Chapter 5 Bourne Shells Scripts

1. Chapter 5 Bourne Shells Scripts By C. Shing ITEC Dept Radford University

2. Objectives • Understand how to use Bourne shell to write shell script

3. Shell Scripts • A sequence of Unix commands • Must be stored in Unix format (no <CR> character) file • The file must be executable (use chmod u+x shellfile) • To execute a shellfile: (type in the shell script filename) such as shellfile

4. Identify Shell Script • If the first line has form: #!shellpath, then shellpath determines which shell the script file is interpreted. • If the first line is just a #, then the current shell is used. • other # form is a comment line (do not start the comment from 1st character of the 1st line, give at least a space if the comment line starts from 1st line) • Otherwise, Bourne shell is used as default shell.

5. Bourne Shells • More Predefined Environment Vaiables:$PS1: shell prompt for 1st line of shell command$PS2: shell prompt for 2nd line or more of the continued shell command

6. Bourne Shells • More Predefined Local Variables:

7. Bourne Shells (Cont.) • More Predefined Local Variables:

8. Bourne Shells (Cont.) • Example: Make sure finish of previous command before proceed to next command

9. Bourne Shells • Assign value to a variable:variable=value • Example: myIncludeDir = /usr/include

10. Bourne Shells • Change from Local to Environment Variable:export variable • Example: export myIncludeDir • read variableRead a line of input and store in $variable

11. Bourne Shells • Evaluate Expression: expr expression

12. Signal - trap • Execute command based on the signals received: • trap command signal1 signal2 ... The shell will execute the command if either one of the signals received. If signal =0, shell executes the command when the the shell script terminates.

13. Signals

14. Redirection • Associate standard input channel with file descriptor n • command <& n • Associate standard error channel with file descriptor n • command >& n Where n: file descriptor n=0: standard input n=1: standard output n=2: standard error

15. Redirection (Cont.) • Example: • (find / -name handler.py > /dev/tty) > & /dev/null This finds the python file and sends errors to /dev/null (drop error message since /dev/null is a pseudo-device), and results to /dev/tty (one’s terminal), so you don’t see error in screen

16. Redirect Standard Error • Redirect Standard Error (2>) • To an error file • command 2> errorfile Example: • gcc beepsyntax.c > beep.out 2>beep.errThen the syntax error will be stored in beep.err • find / -name handler.py –print 2>/dev/null This will show the finding in screen without error messages

17. Redirect Standard Error (Cont.) • Redirect Standard Error (2>) • To Standard Output: • command > outputfile 2>&1 Example: gcc beepsyntax.c > beep.out 2>&1Then the syntax error will be stored in beep.out along with the regular output.

18. Here Document • here document: • command << labelor command >> label The shell copies standard input <<(or output >>) up to, but not including the line with the label into the shell buffer and then execute the command. (Note: label must start a line somewhere down.)

19. Control Structures - if • if structure • Syntax: • if [condition] (or if test condition)then......fi

20. Control Structures - if • Condition: commonly used are as below • -d dirfile: true if dirfile exists as a directory • -f file: true if file exists as a regular file • -r file: true if file exists as readable • -x file: true if file exists as executable • -s file: true if file contains at least one character • str1 = str2 • str1 != str2

21. Control Structures - if • Condition: (Cont.) • str1: true if str1 is not null (you can use this to check whether a process ID still exists) • int1 -eq int2 • int1 -ne int2 • int1 -gt int2 • int1 -ge int2 • int1 -lt int2 • int1 -le int2

22. Control Structures – if (Cont.) • Condition: (Cont.) • !expr • expr1 -a expr2: and • expr1 -o expr2: or • \(expr\): grouping expression

23. Control Structures – if (Cont.) • Example: • If [ -f as5.pl ] then perl as5.pl fi This checks if file as5.pl exists, then run the Perl program.

24. Control Structures – if (Cont.) • Example: (another way) • If [ -f as5.pl ]; then perl as5.pl fi This checks if file as5.pl exists, then run the Perl program.

25. Control Structures – if (Cont.) • Example: • If [ “$1” = “” ] then echo Usage: check.sh uid fi This checks if the 1st command line argument exists. If not, display the error message by Usage clause.

26. Control Structures – if (Cont.) • Example: • If [ ! -f as5.php ] then perl as5.pl fi This checks if file as5.php does not exist, then run the Perl program.

27. Control Structures if … elif • if structure • Syntax: • if [condition] (or if test condition)then...elif ...(many times>then...else...fi

28. Control Structures - case • case structure • Syntax: • case ... invalue1)...;;value2)...;;...*)...;;esac

29. Control Structures - while • while structure • Syntax: • while [condition]do......done

30. Control Structures - while • while structure • Syntax: (another way) • while [condition]; do......done

31. Control Structures - for • for structure • Syntax: • for variable in ...do......done

32. Control Structures - until • until structure • Syntax: • until ...do......done

33. Bourne Shells Example #! /bin/sh # Usage: gradehw 1 If [ $# -eq 1 ] then echo Enter a Grading System Program, Grade HW $1 cd hw$1 for i in `ls` do cd $i cp as$1.c as$1.txt vi as$1.txt cd .. done cd .. else echo Error: Usage: gradehw 1 fi

34. Shell Module • Example: • A Shell Module/Subprogram RunGrading() { … } • Execute the module in a shell script by RunGrading • Check Compilation Example test2.sh

35. More Examples: • track.sh • track.cleanup • track.sed • menu.sh

36. Example of Stealing Superuser if superuser set . As 1st entry in search path (p. 113 Practical Unix) • User: prepare shell script called ls in home directory and do the following: cd chmod 700 . touch ./-f • Superuser: su cd /home/user ls The superuser suggests to user not to use file name –f, didn’t know /bin/sh being stolen as a hidden file .steal

37. Reference • Ch. 5 • Practical Unix & Internet Security by Garfinkel, Spafford & Schwartz, 2ed, O’Reilly