CTEC 1863 – Operating Systems

Shell Scripting

CTEC1863 - 2009F 2

Overview

• How shell works

• Command line parameters– Shift command

• Variables – Including predefined/specials vars

• Functions

CTEC1863 - 2009F 3

Shells

• sh is a command interpreter– It supports a powerful command language– Each invocation of the sh program is called a shell

CTEC1863 - 2009F 4

Shells (2)

• OS’s allow invoking commands one at a time from a terminal

• Shells allow users to:– Combine commands to form new commands– Pass positional parameters to a command– Add or rename commands– Execute commands within loops– Execute commands conditionally– Send commands into the background

CTEC1863 - 2009F 5

How it works:

CTEC1863 - 2009F 6

How it works:

• When you type a command:– Your shell makes a copy of itself through the fork

system call (PID 230)– Your shell puts itself to sleep (via the wait system call)

until its child process (PID 230) calls exit– The child process calls exec(date) to overwrite itself

with the code for date– When the date command is finished, it calls exit()– The child process (230) dies, and your shell wakes up

again, and displays the prompt

CTEC1863 - 2009F 7

Variables

• Spring into existence by being mentioned– May be assigned string values– Syntax: NAME=STRING– Thereafter "$NAME" will yield "STRING"– No spaces around the = sign!!!!!

• Double quotes around the right-hand side allow STRING to contain semicolons, spaces, tabs, and newlines

CTEC1863 - 2009F 8

Examples:

$ echo NAMENAME$ NAME=Howard$ echo $NAMEHoward

$ HIS="/usr/user3"$ ls $HIS

CTEC1863 - 2009F 9

Positional Parameters

• Implicitly created when a shell script runs– These variables are known as: $0, $1, ..., $9

Try this script:echo "Example of positional parameters"echo "$5"echo $1echo $2echo $3 $4$ PARAMS These parameters are passed

Also try changing “$5” to ‘$5’

CTEC1863 - 2009F 10

shift

• Used when more than 10 ($0-$9) parameters– After the shift command is executed, the positional

parameters from $2 ... are renamed $1 ...

• E.g. if commandfile is invoked as:commandfile arg1 arg2 arg3 arg4 arg5 arg6

$0 $1 $2 $3 $4 $5 $6

• After a shift command is executed:commandfile arg1 arg2 arg3 arg4 arg5 arg6

$0 gone $1 $2 $3 $4 $5

CTEC1863 - 2009F 11

Shift exampleTry this script:

echo "Example of shift command"echo $1shiftecho $1shiftecho $1

With:$ ./shiftit shows shift command

CTEC1863 - 2009F 12

set command

• Forces values into the positional parameters• For example:

set -- abc def ghi is equivalent to:$1=abc$2=def$3=ghi

• Note:– Positional parameters are not allowed on the left hand

side of an assignment statement.

Try this script:

set -- THESE THREE VALUESecho $1 $2 $3

CTEC1863 - 2009F 13

Shell Maintained Variables

• HOME: Upon login, the shell assigns a user's login directory

• IFS: Internal field separators, default is blank, tab, newline

• MAIL: Pathname of the file where your mail is deposited

• PATH: Search path used to find commands

CTEC1863 - 2009F 14

Special Variables

$# $? $! $$ $-

$# records the number of arguments passed to a shell procedure

e.g. $ someproc x y z

sets $# to 3

– Tell if correct number of args passed in

CTEC1863 - 2009F 15

Special Variables (2)

$? contains the exit status of the last command executed– Zero indicates successful completion– Anything from 1 to 255 indicates failure

• Often only want to execute next command if last was successful

$! contains the process number of the last process run in the background (using &)

CTEC1863 - 2009F 16

Special Variables (3)

• $$ contains the process number of the currently running process.– Process numbers are unique among all existing

processes.– Often used if a shell procedure needs to create

temporary files:…temp=$HOME/temp.$$ls > $temp…rm $temp…

CTEC1863 - 2009F 17

Special Variables (4)

• $- is string containing the names of all the flags currently turned on in the shell

• Also…– $* is not considered a special variable– Contains $1, $2, ... separated by spaces

CTEC1863 - 2009F 18

Testing Variables

• Test command• Usage: test EXPRESSION

– test evaluates EXPRESSION to true or false– True exit with STATUS 0 – False exit with STATUS !0

• test is used for three categories of testing:– Files– Strings– Numbers

CTEC1863 - 2009F 19

Functions

• Sequence of commands

• Defining:

function_name () {

statements

}

• Use:

function_name

CTEC1863 - 2009F 20

Functions (2)

• Arguments:

print_args () {

echo “Arg 1 is $1”

echo “Arg 2 is $2”

}

• Passing arguments:

print_args arg1 arg2