w FILE                 FILE exists and is writable 

-x FILE                 FILE exists and is executable 

num1 -eq num2           num1 equal to num2            

num1 -ne num2           num1 not equal to num2            

num1 -lt num2           num1 less than num2            

num1 -le num2           num1 less than or equal to num2            

num1 -gt num2           num1 greater than num2            

num1 -ge num2           num1 greater than or equal to num2