1. LINUX System : Lecture 5 (English-Only Lecture)
Bong-Soo Sohn
Assistant Professor
School of Computer Science and Engineering
Chung-Ang University
Acknowledgement : (i) wikipedia.org ,
(ii)

2. Shell Variables Environment Variables (Global) environment variable
Used to provide information to programs
(Global) environment variable
New programs and shells inherit environment variables from their parent shell
Local shell variable
Used only by that shell
Not passed to other processes

3. Environment Variables
“env” or “printenv” command
Display current environment variables
DISPLAY The graphical display to use, e.g. nyssa:0.0
EDITOR The path to your default editor, e.g. /usr/bin/vi
GROUP Your login group, e.g. staff
HOME Path to your home directory, e.g. /home/frank
HOST The hostname of your system, e.g. nyssa
IFS Internal field separators, usually any white space (defaults to tab, space and <newline>)
LOGNAME The name you login with, e.g. frank
PATH Paths to be searched for commands, e.g. /usr/bin:/usr/ucb:/usr/local/bin
PS1 The primary prompt string, Bourne shell only (defaults to $)
PS2 The secondary prompt string, Bourne shell only (defaults to >)
SHELL The login shell you're using, e.g. /usr/bin/csh
TERM Your terminal type, e.g. xterm
USER Your username, e.g. frank

4. Set Shell Variables Mostly set automatically when log in setenv set
$ setenv NAME value # in C Shell
set
$ set name = value # in C Shell

5. 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)

6. 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

7. 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

8. 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

9. 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

10. if-then-else if ( expr ) simple-command if ( expr ) then commandlist-1
endif
C Shell

11. 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

12. Loops
while ( expr )
commandlist
end
foreach var ( worddlist )
C Shell

13. switch C Shell switch ( str ) case string1: commandlist1 breaksw
default
commandlist
endsw
C Shell

14. 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

15. An example script C Shell #! /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 = $< # $< means “read a line”
switch ($ans)
case n: continue
case q: exit
case y: rm -r $name; continue
endsw
end
C Shell