harsh4999 / bashtime

Arguments

Syntax: 
	./file.sh arg1 arg2 ....
Use:
	if [[ -d $1 ]] ---> here $1 is arg1

Variables

Syntax: 
	variableName="value"
Print
	echo $variableName

Conditionals

Syntax:
	if [ $index -lt 5 ]
	then
		echo $index
	else
		echo "Nothing"
	fi


Equal: -eq
Not equal: -ne
Less than or equal: -le
Less than: -lt
Greater than or equal: -ge
Greater than: -gt
Is null: -z

------> When comparing strings use "double quotes" to prevent errors:

For String: 
	echo "Please enter username: "
	read name
	echo $name
	if [ "$name" == "harsh" ];
	then
        echo "Welcom back harsh"
	else
        echo "Welcome not back"
	fi

For Numeric:
	test=3
	if [ $test -lt 44 ]
	then 
	        echo "Harsh"
	else
	        echo "Rahul"
	fi

	if [ $a -eq $b ]
	then
	        echo "You are root"
	elif [ $a -lt $b ]
	then
	        echo "You are god"
	else
	        echo "You are not root"
	fi

Note that while using logical operator use :

[[double brackets ]]

	if [[ $a -eq $b && $a -eq 3 ]]
	then
	        echo "a is 3"
	fi

Bash test:

• File commands

  • "-a" FILE True if file exists
  • "-b" FILE true if a file is a block special
  • "-c" FILE true if a file is character special
  • "-d" FILE true if a file is a directory
  • "-e" FILE true if a file exists
  • "-f" FILE true if a file exists and regular file
  • "-g" FILE true if a file is a set-group-id
  • "-nt" FILE is newer than
  • "-ot" FILE is older than
  • "-r" FILE is readable
  • "-w" FILE can be modified
  • "-x" FILE can be executed
  • "-h" FILE exists and is a symbolic link (same as -L) extra

     -k FILE
            FILE exists and has its sticky bit set
  
     -L FILE
            FILE exists and is a symbolic link (same as -h)
  
     -N FILE
            FILE exists and has been modified since it was last read
  
     -O FILE
            FILE exists and is owned by the effective user ID
  
     -p FILE
            FILE exists and is a named pipe
  
     -r FILE
            FILE exists and read permission is granted
  
     -s FILE
            FILE exists and has a size greater than zero
  
     -S FILE
            FILE exists and is a socket
  
     -t FD  file descriptor FD is opened on a terminal
  
     -u FILE
            FILE exists and its set-user-ID bit is set
  
     -w FILE
            FILE exists and write permission is granted
  
     -x FILE
            FILE exists and execute (or search) permission is granted
  
  FILE1 -nt FILE2
            FILE1 is newer (modification date) than FILE2
  
     	FILE1 -ot FILE2
            FILE1 is older than FILE2
  
       
  

• Number commands

  • "-eq" equal
  • "-ne" not equal
  • "-lt" less than
  • "-le" less than or equal
  • "-gt" greater than
  • "-ge" greater then or equal

• String commands

  • "=" equal
  • "!=" not equal
  • "-z" is empty
  • "-n" is not empty

EXPR

• Add = +
• Sub = -
• Mul = *
• Div = /
• less then = <
• equal = =
• not equal = !=
• substr = expr substr $var 2 3: exctract 3 char from 2 index
• length = expr length $var: give length of var Use: var=expr $x + $y

Case:

Syntax:
	case EXPRESSION in

	  PATTERN_1)
	    STATEMENTS
	    ;;

	  PATTERN_2)
	    STATEMENTS
	    ;;

	  PATTERN_N)
	    STATEMENTS
	    ;;

	  *)
	    STATEMENTS
	    ;;
	esac

Loops:

Syntax:
- [FOR]()
	for((i=0;i<k;i++))
	do
		##
	done
	for a in $str
	do
		echo $a
	done
- [WHILE]()
	while [ $a -lt $k ]
	do
		echo $a
		((a++))
	done