subst man page on OpenDarwin

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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 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	speci‐
       fied,  then  the	 corresponding	substitutions  are not performed.  For
       example, if -nocommands is specified, command substitution is not  per‐
       formed:	 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  com‐ │
       mand  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 suc‐ │

       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}}						       │
       return ``xyz {p} q {r}'', not ``xyz {p\} q \{r}''.		       │

       When command substitution is performed, it includes any	variable  sub‐ │
       stitution necessary to evaluate the script.			       │
	      set a 44							       │
	      subst -novariables {$a [format $a]}			       │
       returns	``$a  44'', not ``$a $a''.  Similarly, when variable substitu‐ │
       tion 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 pre‐ │
       vent substitution of the rest of the command substitution and the  rest │
       of  string  respectively,  giving script authors more options when pro‐ │
       cessing 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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