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CSCI 330 THE UNIX SYSTEM Bash Programming

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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

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

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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

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

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SPECIAL SHELL VARIABLES

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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

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

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

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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

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

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THE IF-THEN-ELSE STATEMENT if [ condition ]; then statements-1 else statements-2 fi executes statements-1 if condition is true executes statements-2 if condition is false

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THE IF…STATEMENT if [ condition ]; then 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

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RELATIONAL OPERATORS

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COMPOUND LOGICAL EXPRESSIONS ! not && and || or

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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

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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

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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

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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

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

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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

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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 read -p "Do you want to continue?" reply if [ $reply = "y" ]; then echo "You entered " $reply fi * "TEST" COMMAND read -p "Do you want to continue?" reply if test $reply = "y"; then echo "You entered " $reply fi

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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

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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

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

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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

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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

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

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

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REPETITION CONSTRUCTS

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

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

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EXAMPLE: USING THE WHILE LOOP #!/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"

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EXAMPLE: USING THE WHILE LOOP #!/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

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

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

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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"

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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

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

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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

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

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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)

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

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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

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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

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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

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

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

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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

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

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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

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

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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

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$EXAMPLE: FUNCTION #!/bin/bash fun () { # A somewhat more complex function. JUST_A_SECOND=1 let i=0 REPEATS=30 echo "And now the fun really...$

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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

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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)

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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

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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

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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

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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

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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

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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 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1 64) SIGRTMAX ^C is 2 - SIGINT

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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

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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

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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

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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

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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

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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

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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