PPT

1. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Shells Scripts
Andrew Nashel
nashel@cs.unc.edu
Department of Computer Science
February 6, 2004
COMP 121-401: UNIX Programming

2. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Course topics
• Shell scripts
• Homework

3. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Shells scripts
Any collection of shell commands can be
stored in a file called a shell script. Scripts
have variables and flow control statements
like other programming languages.
There are two popular classes of shells:
C shell (csh) and variants (tcsh)
Bourne shell (sh) and variants (bash, ksh)

4. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Invoking scripts
There are two ways to launch scripts:
1) Direct interpretation
csh scriptfile [args …]
2) Indirect interpretation
The first line of the file must be
#!/bin/csh
and the file must be executable.
C Shell

5. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Variables
To set variables:
set X [= value]
Variable contents are accessed using ‘$’:
echo $PATH
To count the number of variable elements:
echo $#Y
C Shell

6. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Variables cont’d
To create lists:
set Y = (abc 1 123)
To set a list element:
set Y[2] = 3
To view a list element:
echo $Y[2]
C Shell

7. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Command arguments
A shell script to swap files:
#! /bin/csh –f
set tmp = $argv[1]
cp $argv[2] $argv[1]
cp $tmp $argv[2]
The number of arguments to a script:
$#argv
C Shell

8. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
if-then-else
if ( expr ) simple-command
if ( expr ) then
commandlist-1
[else
commandlist-2]
endif
C Shell

9. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
if-then-else cont’d
An example:
if ($#argv <> 2) then
echo “we need two parameters!“
else
set name1 = $argv[1]
set name2 = $argv[2]
endif
C Shell

10. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Loops
while ( expr )
commandlist
end
foreach var ( worddlist )
commandlist
end
C Shell

11. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
switch
switch ( str )
case string1:
commandlist1
breaksw
case string2:
commandlist2
breaksw
default
commandlist
endsw
C Shell

12. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
goto (Considered harmful!)
To jump unconditionally:
goto label
A label is a line such as:
label:
The classic paper on why not to use goto:
Go To Statement Considered Harmful
Edsger W. Dijkstra, CACM, March 1968
C Shell

13. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
An example script
#! /bin/csh -f
foreach name ($argv)
if ( -f $name ) then
echo -n "delete the file '${name}' (y/n/q)?"
else
echo -n "delete the entire dir '${name}' (y/n/q)? "
endif
set ans = $<
switch ($ans)
case n: continue
case q: exit
case y: rm -r $name; continue
endsw
end
C Shell

14. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Homework
Due Monday:
Write a shell script that reads a file and
replaces a particular text string with
another, all input given interactively.
Hint: Use sed for text replacement.
This is your final graded assignment!

15. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Homework cont’d
Example:
$ ./myscript
File to read: news.txt
Search text: Duke
Replacement text: Dookie
Dookie University
Department of Mathematics
Weekly Calendar
…

16. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Hmmmm.
Okay, shell scripts are useful,
but a little boring.