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 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 substitution  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 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 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}}

       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(3tcl), eval(3tcl), break(3tcl), continue(3tcl)

KEYWORDS

       backslash substitution, command substitution, quoting, substitution, variable substitution

Tcl                                                    7.4                                           subst(3tcl)