CSCI 553: Networking III Unix Network Programming Spring 2006

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CSCI 553 Networking III Unix Network
ProgrammingSpring 2006

• Introduction to Unix Shell Scripts

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Unix Shells

• Provide interactive, command line interface
• I/O redirection
• pipes
• filename wildcards (globbing)
• job control
• command line editing, typing completion and
  history
• Provide scripting/glue language ability

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Unix Shells
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Bourne Shell

• First popular Unix shell
• Proprietary, not available in free distributions
• bash is a reimplementation

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Korn Shell (ksh)

• superset of the bourne shell
• improves bourne shell job control, command line
  editing and programming features
• All of which have been incorporated into free
  implementations, and new implementations of csh
  (tcsh) and bourne shell (bash)

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C Shell (csh)

• Written after the korn shell
• Purpose was to provide scripting language that
  adheres more closely to the C language syntax,
  but in a scripting language

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Bourne Again Shell (bash)

• Free software reimplementation of bourne shell,
  but really borrows from korn and csh
• attempt to creat a best of all shells
• extends the bourne shell (and korn shell) plus
  borrows from c shell
• Because of availability on Linux, may be
  displacing csh and ksh as most used and well
  known

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bash

• More on programming in bash

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Start-up

• Bash is a Unix program.
• When a bash shell starts, it executes commands in
  the file .bashrc in your home directory
• except when started as a login shell, then it
  runs .bash_profile

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Variables

• bash has environment variables, as weve
  discussed, to access

dharter_at_nisl echo PATH /usr/kerberos/bin/us
r/java/j2sdk1.4.2_09/bin/usr/local/bin/bin/usr/
bin/usr/sbin/usr/X11R6/bin/opt/pbs/bin/home/dh
arter/bin/opt/kernel_picker/bin/opt/pvm3/lib/op
t/pvm3/lib/LINUX/opt/pvm3/bin/LINUX/opt/env-swit
cher/bin/opt/c3-4//opt/pbs/lib/xpbs/bin dharter
_at_nisl echo HOME /home/dharter dharter_at_nisl
echo USER dharter
dharter_at_nisl teamname"Pittsburgh
Stealers" dharter_at_nisl echo
teamname Pittsburgh Stealers
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Exporting Variables

• In all shells, variables are local to the
  specific shell and, unless otherwise specified,
  are not passed to subshells.
• You must export a shell variable for its value to
  be still set in a subshell.
• In bash (and bourne shell) use export

dharter_at_nisl export teamname
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Command Shortcuts

• As with C shell and Korn shell, bash allows you
  to define your own commands, using the alias
  built-in

dharter_at_nisl alias dls alh dharter_at_nisl
d total 14M drwx------ 46 dharter dharter
4.0K Feb 7 1541 . drwxr-xr-x 36 root root
4.0K Feb 1 1503 .. -rw-rw-r-- 1 dharter
dharter 24 Feb 1 1735 .aspell.en.prepl -rw-rw-
r-- 1 dharter dharter 167 Feb 1 1735
.aspell.en.pws -rw------- 1 dharter dharter
18K Feb 8 0921 .bash_history -rw-r--r-- 1
dharter dharter 24 Oct 19 2004
.bash_logout -rw-r--r-- 1 dharter dharter 191
Oct 19 2004 .bash_profile -rw------- 1 dharter
dharter 1.9K Oct 24 1828 .bashrc
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Other bash features

• If you have not discovered them, you should also
  exlore
• command history (up arrow is simplest example)
• command editing (can use emacs or vi style)
• set o vi tells the shell to use vi style
  command editing
• Autocompletion (use the tab key to complete
  commands and file names while typing, especially
  useful for long path names)

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Programming the bash shell

• arithmetic is a built-in in bash shell (unlike
  some previous shells)

dharter_at_nisl (( res 5 3 )) dharter_at_nisl
echo res 8
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An example bash program, demonstrating arithmetic
and conditional expressions
dharter_at_nisl labs cat divisor.bash !/bin/bash
declare -i testval20 declare -i count2 while
(( count lt testval )) do (( result
testval count )) if (( result 0 ))
then evenly divisible echo " testval
is evenly divisible by count" fi ((
count )) done dharter_at_nisl labs chmod ux
divisor.bash dharter_at_nisl labs ./divisor.bash
20 is evenly divisible by 2 20 is evenly
divisible by 4 20 is evenly divisible by 5 20
is evenly divisible by 10 20 is evenly divisible
by 20
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String comparisons

• You may manipulate strings in bash, using these
  conditional operators

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File-Oriented Expressions

• Scripting languages are mainly useful for
  manipulating and processing text files. Usually
  have advanced file-oriented handling expressions.

dharter_at_nisl labs cat owner.bash !/bin/bash
if -O /etc/passwd then echo "you are the
owner of /etc/passwd." else echo "you are NOT
the owner of /etc/passwd." fi dharter_at_nisl
labs ./owner.bash you are NOT the owner of
/etc/passwd.
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File-oriented Expressions
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Control Structures

• Control structures allow programming logic
• Similar to those in Bourne and Korn shell

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Control Structures

• case .. in .. esac

case teamnameindex in Dallas Cowboys)
echo Dallas, TX Denver Broncos) echo
Denver, CO Ney York GiantsNew York
Jets) echo New York, NY ) echo Unknown
location esac

• if .. then .. elif .. then .. else .. fi

if teamnameindex Minesota Vikings
then cat vikings.txt elif
teamnameindex Chicago Bears
then cat bears.txt else cat nfl.txt fi
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Control Structures

• for .. do .. done

for file in .txt do grep hello file
done

• while/until .. do .. done

while (( count lt testval )) do (( result
testval count )) if (( result 0 ))
then echo testval is evenly divisible by
count fi (( count )) done
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Functions

• Syntatically the same as Korn shell functions.

function name list of commands or the
keyword function may be omitted name()
list of commands
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Function Example

• dharter_at_corvus cat funcex.bash
• f () two-parameter function
• (( returnValue 1 2 ))
• return returnValue
• f 3 4
• result?
• echo "return value from function was result"
• dharter_at_corvus ./funcex.bash
• return value from function was 12