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: Predefined Local
Explain
quoted list of all command line parameters (other than the command) $#
total # of all command line parameters (don’t count command) $?
exit value of the last command

7. Bourne Shells (Cont.) More Predefined Local Variables: Predefined
Explain $!
PID of the last background command $$
PID of the shell

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 variable Read a line of input and store in $variable

11. Bourne Shells Evaluate Expression: expr expression Expressions Explain
string: regular_expression
return string length if both sides match; return 0 otherwise

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 Signals Explain 1 or SIGHUP hang up (logout) 2 or SIGINT
Press interrupt key
3 or SIGQUIT
quit
9 or SIGKILL
kill

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
To an error file
command 2> errorfile
Example:
gcc beepsyntax.c > beep.out 2>beep.err Then 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.)
To Standard Output:
command > outputfile 2>&1
Example: gcc beepsyntax.c > beep.out 2>&1 Then the syntax error will be stored in beep.out along with the regular output.

18. Here Document here document:
command << label or 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 ... in value1) ... ;; 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
else
echo Error: Usage: gradehw 1 fi

34. Shell Module Example: RunGrading() { } RunGrading
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
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