Linux Variables

• 
  
• $$ = The PID number of the process executing the shell.
• $? = Exit status variable.
• $0 = The name of the command you used to call a program.
• $1 = The first argument on the command line.
• $2 = The second argument on the command line.
• $n = The nth argument on the command line.
• $* = All the arguments on the command line.
• $# The number of command line arguments.

The tests below are test conditions provided by the shell:

    * -b file = True if the file exists and is block special file.
    * -c file = True if the file exists and is character special file.
    * -d file = True if the file exists and is a directory.
    * -e file = True if the file exists.
    * -f file = True if the file exists and is a regular file
    * -g file = True if the file exists and the set-group-id bit is set.
    * -k file = True if the files' "sticky" bit is set.
    * -L file = True if the file exists and is a symbolic link.
    * -p file = True if the file exists and is a named pipe.
    * -r file = True if the file exists and is readable.
    * -s file = True if the file exists and its size is greater than zero.
    * -s file = True if the file exists and is a socket.
    * -t fd = True if the file descriptor is opened on a terminal.
    * -u file = True if the file exists and its set-user-id bit is set.
    * -w file = True if the file exists and is writable.
    * -x file = True if the file exists and is executable.
    * -O file = True if the file exists and is owned by the effective user id.
    * -G file = True if the file exists and is owned by the effective group id.
    * file1 –nt file2 = True if file1 is newer, by modification date, than file2.
    * file1 ot file2 = True if file1 is older than file2.
    * file1 ef file2 = True if file1 and file2 have the same device and inode numbers.
    * -z string = True if the length of the string is 0.
    * -n string = True if the length of the string is non-zero.
    * string1 = string2 = True if the strings are equal.
    * string1 != string2 = True if the strings are not equal.
    * !expr = True if the expr evaluates to false.
    * expr1 –a expr2 = True if both expr1 and expr2 are true.
    * expr1 –o expr2 = True is either expr1 or expr2 is true.

The syntax is :

arg1 OP arg2

where OP is one of –eq, -ne, -lt, -le, -gt, or –ge. Arg1 and arg2 may be positive or negative integers or the special expression "–l string" which evaluates to the length of string.







while [ 1 ]
do
   statement(s)
done




if [ ! -d /mnt ]        # be sure the directory /mnt exists
then
   mkdir /mnt
fi

case $NUM of
1)
   echo The number is 1
   ;;
2)
   echo The number is 2
   ;;
*)
   echo The number is not 1 or 2
   ;;
esac