1. Chapter 9 Shell Programming
CSNB113 SYSTEM ADMINISTRATION
College of Information Technology
Universiti Tenaga Nasional (UNITEN)
SN 2014

2. Objectives
Discover how shell scripts are executed and the role of the interpreter line
Make shell scripts interactive using read
Use positional parameters to read command-line arguments
Understand the significance of the exit status and the exit statement
Learn comprehensive decision making with the if conditional
Use a for loop to iterate with each element of a list
Use a while loop to repeatedly execute a set of commands
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3. Shell Scripts Store a group of commands in a file and then execute it
It can be in any types of file; with, or without extension
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4. Creating a Simple Script
Create the shell script file using vi editor
$ vi my_first_program
#!/bin/bash
echo "I am fine on: "`date`
echo
ls *.pdf
echo -n "I am "; whoami
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5. Comment Use # to indicates comment
# on the first line – interpreter line - indicator for command interpreter
# on the following line - starts the comment
#!/bin/bash
# this is my first program
# written in December 2013
echo "I am fine on: "`date`
echo # creates a new line
ls *.pdf # list all PDFs
echo # bla-bla-bla-lah!
echo -n "I am "; whoami
# done!
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6. Execution Basic three steps; Write command into a file
Make the file executable (‘x’); chmod
Execute the file using “./fileName”
Create a file
$ vi my_first_program
$ chmod 755 my_first_program
$ ./my_first_program
#!/bin/bash
echo "I am fine on: "`date`
echo
ls *.pdf
echo -n "I am "; whoami
Change permission
Execute
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7. Program Availability $ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
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8. Program Availability Make your new program available system-wide
This example shows the relevancy of the 'path'. The path is an environment variable, that the system administrator can set or change
$ sudo cp my_first_program /usr/local/bin/
$ which my_first_program
/usr/local/bin/my_first_program
$ my_first_program
I am fine on: Mon Jan 17 12:26:22 SGT 2011
yahoo.pdf
I am surizal $
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9. read read statement: internal tool for taking input from the user
Input is stored in the variable name
Use quotes to assign multiple words to a single variable
#!/bin/bash
echo –n “Enter Your Name: “
read name
echo “Hello $name. I hope you will enjoy the lecture”
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10. Positional Parameters
Scripts take user input from command-line arguments
$ vi pp_program
$ chmod 755 pp_program
$ ./pp_program CSNB113 SystemAdmin SN010101
#!/bin/bash
echo “You are executing program $0”
echo “Hello $3.”
echo “Your subject code is $1”
echo “Subject name for $1 is $2” $0 $1 $2 $3
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11. exit
exit statement: Shell scripts return exit status; successful or fail
$? : parameter stores the exit status of the last command; 0 (zero) if succeeds, and non-zero if fails
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12. if … then … fi if statement makes two-way decisions
Additional reference: Here
Form 1
Form 2
Form 3
if condition is successful
then
execute commands fi
else
elif command is successful
then …
else …
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13. if … then … fi #!/bin/bash TEST=4 if [ $TEST -lt 5 ]; then
echo "less than five" fi
#!/bin/bash
TEST=6
if [ $TEST -lt 5 ]; then
echo "less than five"
else
echo "five or more" fi
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14. for … do … done for variable in list do commands done
Perform a set of instruction repeatedly
for variable in list do
commands
done
#!/bin/bash
for i in do
echo "This is round $i"
done
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15. while … do … done Perform a set of instruction repeatedly
Doesn’t work with a list
Uses a control command to determine the flow of execution
while condition is true do
commands
done
#!/bin/bash
TEST="no"
while [ "$TEST" != "yes" ]; do
read TEST
done
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16. expr expr value1 operation value2 a=5 b=10 c=`expr $a + $b`
Command used to evaluate an expression – perform calculation
expr value1 operation value2
a=5
b=10
c=`expr $a + $b`
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17. References
Das, S. (2012). Your UNIX/LINUX The Ultimate Guide: Third Edition. McGraw-Hill
Hahn, H. (2008). Harley Hahn's Guide to Unix and Linux. California: McGraw-Hill Higher Education
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