Decision making

• if condition
  • If condition which is used for decision making in shell script, If given condition is true then command1 is executed.
    Syntax:

    if condition
    then
            command1 if condition is true or if exit status
            of condition is 0 (zero)
            ...
            ...
    fi
    

  • Condition is nothing but comparison between two values.
  • For compression you can use test or [ expr ] statements or even exist status can be also used.
  • An expression is nothing but combination of values, relational operator (such as $>,<, <>$ etc) and mathematical operators (such as $+, -, /$ etc ). http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/showfileshowfile

    #!/bin/sh
    #
    #Script to print file
    #
    if cat $1
    then
            echo -e "\n\nFile $1, found and successfully echoed"
    fi
    

• test command or [ expr ]
  • test command or [ expr ] is used to see if an expression is true, and if it is true it return zero (0), otherwise returns nonzero for false.
    Syntax:

    test expression OR [ expression ]
    

  • Following script determine whether given argument number is positive. http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/ispositiveispositive

    #!/bin/sh
    #
    # Script to see whether argument is positive
    #
    if test $1 -gt 0
    then
            echo "$1 number is positive"
    fi
    

  • test or [ expr ] works with
    1. Integer ( Number without decimal point)
    2. File types
    3. Character strings
    
    

    Table 2: Operator in Shell Script

    Mathematical	Operator in Shell
    -eq	is equal to $5 == 6$; if test 5 -eq 6 ; if [ 5 -eq 6 ]
    -ne	is not equal to $5 != 6$; if test 5 -ne 6; if [ 5 -ne 6 ]
    -lt	is less than $5 < 6$; if test 5 -lt 6; if [ 5 -lt 6 ]
    -le	is less than or equal to $5 <= 6$; if test 5 -le 6; if [ 5 -le 6 ]
    -gt	is greater than $5 > 6$; if test 5 -gt 6; if [ 5 -gt 6 ]
    -ge	is greater than or equal to $5 >= 6$; if test 5 -ge 6; if [ 5 -ge 6 ]
    String Comparisons	Operator Meaning
    string1 = string2	string1 is equal to string2
    string1 != string2	string1 is NOT equal to string2
    string1	string1 is NOT NULL or not defined
    -n string1	string1 is NOT NULL and does exist
    -z string1	string1 is NULL and does exist
    File and directory	Test Meaning
    -s file	Non empty file
    -f file	Is File exist or normal file and not a directory
    -d dir	Is Directory exist and not a file
    -w file	Is writeable file
    -r file	Is read-only file
    -x file	Is file is executable
    Logical Operators	Operator Meaning
    ! expression	Logical NOT
    expression1 -a expression2	Logical AND
    expression1 -o expression2	Logical OR

    
    
• if...else...fi
  • If given condition is true then command1 is executed otherwise command2 is executed. Syntax:

    if condition
    then
            condition is zero (true - 0)
            execute all commands up to else statement
    else
            if condition is not true then
            execute all commands up to fi
    fi
    

  • Script to see whether argument is positive or negative; http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/isnumpnisnumpn

    #!/bin/sh
    #
    # Script to see whether argument is positive or negative
    #
    if [ $# -eq 0 ]
    then
            echo "$0 : You must give/supply one integers"
            exit 1
    fi
    if test $1 -gt 0
    then
            echo "$1 number is positive"
    else
            echo "$1 number is negative"
    fi
    

• Nested if-else-fi
  • You can write the entire if-else construct within either the body of the if statement of the body of an else statement. This is called the nesting of ifs. Syntax:

    if condition
    then
            if condition
            then
                    do this
            else
                    do this
            fi
    else
            do this
    fi
    

    http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/nestedifnestedif

    #!/bin/sh
    #
    # Script to see nesting of ifs
    #
    osch=0
    
    echo "1. Unix (Sun Os)"
    echo "2. Linux (Red Hat)"
    echo -n "Select your os choice [1 or 2]? "
    read osch
    
    if [ $osch -eq 1 ] ; 
    then
            echo "You Pick up Unix (Sun Os)"
            else # nested if i.e. if within if #
            if [ $osch -eq 2 ] ;
            then
                    echo "You Pick up Linux (Red Hat)"
            else
                    echo "What you don't like Unix/Linux OS."
            fi
    fi
    

• Multilevel if-then-else Syntax:

  if condition
  then
          condition is zero (true - 0)
          execute all commands up to elif statement
  elif condition1
  then
          condition1 is zero (true - 0)
          execute all commands up to elif statement
  elif condition2
  then
          condition2 is zero (true - 0)
          execute all commands up to elif statement
  else
          None of the above condition,condition1,condition2 are true (i.e.
          all of the above nonzero or false)
          execute all commands up to fi
  fi
  

  For multilevel if-then-else statement try the following script: http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/elfelf

  #!/bin/sh
  # Script to test if..elif...else
  #
  if [ $1 -gt 0 ]; 
  then
          echo "$1 is positive"
  elif [ $1 -lt 0 ]
  then
          echo "$1 is negative"
  elif [ $1 -eq 0 ]
  then
          echo "$1 is zero"
  else
          echo "Opps! $1 is not number, give number"
  fi
  

  Integer comparison is expected.