Shell Built-In Commands

• The set command
  Can be used to produce a list of currently defined shell variables (see section )
  Can be used to assign values to the positional parameters (see section )
  Can be used to set options for the shell:
  set -v for example tells the shell to echo each subsequent command before it is executed
• The Do-Nothing Command: :
  The colon is the shell's do-nothing command
  It can have arguments, which, though the command doesn't do anything about them, themselves can do something
  Example:
  : $DEBUG? 'Variable DEBUG not defined. Exiting...'
  will cause the script to terminate, if the variable DEBUG isn't defined
• The dot command: . The command .commandlist reads and executes commands contained in the file commandlist. The two differences between the dot command and a standard shell script are:
  • The dot command does not create a new subshell
  • The dot command does not allow command line arguments
  Note: A script executed via the dot command can change the value of shell variables in the current shell!
• The export command
  export can be used to export the value of a variable to a subshell
  export without arguments prints out all variables that are already exported
• The shift command
  The shift command shifts the positional parameters: $2 to $1, $3 to $2 and so on...
  Note: $1 is lost!!!