1. Shell Programming

2. Creating Shell Scripts: Some Basic Principles A script name is arbitrary. Choose names that make it easy to quickly identify file function. Use.sh as an extension to denote shell script files. –Example: quarterly_report.sh Make #!/bin/sh be the 1st line of your script (she-bang). Add comments to your scripts using #. Everything to the right of # is for the benefit of humans. Use chmod +x to make the shell script executable. To execute in current directory use./script.sh

3. Interactive vs. Non-Interactive Scripts Scripts should be both interactive (accept user input) and non- interactive (for use with pipes, redirection, cron jobs, …) The read statement enables a script to take input from user. Positional parameters allow scripts to be non-interactive. –command line arguments are referred to as $0 (the command name), $1, $2, … –$# refers to the total number of arguments on the command line –$* is a string representing all the parameters

4. An Example Script Interactive Version: echo “Enter pattern to be searched: \c” # \c = no newline read var1 echo “Enter file to be used: \c” read var2 echo “Searching for $var1 from file $var2” grep “$var1” $var2 # quote multiword strings Non-Interactive Version: echo “Program: $0” # $0 always = script name echo “The number of arguments specified is $#” echo “The arguments are $*” # all arguments stored in $* grep “$1” $2 echo “\nJob Over”

5. All scripts have an exit status indicating whether or not the script succeeded or failed. Use the exit command to indicate the script status. exit values – exit - everything OK, default status is 0 – exit 0 - True, meaning everything ran OK – exit n - False, meaning error was encountered ( n > =1) The variable $? stores the exit status of the last command. The programmer of the script determines success or failure. The Exit Status of a Script

6. && is the logical AND operator that connects 2 commands: cmd1 && cmd2 means cmd2 is executed if cmd1 succeeds || is the logical OR operator that connects 2 commands: cmd1 || cmd2 means cmd2 is executed if cmd1 fails Examples: $ grep ‘manager’ emplist && echo “Pattern found” # display success message if grep works $ grep “$1” $2 || { echo “not found”; exit 2; } # quit if search fails Making Simple Decisions with 2 Logical Operators

7. Making Decisions with if-then-else Statements if command is successful then excute commands fi if command is successful then excute commands else excute commands fi if command is successful then excute commands elif command is successful then excute commands else excute commands fi Form 1: Form 2: Form 3:

8. An if-then-else Example $ pico ifthen.sh if grep “^$1[: ]” /etc/passwd # did grep succeed? then echo “Pattern found – Job over” # yes, grep worked else echo “Pattern not found” # no, grep failed fi $ chmod 755 ifthen.sh# make script executable $ ifthen.sh rick# test script

9. test is used for making comparisons. Can be used in if- then-else statements and looping statements. Examples: Compare 2 numbers: test $x –gt $y Compare 2 strings: test $x != $y Check a file’s attributes: test -f $file1 Shorthand notation uses [] instead of test : [ $x –gt $y ] # note the whitespace!! [ $x != $y ] [ -f $file ] Relational Testing Using test

10. NumericString -eq Equal to = Equal to -ne Not equal to != Not equal to -gt Greater than -n string String is not null -ge Greater or equal to -z string String is null -lt Less than string String assigned & not null -le Less or equal to == Equal to (Bash & Korn) File -f fName fName exists and is a regular file -r fName fName exists and is readable -s fName fName exists and is its size greater than 0 Some of the Relational Operators (see text)

11. Good for compound conditions in if-then-else Each logical operator has 2 forms. Logical OR written as || or -o if [“$0” = “lm”] || [“$0” = “./lm”]; then if [“$0” = “lm”] -o [“$0” = “./lm”]; then Logical AND written as && or -a if [“$1” = “list1”] && [“$2” = “list2”]; then if [“$1” = “list1”] -a [“$2” = “list2”]; then Combining Logical and Relational Operators

12. # studentinfo.sh if [ $# -eq 0 ]; then#interactive part echo -n “Enter student name: ” read sname if [ -z “$sname” ]; then echo “Student name is null” ; exit 2 fi grep “$sname” “/etc/passwd” else grep “$1” “/etc/passwd”#non-interactive part fi Example: An Interactive/Non-interactive Script

13. The case statement is good for decision making that involves more than two-way branching. The general form is: case expression in pattern 1 )commands 1 ;; pattern 2 )commands 2 ;; pattern 3 )commands 3 ;;... pattern n )commands n ;; esac Patterns may have multiple values separated by | e.g., Mon|Tue) The case Statement

14. # Use of a case statement to offer a 5 item menu echo “ Menu\n1. List of files \n2. Processes of user\n3. Today’s date\n4. Users of system\n5. Quit to Unix\nEnter your option #: \c” read choice case “$choice” in 1)ls -l;; 2)ps -f;; 3)date;; 4)who ;; 5)exit ;; *)echo “Invalid option” # ;; not needed for last option esac Example: A Menu Script

15. expr allows for the 4 basic arithmetic operations on numbers: +, -, \*, and / (Note that the multiplication symbol, *, is escaped.) All arithmetic is integer: the decimal parts are truncated Some Examples: $ x=3; y=5 $ expr 3 + 5 $ expr $x - $y $ expr $x \* $y # escape the asterisk $ expr $y / $x # decimal is truncated $ z=`expr $x \* $y`; echo $z Arithmetic Computation with expr

16. When using expr on strings, separate 2 strings with a : (whitespace) expr with the regular expression.* calculates the length of a string. Some Examples: $ length=expr “rick bournique” : ‘.*’ String Handling with expr and basename if [ `expr “$name” : ‘.*’` -gt 20 ]; then basename with an absolute filename extracts the base filename basename /etc/passwd # yields passwd basename with 2 nd argument extracts 2 nd string from 1 st string. basename this.is.a.test.a.test # yields this.is