21.1.4 Types of Tcl Commands

Built-in commands are provided by the Tcl interpreter. They are available in all Tcl applications.

• Tcl provides several commands for manipulating file names, reading and writing file attributes, copying files, deleting files, creating directories, and so on.
• You can execute an external program using exec. Upon execution, the return value is the output (on stdout) from the external program, for example:

  set tmp [ exec myprog ] puts stdout $tmp

• You can easily create collections of values (lists) and manipulate them in a variety of ways.
• You can create arrays - structured values consisting of name-value pairs with arbitrary string values for the names and values.
• You can manipulate the time and date variables.
• You can write scripts that can wait for certain events to occur, such as an elapsed time or the availability of input data on a network socket.

You use the proc command to declare a procedure. You can then use the name of the procedure as a Tcl command.

The following sample script consists of a single command named proc. The proc command takes three arguments:

• The name of a procedure (myproc)
• A list of argument names (arg1 arg2)
• The body of the procedure, which is a Tcl script

  proc myproc { arg1 arg2 } { 
  # procedure body
  }
  myproc a b