Shell script/program Basic shell scripting - …

1 Basic Shell scriptingCS 2204 Class meeting 5 Created by Doug Bowman, 2001 Modified by Mir Farooq Ali, 2002(C) Doug Bowman, Virginia Tech, 20012 Shell script/program A series of Shell commands placed in an ASCII text file Commands include Anything you can type on the command line Shell variables Control statements (if, while, for)(C) Doug Bowman, Virginia Tech, 20013 Resources Review UIAN chapter 4 Online Example scripts with comments advanced bash - scripting guide bash Reference Manual (C) Doug Bowman, Virginia Tech, 20014 Script execution Provide script as an argument to the Shell program ( bash my_script) Or specify which Shell to use within the script First line of script is #!

2 /bin/ bash Make the script executable using chmod Run directly from the command line No compilation is necessary!(C) Doug Bowman, Virginia Tech, 20015 Simple example script#!/bin/bashecho Hello world! cd ~pwdOutput:Hello world!/home/gradstudents/m/miali(C) Doug Bowman, Virginia Tech, 20016 Quoting Quoting is necessary to use special characters in a variable s value or string - Shell only interprets $and $ - variable substitution ` - Command substitution - End of double quote - Shell doesn t interpret any special characters \ is used to escape characters echo \$t = $t will print $t = 100 (C) Doug Bowman, Virginia Tech, 20017 Shell variables varrefers to the name, $varto the value Remove a variable with unset var Names begin with alpha characters and include alpha, numeric, or underscore Numeric Strings Arrays Command line arguments Functions Read only(C)

3 Doug Bowman, Virginia Tech, 20018 Numeric variables Integer variables are the only pure numeric variables that can be used in bash Declaration and setting value: declare -i var=100 Expressions in the style of C: (( expression )) (( var+=1 )) +, -, *, /, %, &, |, ~, <, >, <=, >=, ==, !=, &&, ||(C) Doug Bowman, Virginia Tech, 20019 String variables If you do not use the declare keyword with option i when using a variable for the first time, it will be a string var=100makes varthe string 100 .(C) Doug Bowman, Virginia Tech, 200110 Array variables Array is a list of values Reference a value by ${name[index]} ${a[3]} $a(same as ${a[0]}) Use the declare -acommand to declare an array declare a sports(C) Doug Bowman, Virginia Tech, 200111 Arrays Array initialization sports=(football basketball) moresports=(${sports[*]} tennis) ${array[@]} or${array[*]} refers to the entire array contents echo ${moresports[*]} producesfootball basketball tennis(C) Doug Bowman, Virginia Tech, 200112 Command line arguments If arguments are passed to a script, they can be referenced by $1, $2, $3.

4 $0refers to the name of the script $@- array filled with arguments excluding $0 $#- number of arguments (C) Doug Bowman, Virginia Tech, 200113 Exporting variables The exportcommand, when used with a variable name, allows child processes of the Shell to access the variable(C) Doug Bowman, Virginia Tech, 200114 Output We have already seen echo More common in other shells including ksh is print (does not exist in bash ) echo ndoes not print newline after output(C) Doug Bowman, Virginia Tech, 200115 Return values Scripts can return an integer value Use return N The variable $?will contain the return value of the last command run Can be used to test conditions(C) Doug Bowman, Virginia Tech, 200116 Conditions If using integers: (( condition )) If using strings: [[ condition ]] Examples: (( a == 10 )) (( b >= 3 )) [[ $1 = -n ]] [[ ($v !)]]

5 = fun) && ($v != games) ]] Special conditions for file existence, file permissions, ownership, file type, etc.(C) Doug Bowman, Virginia Tech, 200117 Conditions ( ) [[ -e $file]] File exists? [[ -f $file]] Regular file? [[ -d $file]] Directory? [[ -L $file]] Symbolic link? [[ -r $file]] File has read permission? [[ -w $file]] File has write permission? [[ -x $file]] File has execute perm? [[ -p $file]] File is a pipe?(C) Doug Bowman, Virginia Tech, 200118If statements Syntax:if conditionthenstatementselifconditionthen statementselsestatementsfioptional (C) Doug Bowman, Virginia Tech, 200119If statement Exampleif [[ -r $fname ]]thenecho $fname is readable elif [[ -w $fname && -x $fname ]]thenecho $fname is writeable and executable fi(C) Doug Bowman, Virginia Tech, 200120 For loops Syntax:for var[in list]dostatementsdone If listis omitted, $@is assumed Otherwise ${list[*]} where listis an array variable.

6 (C) Doug Bowman, Virginia Tech, 200121 For loop examplefor colors in Red Blue Green Yellow Orange Black Gray Whitedoecho $colorsdoneecho (C) Doug Bowman, Virginia Tech, 200122 While loops Syntax:while conditiondostatementsdone The keywords break, continue, and returnhave the same meaning as in C/C++(C) Doug Bowman, Virginia Tech, 200123 Case statements Syntax:case expressioninpattern1)statements ;;pattern2)statements ;;..*)statements ;;esac(C) Doug Bowman, Virginia Tech, 200124 Case statement examplecase $1 in-a)statements related to option a ;;-b)statements related to option b ;;*)all other options ;;esac (C) Doug Bowman, Virginia Tech, 200125 Command substitution Use the output of a command in a variable, condition, etc.

7 By: `command` $(command)(C) Doug Bowman, Virginia Tech, 200126 Examples Arguments printed in a for loop Reformatting the wc command Performing a depth-first search