1. Shell Scripting September 27, 2004 Class Meeting 6, Part II * Notes adapted by Lenwood Heath from previous work by other members of the CS faculty at Virginia Tech

2. Lenwood Heath, Virginia Tech, Fall, 20042 Shell Script (Program) What is a shell script? Shell commands in a text file that is invoked as its own command Commands include... anything you can type on the command line shell variables control statements ( if, while, for,... )

3. Lenwood Heath, Virginia Tech, Fall, 20043 Script Execution Provide script as an argument to a shell command — bash my_script Or specify shell on the first line of the script — #!/bin/bash Make sure that the script is executable Run my_script directly from the command line No compilation; interpreted by shell

4. Lenwood Heath, Virginia Tech, Fall, 20044 Simple Script #!/bin/bash echo "Hello, World! " system=$(uname -a) echo $system Result: Hello, World! Linux cashew.cslab 2.6.3-14mdk #1 Fri Aug 13 11:14:25 EDT 2004 i686 unknown unknown GNU/Linux

5. Lenwood Heath, Virginia Tech, Fall, 20045 Shell Variables Numeric Strings Arrays var refers to the name of the variable, $var refers to the value var=100 # sets the value to 100 (( var2=1+$var )) Variable names begin with a letter and can include letters, digits, and underscores

6. Lenwood Heath, Virginia Tech, Fall, 20046 Numeric Variables Integer variables are the only pure numeric variables that can be used in bash Declaration and setting value declare –i var=100 Numeric expressions are enclosed in double parentheses (( var+=1 )) Operators are the same as in C/C++ +, -, *, /, %, &, |,, =, ==, !=, &&, ||

7. Lenwood Heath, Virginia Tech, Fall, 20047 String Variables Unless explicitly declared as another type, variables are strings var=100 makes the var the string "100" However, placing the variable within double parentheses will treat it as an integer (( var2=1+$var ))

8. Lenwood Heath, Virginia Tech, Fall, 20048 String Variables (continued) Using substrings ${string:n} ${string:5} # first five chars ${string:-2} # last two chars ${string:n:m} ${string:0:4} # first to fifth ${string:1:3} # second to fourth ${#string} # length of string Concatenating strings string_var1="$string_var1 $string_var2"

9. Lenwood Heath, Virginia Tech, Fall, 20049 Array Variables Array is a list of values — do not have to declare size Reference a value by ${a[index]} ${a[3]} # value in fourth position $a # same as ${a[0]} Use the declare –a command to declare an array declare –a sports sports=(basketball football soccer) sports[3]=hockey

10. Lenwood Heath, Virginia Tech, Fall, 200410 Array Variables (cont) Array initialization sports=(football basketball) moresports=($sports tennis) ${array[@]} or ${array[*]} refers to the entire contents of the array echo ${moresports[*]} Output: football basketball tennis

11. Lenwood Heath, Virginia Tech, Fall, 200411 Command Line Arguments If arguments are passed to a script, they have the values $1, $2, $3, etc. $0 is the name of the script $* is a string of all of the arguments separated by spaces $@ is an array of all of the arguments $# is the number of arguments

12. Lenwood Heath, Virginia Tech, Fall, 200412 Output and Quoting echo a message # print to stdout echo –n "yes/no? " # a prompt Shell interprets $ and `` within double quotes $ — variable substitution ` — command substitution echo "`date +%D`" # 09/27/04 Shell does not interpret special characters within single quotes echo '`date +%D`' # `date +%D`

13. Lenwood Heath, Virginia Tech, Fall, 200413 Redirecting Output to File Redirecting output to a file is the same as on the command line Examples echo "$var" > $OUTFILE # overwrite echo "$var" >> $OUTFILE # append String expansion echo $'\n\n\n' # 3 blank lines echo $'\t' indent # indented text

14. Lenwood Heath, Virginia Tech, Fall, 200414 Control Testing conditions if — Conditional execution for — Loop over the elements of an array while — Loop while a condition is true case — Multiway conditional on variable value Define and invoke functions

15. Lenwood Heath, Virginia Tech, Fall, 200415 Conditions If using integers: (( condition )) If using strings: [[ condition ]] Examples (( a == 10 )) (( b >= 3 )) [[ $1 = -n ]] [[ ($v != fun) && ($v != games) ]] Special conditions for file existence, file permissions, ownership, and file type

16. Lenwood Heath, Virginia Tech, Fall, 200416 Conditions (cont) [[ -e $file ]] – File exists? [[ -f $file ]] – Regular file? [[ -d $file ]] – Directory? [[ -L $file ]] – Symbolic link? [[ -r $file ]] – Read permission? [[ -w $file ]] – Write permission? [[ -x $file ]] – Execute permission?

17. Lenwood Heath, Virginia Tech, Fall, 200417 If Statement Syntax if condition1 then statements elif condition2 then statements else statements fi optional

18. Lenwood Heath, Virginia Tech, Fall, 200418 If Statement — Example file=readings.php if [[ -r $file ]] then echo " $file is readable " elif [[ (-w $file) && (-x $file) ]] then echo "$file is writable and executable " fi Result: readings.php is readable

19. Lenwood Heath, Virginia Tech, Fall, 200419 For Loops Syntax for var [in list] do statements done If list is omitted, $@ is assumed Otherwise ${list[*]}, where list is an array variable

20. Lenwood Heath, Virginia Tech, Fall, 200420 For Loops — Example colors=(yellow red blue green orange) for color in ${colors[*]} do echo $color done Result: yellow red blue green orange

21. Lenwood Heath, Virginia Tech, Fall, 200421 While Loops Syntax while condition do statements done The keywords break, continue, and return have the same meaning as in C/C++

22. Lenwood Heath, Virginia Tech, Fall, 200422 While Loops — Example #!/bin/bash # A shell script that only prints # its arguments while [[ $1 ]] do echo $1 shift done

23. Lenwood Heath, Virginia Tech, Fall, 200423 Case Statements Syntax case expression in pattern1) statements ;; pattern2) statements ;;... *) statements ;; esac

24. Lenwood Heath, Virginia Tech, Fall, 200424 Case Statements — Example case $1 in -a) # statements related to option a ;; -b) # statements related to option b ;; *) # all other options ;; esac

25. Lenwood Heath, Virginia Tech, Fall, 200425 Functions Definition function function_name { statements } Function arguments are $1, $2, $3, etc.

26. Lenwood Heath, Virginia Tech, Fall, 200426 Functions (continued) Example definition function add_two{ (( $sum=$1+$2 )) return $sum } Invoking the function add_two 1 3 echo $? $? value returned by last function call or command Function definition must occur before the function is called in the script