Bash Programming

CSCI 330
THE UNIX SYSTEM
Bash Programming

BASIC SHELL PROGRAMMING
 A script is a file that contains shell commands
 data structure: variables
 control structure: sequence, decision, loop
 Shebang line for bash shell script:
#! /bin/bash
#! /bin/sh
 to run:
 make executable: % chmod +x script
 invoke via: % ./script
2
CSCI330-TheUnixSystem

BASH SHELL PROGRAMMING
 Input
 prompting user
 command line arguments
 Decision:
 if-then-else
 case
 Repetition
 do-while, repeat-until
 for
 select
 Functions
 Traps 3

USER INPUT
 shell allows to prompt for user input
Syntax:
read varname [more vars]
 or
read –p "prompt" varname [more vars]
 words entered by user are assigned to
varname and “more vars”
 last variable gets rest of input line 4

USER INPUT EXAMPLE
#! /bin/sh
read -p "enter your name: " first last
echo "First name: $first"
echo "Last name: $last"
5

SPECIAL SHELL VARIABLES
6
Parameter Meaning
$0 Name of the current shell script
$1-$9 Positional parameters 1 through 9
$# The number of positional parameters
$* All positional parameters, “$*” is one string
$@ All positional parameters, “$@” is a set of strings
$? Return status of most recently executed command
$$ Process id of current process

EXAMPLES: COMMAND LINE
ARGUMENTS
% set tim bill ann fred
$1 $2 $3 $4
% echo $*
tim bill ann fred
% echo $#
4
% echo $1
tim
% echo $3 $4
ann fred 7
The ‘set’
command can
be used to
assign values to
positional
parameters

BASH CONTROL STRUCTURES
 if-then-else
 case
 loops
 for
 while
 until
 select
8

IF STATEMENT
if command
then
statements
fi
 statements are executed only if command
succeeds, i.e. has return status “0”
9

TEST COMMAND
Syntax:
test expression
[ expression ]
 evaluates ‘expression’ and returns true or false
Example:
if test –w "$1"
then
echo "file $1 is write-able"
fi 10

THE SIMPLE IF STATEMENT
if [ condition ]; then
statements
fi
 executes the statements only if condition is
true
11

THE IF-THEN-ELSE STATEMENT
statements-1
else
statements-2
fi
 executes statements-1 if condition is true
 executes statements-2 if condition is false
12

THE IF…STATEMENT
statements
elif [ condition ]; then
statement
else
statements
fi
 The word elif stands for “else if”
 It is part of the if statement and cannot be used
by itself
13

RELATIONAL OPERATORS
Meaning Numeric String
Greater than -gt
Greater than or equal -ge
Less than -lt
Less than or equal -le
Equal -eg = or ==
Not equal -ne !=
str1 is less than str2 str1 < str2
str1 is greater str2 str1 > str2
String length is greater than zero -n str
String length is zero -z str
14

COMPOUND LOGICAL EXPRESSIONS
! not
&& and
|| or
15
and, or
must be enclosed within
[[ ]]

EXAMPLE: USING THE ! OPERATOR
#!/bin/bash
read -p "Enter years of work: " Years
if [ ! "$Years" -lt 20 ]; then
echo "You can retire now."
else
echo "You need 20+ years to retire"
fi
16

EXAMPLE: USING THE &&
OPERATOR
#!/bin/bash
Bonus=500
read -p "Enter Status: " Status
read -p "Enter Shift: " Shift
if [[ "$Status" = "H" && "$Shift" = 3 ]]
then
echo "shift $Shift gets $$Bonus bonus"
else
echo "only hourly workers in"
echo "shift 3 get a bonus"
fi 17

EXAMPLE: USING THE ||
OPERATOR
#!/bin/bash
read -p "Enter calls handled:" CHandle
read -p "Enter calls closed: " CClose
if [[ "$CHandle" -gt 150 || "$CClose" -gt 50 ]]
then
echo "You are entitled to a bonus"
else
echo "You get a bonus if the calls"
echo "handled exceeds 150 or"
echo "calls closed exceeds 50"
fi 18

FILE TESTING
Meaning
-d file True if ‘file’ is a directory
-f file True if ‘file’ is an ord. file
-r file True if ‘file’ is readable
-w file True if ‘file’ is writable
-x file True if ‘file’ is executable
-s file True if length of ‘file’ is nonzero
19

EXAMPLE: FILE TESTING
#!/bin/bash
echo "Enter a filename: "
read filename
if [ ! –r "$filename" ]
then
echo "File is not read-able"
exit 1
fi
20

EXAMPLE: FILE TESTING
#! /bin/bash
if [ $# -lt 1 ]; then
echo "Usage: filetest filename"
exit 1
fi
if [[ ! -f "$1" || ! -r "$1" || ! -w "$1" ]]
then
echo "File $1 is not accessible"
exit 1
fi
21

EXAMPLE: IF… STATEMENT
# The following THREE if-conditions produce the same result
* DOUBLE SQUARE BRACKETS
read -p "Do you want to continue?" reply
if [[ $reply = "y" ]]; then
echo "You entered " $reply
fi
* SINGLE SQUARE BRACKETS
if [ $reply = "y" ]; then
fi
* "TEST" COMMAND
if test $reply = "y"; then
fi
22

EXAMPLE: IF..ELIF... STATEMENT
#!/bin/bash
read -p "Enter Income Amount: " Income
read -p "Enter Expenses Amount: " Expense
let Net=$Income-$Expense
if [ "$Net" -eq "0" ]; then
echo "Income and Expenses are equal -
breakeven."
elif [ "$Net" -gt "0" ]; then
echo "Profit of: " $Net
else
echo "Loss of: " $Net
fi
23

THE CASE STATEMENT
 use the case statement for a decision that is
based on multiple choices
Syntax:
case word in
pattern1) command-list1
;;
pattern2) command-list2
;;
patternN) command-listN
;;
esac
24

CASE PATTERN
 checked against word for match
 may also contain:
*
?
[ … ]
[:class:]
 multiple patterns can be listed via:
|
25

EXAMPLE 1: THE CASE STATEMENT
#!/bin/bash
echo "Enter Y to see all files including hidden files"
echo "Enter N to see all non-hidden files"
echo "Enter q to quit"
read -p "Enter your choice: " reply
case $reply in
Y|YES) echo "Displaying all (really…) files"
ls -a ;;
N|NO) echo "Display all non-hidden files..."
ls ;;
Q) exit 0 ;;
*) echo "Invalid choice!"; exit 1 ;;
esac
26

EXAMPLE 2: THE CASE STATEMENT
#!/bin/bash
ChildRate=3
AdultRate=10
SeniorRate=7
read -p "Enter your age: " age
case $age in
[1-9]|[1][0-2]) # child, if age 12 and younger
echo "your rate is" '$'"$ChildRate.00" ;;
# adult, if age is between 13 and 59 inclusive
[1][3-9]|[2-5][0-9])
echo "your rate is" '$'"$AdultRate.00" ;;
[6-9][0-9]) # senior, if age is 60+
echo "your rate is" '$'"$SeniorRate.00" ;;
esac 27

BASH PROGRAMMING: SO FAR
 Data structure
 Variables
 Numeric variables
 Arrays
 User input
 Control structures
 if-then-else
 case
28

BASH PROGRAMMING: STILL TO
COME
 Control structures
 Repetition
 do-while, repeat-until
 for
 select
 Functions
 Trapping signals
29

REPETITION CONSTRUCTS
30

THE WHILE LOOP
 Purpose:
To execute commands in “command-list” as long
as “expression” evaluates to true
Syntax:
while [ expression ]
do
command-list
done
31

EXAMPLE: USING THE WHILE LOOP
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]
do
echo The counter is $COUNTER
let COUNTER=$COUNTER+1
done
32

#!/bin/bash
Cont="Y"
while [ $Cont = "Y" ]; do
ps -A
read -p "want to continue? (Y/N)" reply
Cont=`echo $reply | tr [:lower:] [:upper:]`
done
echo "done"
33

#!/bin/bash
# copies files from home- into the webserver- directory
# A new directory is created every hour
PICSDIR=/home/carol/pics
WEBDIR=/var/www/carol/webcam
while true; do
DATE=`date +%Y%m%d`
HOUR=`date +%H`
mkdir $WEBDIR/"$DATE"
while [ $HOUR -ne "00" ]; do
DESTDIR=$WEBDIR/"$DATE"/"$HOUR"
mkdir "$DESTDIR"
mv $PICSDIR/*.jpg "$DESTDIR"/
sleep 3600
HOUR=`date +%H`
done
done
34

THE UNTIL LOOP
 Purpose:
To execute commands in “command-list” as long
as “expression” evaluates to false
Syntax:
until [ expression ]
do
command-list
done
35

EXAMPLE: USING THE UNTIL LOOP
#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]
do
echo $COUNTER
let COUNTER-=1
done
36

EXAMPLE: USING THE UNTIL LOOP
#!/bin/bash
Stop="N"
until [ $Stop = "Y" ]; do
ps -A
read -p "want to stop? (Y/N)" reply
Stop=`echo $reply | tr [:lower:] [:upper:]`
done
echo "done"
37

THE FOR LOOP
 Purpose:
To execute commands as many times as the
number of words in the “argument-list”
Syntax:
for variable in argument-list
do
commands
done
38

EXAMPLE 1: THE FOR LOOP
#!/bin/bash
for i in 7 9 2 3 4 5
do
echo $i
done
39

EXAMPLE 2: USING THE FOR LOOP
#!/bin/bash
# compute the average weekly temperature
for num in 1 2 3 4 5 6 7
do
read -p "Enter temp for day $num: " Temp
let TempTotal=$TempTotal+$Temp
done
let AvgTemp=$TempTotal/7
echo "Average temperature: " $AvgTemp 40

LOOPING OVER ARGUMENTS
 simplest form will iterate over all command line
arguments:
#! /bin/bash
for parm
do
echo $parm
done
41

SELECT COMMAND
 Constructs simple menu from word list
 Allows user to enter a number instead of a word
 User enters sequence number corresponding to
the word
Syntax:
select WORD in LIST
do
RESPECTIVE-COMMANDS
done
 Loops until end of input, i.e. ^d (or ^c)
42

SELECT EXAMPLE
#! /bin/bash
select var in alpha beta gamma
do
echo $var
done
 Prints:
43
1) alpha
2) beta
3) gamma
#? 2
beta
#? 4
#? 1
alpha

SELECT DETAIL
 PS3 is select sub-prompt
 $REPLY is user input (the number)
#! /bin/bash
PS3="select entry or ^D: "
select var in alpha beta
do
echo "$REPLY = $var"
done
44
Output:
select ...
1) alpha
2) beta
? 2
2 = beta
? 1
1 = alpha

SELECT EXAMPLE
#!/bin/bash
echo "script to make files private"
echo "Select file to protect:"
select FILENAME in *
do
echo "You picked $FILENAME ($REPLY)"
chmod go-rwx "$FILENAME"
echo "it is now private"
done 45

BREAK AND CONTINUE
 Interrupt for, while or until loop
 The break statement
 transfer control to the statement AFTER the done
statement
 terminate execution of the loop
 The continue statement
 transfer control to the statement TO the done
statement
 skip the test statements for the current iteration
 continues execution of the loop
46

THE BREAK COMMAND
while [ condition ]
do
cmd-1
break
cmd-n
done
echo "done"
47
This iteration is over
and there are no
more iterations

THE CONTINUE COMMAND
while [ condition ]
do
cmd-1
continue
cmd-n
done
echo "done"
48
This iteration is
over; do the next
iteration

EXAMPLE:
for index in 1 2 3 4 5 6 7 8 9 10
do
if [ $index –le 3 ]; then
echo "continue"
continue
fi
echo $index
if [ $index –ge 8 ]; then
echo "break"
break
fi
done 49

DONE !
BASH SHELL PROGRAMMING
 Sequence
 Decision:
 if-then-else
 case
 Repetition
 for
 select
 Functions
 Traps
50
still to come

SHELL FUNCTIONS
 A shell function is similar to a shell script
 stores a series of commands for execution later
 shell stores functions in memory
 shell executes a shell function in the same shell that
called it
 Where to define
 In .profile
 In your script
 Or on the command line
 Remove a function
 Use unset built-in
51

SHELL FUNCTIONS
 must be defined before they can be referenced
 usually placed at the beginning of the script
Syntax:
function-name () {
statements
}
52

EXAMPLE: FUNCTION
#!/bin/bash
funky () {
# This is a simple function
echo "This is a funky function."
echo "Now exiting funky function."
}
# declaration must precede call:
funky 53

EXAMPLE: FUNCTION
#!/bin/bash
fun () { # A somewhat more complex function.
JUST_A_SECOND=1
let i=0
REPEATS=30
echo "And now the fun really begins."
while [ $i -lt $REPEATS ]
do
echo "-------FUNCTIONS are fun-------->"
sleep $JUST_A_SECOND
let i+=1
done
}
fun 54

FUNCTION PARAMETERS
 Need not be declared
 Arguments provided via function call are
accessible inside function as $1, $2, $3, …
$# reflects number of parameters
$0 still contains name of script
(not name of function)
55

EXAMPLE: FUNCTION WITH
PARAMETER
#! /bin/sh
testfile() {
if [ $# -gt 0 ]; then
if [[ -f $1 && -r $1 ]]; then
echo $1 is a readable file
else
echo $1 is not a readable file
fi
fi
}
testfile .
testfile funtest 56

EXAMPLE: FUNCTION WITH
PARAMETERS
#! /bin/bash
checkfile() {
for file
do
if [ -f "$file" ]; then
echo "$file is a file"
else
if [ -d "$file" ]; then
echo "$file is a directory"
fi
fi
done
}
checkfile . funtest
57

LOCAL VARIABLES IN FUNCTIONS
 Variables defined within functions are global,
i.e. their values are known throughout the entire
shell program
 keyword “local” inside a function definition
makes referenced variables “local” to that
function
58

EXAMPLE: FUNCTION
#! /bin/bash
global="pretty good variable"
foo () {
local inside="not so good variable"
echo $global
echo $inside
global="better variable"
}
echo $global
foo
echo $global
echo $inside
59

HANDLING SIGNALS
 Unix allows you to send a signal to any process
 -1 = hangup kill -HUP 1234
 -2 = interrupt with ^C kill -2 1235
 no argument = terminate kill 1235
 -9 = kill kill -9 1236
 -9 cannot be blocked
 list your processes with
ps -u userid
60

SIGNALS ON LINUX
% kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE
9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2
13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT
17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU
25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH
29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN
35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4
47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14
51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10
63) SIGRTMAX-1 64) SIGRTMAX
 ^C is 2 - SIGINT 61

HANDLING SIGNALS
 Default action for most signals is to end process
 term: signal handler
 Bash allows to install custom signal handler
Syntax:
trap 'handler commands' signals
Example:
trap 'echo do not hangup' 1 2
62

EXAMPLE: TRAP HANGUP
#! /bin/bash
# kill -1 won’t kill this process
# kill -2 will
trap 'echo dont hang up' 1
while true
do
echo "try to hang up"
sleep 1
done
63

EXAMPLE: TRAP MULTIPLE
SIGNALS
#! /bin/sh
# plain kill or kill -9 will kill this
trap 'echo 1' 1
trap 'echo 2' 2
while true; do
echo -n .
sleep 1
done
64

EXAMPLE: REMOVING TEMP FILES
#! /bin/bash
trap 'cleanup; exit' 2
cleanup () {
/bin/rm -f /tmp/tempfile.$$.?
}
for i in 1 2 3 4 5 6 7 8
do
echo "$i.iteration"
touch /tmp/tempfile.$$.$i
sleep 1
done
cleanup
65

RESTORING DEFAULT HANDLERS
 trap without a command list will remove a signal
handler
 Use this to run a signal handler once only
#! /bin/sh
trap 'justonce' 2
justonce() {
echo "not yet"
trap 2 # now reset it
}
while true; do
echo -n "."
sleep 1
done
66

DEBUG SHELL PROGRAMS
 Debugging is troubleshooting errors that may
occur during the execution of a program/script
 The following two commands can help you debug
a bash shell script:
 echo
use explicit output statements to trace execution
 set
67

DEBUGGING USING “SET”
 The “set” command is a shell built-in command
 has options to allow flow of execution
–v option prints each line as it is read
–x option displays the command and its arguments
–n checks for syntax errors
 options can turned on or off
 To turn on the option: set -xv
 To turn off the options: set +xv
 Options can also be set via she-bang line
#! /bin/bash -xv 68

DONE !
SUMMARY: BASH SHELL
PROGRAMMING
 Sequence
 Decision:
 if-then-else
 case
 Repetition
 for
 select
 Functions
 Traps
69

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