COMP 4/6262:
Programming UNIX
Lecture 12
shells, shell programming: passing arguments, if,
debug
March 13, 2006
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Shells, Shell Programming - Lecture Notes | COMP 6262, Study notes of Computer Science
Material Type: Notes; Class: Programming Unix; Subject: COMP Computer Science; University: University of Memphis; Term: Spring 2006;
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- COMP 4/6262: Programming UNIX Lecture 12 shells, shell programming: passing arguments, if,debug March 13,
Outline {^
shells
{^
shell programming^ z
passing arguments (KW Ch.7) z exit status z if (KW Ch.8)
{^ test z^
debug
core shell functionality {^
built-in commands {^
scripts {^
variables (local, environment) {^
redirection {^
wildcards {^
pipes {^
sequences {^
subshells {^
background processing {^
command substitution
common shells {^
the shell you use is a matter of taste, power,compatibility, and availability {^
C shell is better than Bourne shell for interactivework, but slightly worse in some respects forscript programming {^
Korn shell incorporates the best of features of theBourne shell and C shells {^
Bash also “best of all worlds”, includes featuresfrom all the other major shells {^
Bourne shell comes with every version of UNIX {^
Bash is the newest, is becoming the mostpopular, open software
changing shell {^
chsh
{^
tcsh
{^
ksh
So Many Commands… {^
Unix has a lot of commands, but there isno way it has everything {^
What do you do if no command existsthat does what you want?^ z^
Build it yourself! z^
The shell itself is actually a programminglanguage z^
A shell program/script consists of a sequentiallist of commands^ {
think of it as a listing of programs to run inorder
Example {^
Check whether a user is logged on
Command Line Parameters {^
Just like all other Unix programs,shell scripts can read parameters offof the command line
{^
Different from user input^ z
Value is known before execution, nottyped in during execution
{^
Example:^ >commandsFile.sh param1.test
Example {^
Check whether a user is logged on
{^
Command file
ison
z^
who
|^
grep $
{^
Usage^ z
./ison 6262-
Shift {^
only first 9 arguments are directly accessible via$1, $2, …, $ {^
for 10 or above use ${10}, ${11}, etc. {^
if more are supplied use shift command to accessthem^ z^
Shifts the arguments to the left and brings newvalues in from the right
z^
z^
z^
z^
z^
z^
$# is automatically decremented by one
{^
example: testShift
exit status {^
Every program and shell returns aninteger value to indicate success orfailure^ z
0 - Success z Any other number – Failure z Used to inform other programs howthings went
exit status {^
variable $?^ z^
the exit status of the last command executed {^
in a pipeline, the exist status is that ofthe last command in the pipe {^
examples^ z^
who | grep 6262- z^
echo $? z^
who | grep good z^
echo $? z^
echo $?
The test Command {^
built-in shell command
{^
to check if certain conditions in an ifcommand
{^
Usage:
test
expression
z^
test evaluate
expression
z^
if the result is true, returns an exitstatus of 0 z^
otherwise, the result is false, returns anonzero exit status
Shortcut For Test {^
Because this is used very often, ashortcut exists {^
[]^ z^
[ is the name of the command z^
still initiates execution of the same testcommand, only that test expects to see aclosing ] z^
spaces after [ and before ] {^
example^ z^
test –f tFile z^
[ -f tFile ]