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

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NAME

       subst - Perform backslash, command, and variable substitutions

SYNOPSIS

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

       This  command  performs	variable  substitutions, command substitutions, and backslash substitutions on its string argument and returns the
       fully-substituted result.  The substitutions 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 corresponding substitutions are not performed.  For
       example, if -nocommands is specified, command substitution is not performed:  open and close brackets are treated  as  ordinary	characters
       with no special interpretation.

       Note  that  the substitution of one kind can include substitution of other kinds.  For example, even when the -novariables option is speci-
       fied, 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  substi-
       tution, the result of the whole substitution will be the string (as substituted) up to the start of the substitution that raised the excep-
       tion.  If a continue exception occurs during the evaluation of a command or variable 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.

EXAMPLES

       When it performs its substitutions, subst does not give any special treatment to double quotes or curly braces (except within command  sub-
       stitutions) 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}}
       returns "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".

SEE ALSO

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

KEYWORDS

       backslash substitution, command substitution, variable substitution

Tcl									7.4								  subst(n)