LINUX System : Lecture 5 (English-Only Lecture)

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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) http//www.doc.ic.ac.uk/wj
k/UnixIntro
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Shell Variables

• Environment Variables
• 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

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Environment Variables

• env or printenv command
• Display current environment variables
• DISPLAY The graphical display to use, e.g.
  nyssa0.0
• 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 ltnewlinegt)
• 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 gt)
• 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

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Set Shell Variables

• Mostly set automatically when log in
• setenv
• setenv NAME value in C Shell
• set
• set name value in C Shell

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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)

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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
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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
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Variables contd

• To create lists
• set Y (abc 1 123)
• To set a list element
• set Y2 3
• To view a list element
• echo Y2

C Shell
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Command arguments

• A shell script to swap files
• ! /bin/csh f
• set tmp argv1
• cp argv2 argv1
• cp tmp argv2
• The number of arguments to a script
• argv

C Shell
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if-then-else

• if ( expr ) simple-command
• if ( expr ) then
• commandlist-1
• else
• commandlist-2
• endif

C Shell
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if-then-else contd

• An example
• if (argv ltgt 2) then
• echo we need two parameters!
• else
• set name1 argv1
• set name2 argv2
• endif

C Shell
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Loops

• while ( expr )
• commandlist
• end
• foreach var ( worddlist )
• commandlist
• end

C Shell
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switch

• switch ( str )
• case string1
• commandlist1
• breaksw
• case string2
• commandlist2
• breaksw
• default
• commandlist
• endsw

C Shell
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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 HarmfulEdsger W.
  Dijkstra, CACM, March 1968

C Shell
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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 lt lt means read a line
• switch (ans)
• case n continue
• case q exit
• case y rm -r name continue
• endsw
• end

C Shell