redhat man page for subst

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, no command substitution is performed:  open and close brackets are  treated  as  ordinary	characters
       with no special interpretation.

       Note:  when  it	performs  its substitutions, subst does not give any special treatment to double quotes or curly braces.  For example, the
       script
	      set a 44
	      subst {xyz {$a}}
       returns ``xyz {44}'', not ``xyz {$a}''.

SEE ALSO
       eval(n)

KEYWORDS
       backslash substitution, command substitution, variable substitution

Tcl									7.4								  subst(n)