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