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03-60-256-01 System Programming Shell Programming

03-60-256-01 System Programming Shell Programming. Quazi Rahman [Modified from slides by Dr. B. Boufama ] School of Computer Science University of Windsor Winter, 2013. Outline. Shell Programs: Scripts Shell and Environment Variables Quoting BASH as a Programming Language Exercises.

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03-60-256-01 System Programming Shell Programming

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  1. 03-60-256-01System ProgrammingShell Programming QuaziRahman [Modified from slides by Dr. B. Boufama] School of Computer Science University of Windsor Winter, 2013

  2. Outline • Shell Programs: Scripts • Shell and Environment Variables • Quoting • BASH as a Programming Language • Exercises

  3. Shell programs: Scripts • Shells are more than just command interpreters • They have their own programming languages. • Shell programming (shell scripts) are excellent for concise file system operations and scripting the combination of existing functionality in filters and command line tools via pipes. • A shell script, (shell program),is a file that contains various shell commands. • A shell language can also: • Define, read and write variables (shell variables) • Implement control structures such as for and while loops and, if and switch • Define and use functions Shell Scripts

  4. Shell programs: Scripts export EDITOR=emacs if [[ $REMOTEHOST ]]; then if [[ $REMOTEHOST == "csgate.uwindsor.ca“ ]]; then export DISPLAY="U96.lamf.uwindsor.ca:0.0" else export DISPLAY=$REMOTEHOST":0.0" fi else export DISPLAY=$HOST":0.0" fi Shell Scripts: Example

  5. Shell programs: Scripts • Make the file executable: • chmod +x script_file • The name script_file becomes like a command. To execute: • bravo:~/ script_file • Different shells have different syntax. • A Bash shell script won’t be run by C-shell or Korn shell. • The shell to be used for a script is chosen as follow: • If the first line of the script consists of the only character #, then the script is interpreted by the shell from which it has been called. • If the first line of the script is of the for #!fullPathName, then the program fullPathName is used. • Otherwise, the Bourne shell is used. How to Run a Script?

  6. Shell programs: Scripts • #!/bin/csh#This is a sample C-shell script echo -n the date of today is’ ’# -n omits new line date • #!/bin/ksh #This is a sample K-shell script echo “the date of today is \c” # \c omits new line Date • #!/bin/bash #This is a sample BASH script echo -n “the date of today is ”# -n omits new line date Example

  7. Shell programs: Scripts #!/bin/bash # This is a simple bash shell script to print the date. # If you put a command in back ticks, you can save it to a variable #+ to use later, or you can put it directly into another command. todays_date=`date` # Use a dollar sign ($), to access the value of variables. echo "The date of today is $todays_date" echo "The date of today is still `date`" # If you need to omit the newline on the echo, use the -n option #+ and run the date command on the following line. echo -n "And once again, the date is " date Tips on BASH Scripts

  8. Shell programs: Scripts $0- Name of the program that is running $1...$9 - Values of command line arguments 1 through 9 $* - Values of all command line arguments $# - Total number of command line arguments $$ - Process ID of current process $? - Exit status of most recent command $! - PID of most recent background process Some read-only shell/environment variables

  9. Quoting • The shell’s wildcard/variable/command substitution mechanism can be inhibited using quotes: • Single quotes (‘ ’) inhibit wildcard/variable/command substitution. • Double quotes (“ ”) inhibit wildcard replacement only. • Back quotes (` `) used for command execution • When quotes are nested, only the outer quotes matter. • Example echo 3 * 5 = 15 won’t work because * is a wildcard (although echo3*5=15 might work! Why?) echo ’3 * 5 = 15’  3 * 5 = 15 echo ’I am $USER’  I am $USER echo “I am $USER”  I am rahmanq echo “Today is `date`”  Today is Sat Jan 12 16:09:00 EST 2013 Quoting

  10. BASH as a Programming Language • Built-in programming languages • In addition to the basic facilities, shells have built-in programming languages that support for: • conditions, • loops, • input/output • basic arithmetic • It supports user defined variables and commands (like functions in C) BASH as a Programming Language

  11. BASH as a Programming Language Example 1: #!/bin/bash echo -n "Enter a value> " read a echo -n "Enter another value> " read b echo "Doing arithmetic> " # When assigning variables, no space on either side of the #+ equal sign. To do arithmetic in bash, surround the #+ expression with $(( and )). sum=$(( $a + $b )) echo "The sum a + b is $sum" BASH as a Programming Language

  12. BASH as a Programming Language Example 1 (cont’d): difference=$(( $a - $b )) echo "The difference a - b is $difference" product=$(($a * $b)) echo "The product a * b is $product" if [[ $b -ne 0 ]]; then quotient=$(($a / $b)) echo "The division a / b is $quotient" else echo "The division a/b is impossible" fi BASH as a Programming Language

  13. BASH as a Programming Language Example 1 (enhanced): #!/bin/bash if [ $#!= 2 ]; # or, if ( test $# != 2 ) then echo “Usage: $0 integer1 integer2” else echo “Doing arithmetic> “ r=$(($1 + $2)) ; echo “the sum "$1" + "$2“ is $r” r=$(($1 - $2)) ; echo "the subtraction "$1" - "$2" is $r“ r=$(($1 * $2)) ; echo "the product $1 * $2 is $r“ if [ $2 -ne 0 ] ; then r=$(($1 / $2)) ; echo "the division $1 / $2 is $r" else echo "the division $1 / $2 is impossible" fi fi BASH as a Programming Language

  14. BASH as a Programming Language • Accessing a simple variable: • $VAR # access the value of variable VAR • Example: dir=“/export/home/” echo my home is ${dir}rahmanq/ my home is /export/home/rahmanq/ • List variables: name=(arg1 arg2 ...) names=( Windsor Toronto Ottawa ) echo ${names[0]}  Windsor echo ${names[@]:1:2}  Toronto Ottawa echo ${names[*]}  Windsor Toronto Ottawa echo ${#names[@]}  3 #number of elements names=(${names[@]} London) #add a new element names[1]=Quebec #change element 1 (2nd element) echo $names[@]  Windsor Quebec Ottawa London Accessing variables

  15. BASH as a Programming Language • String expressions: • in addition to == and != we have • =~ like == but right side may contain wildcards • !~like != but right side may contain wildcards • Arithmetic expressions: • Similar to the C arithmetic operators, however, only integers are supported. String and Arithmetic expressions

  16. BASH as a Programming Language • Example: (Use of “let”) #!/bin/bash if [[ $1 > 0 && $(($2 % 10)) != 0 ]]; then echo Operands are valid let “a = $2 % 10” let “r = $(($1 * $2)) / $a” echo "expression value is $r" else echo "Operand problem" fi String and Arithmetic expressions

  17. BASH as a Programming Language #!/bin/bash if [ $# -ne 2 ] # Argument check then echo "Usage: $0 first-number second-number" exit 1 fi gcd () { dividend=$1; divisor=$2; remainder=1 until [ "$remainder" -eq 0 ] do let "remainder = $dividend % $divisor" dividend=$divisor; divisor=$remainder done } gcd $1 $2 echo; echo "GCD of $1 and $2 = $dividend"; echo Example: gcd

  18. BASH as a Programming Language • bravo:~/ test expression, or [ expression ] • The value is 1 if the selected option is true and 0 otherwise. • Available options: • For expression as: fileName1 –option fileName2 • nt – fileName1 newer than fileName2 • ot – fileName1 older than fileName2 • For expression as: –option fileName r – Shell has the permission to read from the fileName w – Shell has the permission to write into the fileName x – Shell has the permission to execute the fileName e – fileName exists o – fileName is owned by shell’s user z – file exists but is of size 0 f – fileName is a regular file not a directory d – fileName is a directory Expressions for File `test`

  19. BASH as a Programming Language #!/bin/bash echo -n "Enter file name> " read file # Use elif in bash for the “else if” construct. # The “>>” in the example is output redirection with appending. # The output of the ls command will be appended to the file. if [ -w "$file" ]; then ls >> $file echo "More input has been appended" elif [ -e "$file" ]; then echo "You have no write permission on $file" else echo "$file does not exist" fi Example

  20. BASH as a Programming Language • Bash supports several control structures, in particular: • if statement: 1. if [ expr]; then #–> “if” {code} fi 2. if [ expr] #–> “if-else” then {code} else {code} fi 3. if [ expr1 ]; then #–> “if-else-if” {code} elif [ expr2 ] then {code} else {code} fi • if [ expr] • then {code} • fi Control structures if [expr]; then {code}; fi if [expr]; then {code}; else {code}; fi Try This one your selves.

  21. BASH as a Programming Language #!/bin/bash echo -n "Enter file name> " read file if [ ! -e $file ]; then echo "Sorry, $file does not exist." elif [ ! -w $file ]; then echo "You have no write permission on $file“ if [ -o $file ]; then chmodu+w $file #(grant write permission) echo "Write permission granted" else echo "Write permission cannot be granted" echo "because you don't own this file" fi else ls >> $file echo "More input has been appended" fi Example: append to a file

  22. BASH as a Programming Language • while loops have the style of [tests statements]do and they end with done #!/bin/bash secretCode=zoom99 echo -n "Guess the code> " read yourGuess while [ $secretCode != $yourGuess ]; do echo "Good guess but wrong, try again" echo -n "Enter your guess> " read yourGuess done echo "BINGO!" exit 0 • An infinite loop with a while can be done by using a colon (:) as the test condition. while : ; do {code} done While-do

  23. BASH as a Programming Language • Case statements have the following structure. case $choice in [cC]) exec /bin/csh ;; [bB]) exec /bin/bash ;; [kK]) exec /bin/ksh ;; *) echo "Wrong choice, try again“ read choice esac Case (Switch) Statement

  24. BASH as a Programming Language • There are a few ways to write for-loop for VAR in {VAR value list} ; do { code } done for (( i=0; i<5; i++ )) do { code } done for people in $1 $2 $3 $4; do # using command line arguments echo $people done for people in $*; do # using all command line arguments echo $people done For Loop

  25. BASH as a Programming Language • Syntax: until-do • until [ i -eq 10 ]do   {code}   let i++done declare -a array #declare an array without initialization for name in $*; do array=("${array[@]}" $name) done echo ${array[@]} #print all the array elements i=0 until [ $i -eq $# ]; do echo -n ${array[$i]} #print one array element echo let i++ done Repeat Until

  26. BASH as a Programming Language • Use the trap command to handle signals in bash. • Unlike csh, you can trap any signal (except for SIGKILL of course) #!/bin/bash trap ' { echo "CTRL-C by user, Finishing, bye bye :)" exit 1 }' INT while : ;do echo "Infinite loop!!!!" sleep 5 done • To ignore any interrupt, use trap ‘ ’ INT • Notice that csh uses – with no argument to ignore a signal. • Since SIGINT won't work (CTRL-C), you can use (CTRL-Z) to pause the job. • ``ps'' to find the running processes and ``kill -9 <pid>'' to kill the process. #!/bin/bash trap '' INT while : ;do echo "Infinite loop!!!!" sleep 5 done Trap Command

  27. BASH as a Programming Language #!/bin/bash trap '' INT clear stty –echo #turn off the echo mechanism echo -n "Enter your passwd here> " read secretPass echo " " echo -n "Confirm your passwd here> " read confirmPass echo " " if [ "$secretPass" != "$confirmPass" ]; then echo "Work on your short-term memory first" exit 1 fi yourGuess="" while [ "$yourGuess" != "$secretPass" ]; do clear echo -n "Enter password to unlock screen> " read yourGuess echo " " done clear echo "You're back in the system!" • stty echo #turn on the echo mechanism exit 0 Example

  28. Summary • When you log onto a Unix machine, the operating system runs a program, called shell. • A shell provides a prompt and waits for you to type in commands. • A shell command can be a built-in (internal) or an external command. • To execute an external command, the shell searches for its binary file in several directories, the search path. • The search path is stored in a shell variable called PATH • There are several shells, the most common ones are the Bourne Again shell, the Korn shell and the C shell. • A shell has the built-in capability for numeric, string and file expressions. • A shell has most control structures found in a programming language • A shell has means to define and use simple variables and list variables(arrays). • The shell makes it easy for a programmer to write useful programs. • You can debug a Bash shell program by using: bash –xv script_file Summary

  29. Exercises • Write a bash-shell script to separately list directories and files. • Write a bash-shell script to list all regular files in the current directory that are smaller than n bytes (n is an integer argument). • Write a bash-shell script to replace all occurrences of a word by another word in a file. • Synopsis: replaceWord < inFile > < word > < repWord > < outFile > • Write a bash-shell script to calculate the Fibonacci numbers • given by: 0, if n = 0 Fn = 1, if n = 1 Fn-1 + Fn-2, if n > 1 • Synopsis: fibonacci < integer > Exercises

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