Bash programming. CSCI 330 the unix system

BASIC SHELL PROGRAMMING

A script is a file that contains shell commands
data

BASH SHELL PROGRAMMING

Input
prompting user
command line arguments
Decision:
if-then-else
case
Repetition
do-while, repeat-until
for
select
Functions
Traps

CSCI 330 - The

USER INPUT

shell allows to prompt for user input
Syntax:
read varname [more vars]
or
read

USER INPUT EXAMPLE

#! /bin/sh
read -p "enter your name: " first last
echo

SPECIAL SHELL VARIABLES

CSCI 330 - The Unix System

EXAMPLES: COMMAND LINE ARGUMENTS

% set tim bill ann fred
$1 $2

BASH CONTROL STRUCTURES

if-then-else
case
loops
for
while
until
select

CSCI 330 - The Unix System

IF STATEMENT

if command
then
statements
fi
statements are executed only if command succeeds, i.e. has

TEST COMMAND

Syntax:
test expression
[ expression ]
evaluates ‘expression’ and returns true or

THE SIMPLE IF STATEMENT

if [ condition ]; then
statements
fi
executes the statements only

THE IF-THEN-ELSE STATEMENT

if [ condition ]; then
statements-1
else
statements-2
fi
executes statements-1 if condition

THE IF…STATEMENT

if [ condition ]; then
statements
elif [ condition ]; then

RELATIONAL OPERATORS

CSCI 330 - The Unix System

COMPOUND LOGICAL EXPRESSIONS

! not
&& and
|| or

CSCI 330 - The Unix System

and, or
must be

EXAMPLE: USING THE ! OPERATOR

#!/bin/bash
read -p "Enter years of work: "

EXAMPLE: USING THE && OPERATOR

#!/bin/bash
Bonus=500
read -p "Enter Status: " Status
read

EXAMPLE: USING THE || OPERATOR

#!/bin/bash
read -p "Enter calls handled:" CHandle
read -p

FILE TESTING

Meaning
-d file True if ‘file’ is a directory
-f file True if

EXAMPLE: FILE TESTING

#!/bin/bash
echo "Enter a filename: "
read filename
if [ ! –r

EXAMPLE: FILE TESTING

#! /bin/bash
if [ $# -lt 1 ]; then
echo

EXAMPLE: IF… STATEMENT

# The following THREE if-conditions produce the same result
*

EXAMPLE: IF..ELIF... STATEMENT

#!/bin/bash
read -p "Enter Income Amount: " Income
read -p "Enter

THE CASE STATEMENT

use the case statement for a decision that is

CASE PATTERN

checked against word for match
may also contain:
*
?
[ … ]
[:class:]
multiple patterns

EXAMPLE 1: THE CASE STATEMENT

#!/bin/bash
echo "Enter Y to see all files

EXAMPLE 2: THE CASE STATEMENT

#!/bin/bash
ChildRate=3
AdultRate=10
SeniorRate=7
read -p "Enter your age: " age
case

BASH PROGRAMMING: SO FAR

Data structure
Variables
Numeric variables
Arrays
User input
Control structures
if-then-else
case

CSCI 330 - The

BASH PROGRAMMING: STILL TO COME
Control structures
Repetition
do-while, repeat-until
for
select
Functions
Trapping signals

CSCI 330 - The

REPETITION CONSTRUCTS

CSCI 330 - The Unix System

THE WHILE LOOP

Purpose:
To execute commands in “command-list” as long as “expression”

EXAMPLE: USING THE WHILE LOOP

#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10

EXAMPLE: USING THE WHILE LOOP

#!/bin/bash
Cont="Y"
while [ $Cont = "Y" ]; do

EXAMPLE: USING THE WHILE LOOP

#!/bin/bash
# copies files from home- into

THE UNTIL LOOP

Purpose:
To execute commands in “command-list” as long as “expression”

EXAMPLE: USING THE UNTIL LOOP

#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10

EXAMPLE: USING THE UNTIL LOOP

#!/bin/bash
Stop="N"
until [ $Stop = "Y" ]; do

THE FOR LOOP

Purpose:
To execute commands as many times as the

EXAMPLE 1: THE FOR LOOP

#!/bin/bash
for i in 7 9 2 3

EXAMPLE 2: USING THE FOR LOOP

#!/bin/bash
# compute the average weekly temperature
for

LOOPING OVER ARGUMENTS

simplest form will iterate over all command line arguments:
#!

SELECT COMMAND

Constructs simple menu from word list
Allows user to enter a

SELECT EXAMPLE

#! /bin/bash
select var in alpha beta gamma
do
echo $var
done
Prints:

CSCI 330

SELECT DETAIL

PS3 is select sub-prompt
$REPLY is user input (the number)
#! /bin/bash
PS3="select

SELECT EXAMPLE

#!/bin/bash
echo "script to make files private"
echo "Select file to protect:"
select

BREAK AND CONTINUE

Interrupt for, while or until loop
The break statement

THE BREAK COMMAND

while [ condition ]
do
cmd-1
break
cmd-n
done
echo "done"

CSCI 330

THE CONTINUE COMMAND

while [ condition ]
do
cmd-1
continue
cmd-n
done
echo "done"

CSCI 330

EXAMPLE:

for index in 1 2 3 4 5 6 7 8

DONE !

BASH SHELL PROGRAMMING

Sequence
Decision:
if-then-else
case
Repetition
do-while, repeat-until
for
select
Functions
Traps

CSCI 330 - The Unix

SHELL FUNCTIONS

A shell function is similar to a shell script
stores a

SHELL FUNCTIONS

must be defined before they can be referenced
usually placed at

EXAMPLE: FUNCTION

#!/bin/bash
funky () {
# This is a simple function
echo

EXAMPLE: FUNCTION

#!/bin/bash
fun () { # A somewhat more complex function.
JUST_A_SECOND=1

FUNCTION PARAMETERS

Need not be declared
Arguments provided via function call are accessible

EXAMPLE: FUNCTION WITH PARAMETER

#! /bin/sh
testfile() {
if [ $# -gt 0

EXAMPLE: FUNCTION WITH PARAMETERS

#! /bin/bash
checkfile() {
for file
do
if [

LOCAL VARIABLES IN FUNCTIONS

Variables defined within functions are global,
i.e. their values

EXAMPLE: FUNCTION

#! /bin/bash
global="pretty good variable"
foo () {
local inside="not so good

HANDLING SIGNALS

Unix allows you to send a signal to any process
-1

SIGNALS ON LINUX

% kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT

HANDLING SIGNALS

Default action for most signals is to end process
term: signal

EXAMPLE: TRAP HANGUP

#! /bin/bash
# kill -1 won’t kill this process
# kill

EXAMPLE: TRAP MULTIPLE SIGNALS

#! /bin/sh
# plain kill or kill -9 will

EXAMPLE: REMOVING TEMP FILES

#! /bin/bash
trap 'cleanup; exit' 2
cleanup () {
/bin/rm

RESTORING DEFAULT HANDLERS

trap without a command list will remove a signal

DEBUG SHELL PROGRAMS

Debugging is troubleshooting errors that may occur during the

DEBUGGING USING “SET”

The “set” command is a shell built-in command
has options