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OpenDarwin 7.2.1 - man page for subst (opendarwin section n)

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subst(n)			      Tcl Built-In Commands				 subst(n)


       subst - Perform backslash, command, and variable substitutions

       subst ?-nobackslashes? ?-nocommands? ?-novariables? string

       This command performs variable substitutions, command substitutions, and backslash substi-
       tutions on its string argument and returns the fully-substituted  result.   The	substitu-
       tions  are performed in exactly the same way as for Tcl commands.  As a result, the string
       argument is actually substituted twice, once by the Tcl parser in the  usual  fashion  for
       Tcl commands, and again by the subst command.

       If  any of the -nobackslashes, -nocommands, or -novariables are specified, then the corre-
       sponding substitutions are not performed.  For example, if -nocommands is specified,  com-
       mand substitution is not performed:  open and close brackets are treated as ordinary char-
       acters with no special interpretation.

       Note that the substitution of one kind can include substitution of other kinds.	For exam- |
       ple,  even  when  the  -novariables option is specified, command substitution is performed |
       without restriction.  This means that any variable substitution necessary to complete  the |
       command	substitution will still take place.  Likewise, any command substitution necessary |
       to complete a variable substitution will take place, even when -nocommands  is  specified. |
       See the EXAMPLES below.									  |

       If  an  error  occurs  during substitution, then subst will return that error.  If a break |
       exception occurs during command or variable substitution, the result of the whole  substi- |
       tution will be the string (as substituted) up to the start of the substitution that raised |
       the exception.  If a continue exception occurs during the evaluation of a command or vari- |
       able substitution, an empty string will be substituted for that entire command or variable |
       substitution (as long as it is well-formed Tcl.)  If a return  exception  occurs,  or  any |
       other  return  code is returned during command or variable substitution, then the returned |
       value is substituted for that substitution.  See the EXAMPLES below.   In  this	way,  all |
       exceptional  return  codes  are ``caught'' by subst.  The subst command itself will either |
       return an error, or will complete successfully.

       When it performs its substitutions, subst does not give any special  treatment  to  double
       quotes or curly braces (except within command substitutions) so the script
	      set a 44
	      subst {xyz {$a}}
       returns ``xyz {44}'', not ``xyz {$a}'' and the script					  |
	      set a "p\} q \{r" 								  |
	      subst {xyz {$a}}									  |
       return ``xyz {p} q {r}'', not ``xyz {p\} q \{r}''.					  |

       When command substitution is performed, it includes any variable substitution necessary to |
       evaluate the script.									  |
	      set a 44										  |
	      subst -novariables {$a [format $a]}						  |
       returns ``$a 44'', not ``$a $a''.  Similarly, when variable substitution is performed,  it |
       includes any command substitution necessary to retrieve the value of the variable.	  |
	      proc b {} {return c}								  |
	      array set a {c c [b] tricky}							  |
	      subst -nocommands {[b] $a([b])}							  |
       returns ``[b] c'', not ``[b] tricky''.							  |

       The  continue  and break exceptions allow command substitutions to prevent substitution of |
       the rest of the command substitution and the rest of string  respectively,  giving  script |
       authors more options when processing text using subst.  For example, the script		  |
	      subst {abc,[break],def}								  |
       returns ``abc,'', not ``abc,,def'' and the script					  |
	      subst {abc,[continue;expr 1+2],def}						  |
       returns ``abc,,def'', not ``abc,3,def''. 						  |

       Other exceptional return codes substitute the returned value				  |
	      subst {abc,[return foo;expr 1+2],def}						  |
       returns ``abc,foo,def'', not ``abc,3,def'' and						  |
	      subst {abc,[return -code 10 foo;expr 1+2],def}					  |
       also returns ``abc,foo,def'', not ``abc,3,def''.

       Tcl(n), eval(n), break(n), continue(n)

       backslash substitution, command substitution, variable substitution

Tcl					       7.4					 subst(n)
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