nix

• if condition
  Syntax:
  if condition
  then
             command1 if condition is true or if exit status of condition is 0 (zero)
             …
             …
  fi
  Ex:
  #!/bin/sh
  # Script to test rm command and exist status
  if rm $1
  then
      echo "$1 file deleted"
  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.
• 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 ]
  Ex:
  #!/bin/sh
  # Script to see whether argument is positive
  if test $1 -gt 0        /* Or, if [ $1 –gt 0 ] */
  then
      echo "$1 number is positive"
  fi
• test or [ expr ] works with
  • Integer ( Number without decimal point)
  • File types
  • Character strings
• Operators in shell script

Mathematical operator in shell script	Meaning	Normal arithmetical / mathematical statements	In shell
			test statement	[ expr ] statement
-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 comparison

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

• Tests for file and directory types

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
  Syntax:
  if condition
  then
             condition is zero (true), execute all commands up to else statement
  else
             if condition is not true then execute all commands up to fi
  fi
  Ex:
  #!/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
• Multilevel if-then-else
  Syntax:
  if condition1
  then
             condition1 is zero (true), execute all commands up to elif statement
  elif condition2
  then
             condition2 is zero (true), execute all commands up to elif statement
  elif condition3
  then
             condition3 is zero (true), execute all commands up to elif statement
  else
             None of the above condition1, condition2, condition3 is true execute all commands up to fi
  fi
• Computer can repeat particular instruction again and again, until particular condition satisfies. A group of instruction that is executed repeatedly is called a loop.
• for loop
  Syntax:
  for { variable_name } in { list }
  do
             execute one for each item in the list until the list is not finished (And repeat all statement between do and done)
  done
  Or
  for (( expr1; expr2; expr3 ))
  do
             repeat all statements between do and done until expr2 is TRUE
  done
  Ex:
  #!/bin/sh
  #Script to test for loop
  if [ $# -eq 0 ]
  then
      echo "Error - Number missing form command line argument"
      echo "Syntax : $0 number"
      echo "Use to print multiplication table for given number"
      exit 1
  fi
  n=$1
  for i in 1 2 3 4 5 6 7 8 9 10
  do
      echo "$n * $i = `expr $i \* $n`"
  done
  Ex:
  #!/bin/sh
  for (( i = 1; i <= 9; i++ ))             ### Outer for loop ###
  do
      for (( j = 1 ; j <= 9; j++ ))            ### Inner for loop ###
      do
          tot=`expr $i + $j`
          tmp=`expr $tot % 2`
          if [ $tmp -eq 0 ]; then
              echo -e -n "\033[47m "
          else
              echo -e -n "\033[40m "
          fi
      done
      echo -e -n "\033[40m"              #### set back background color to black
      echo ""                                         #### print the new line ###
  done
• while loop
  Syntax:
  while [ condition ]
  do
             command1
             …
             …
  done
  Ex:
  #!/bin/sh
  #Script to test while statement
  if [ $# -eq 0 ]
  then
      echo "Error - Number missing form command line argument"
      echo "Syntax : $0 number"
      echo " Use to print multiplication table for given number"
      exit 1
  fi
  n=$1
  i=1
  while [ $i -le 10 ]
  do
      echo "$n * $i = `expr $i \* $n`"
      i=`expr $i + 1`
  done
• The case statement
  Syntax:
  case $variable_name in          /* the $variable_name is compared against the patterns until a match is found. */
             pattern1)     command
                                  …
                                  command;;
             pattern2)     command
                                  …
                                  command;;
             patternN)    command
                                  …
                                  command;;
             *)                  command
                                  …
                                  command;;
  esac
  Ex:
  # if no vehicle name is given i.e. -z $1 is defined and it is NULL, if no command line arg
  if [ -z $1 ]
  then
      rental="*** Unknown vehicle ***"
  elif [ -n $1 ]
  then   # otherwise make first arg as rental
      rental=$1
  fi
  case $rental in
      "car")           echo "For $rental Rs.20 per k/m";;
      "van")          echo "For $rental Rs.10 per k/m";;
      "jeep")        echo "For $rental Rs.5 per k/m";;
      "bicycle")    echo "For $rental 20 paisa per k/m";;
      *)                  echo "Sorry, I can not gat a $rental for you";;
  esac
• Debug the shell script.
  sh option { shell-script-name }
  • -v Print shell input lines as they are read.
  • -x After expanding each simple-command, bash displays the expanded value of PS4 system variable, followed by the command and its expanded arguments.