subst(n) Tcl Built-In Commands subst(n)
NAME
subst - Perform backslash, command, and variable substitutions
SYNOPSIS
subst ?-nobackslashes? ?-nocommands? ?-novariables? string
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 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-
cessfully.
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 {{$$a}}}}
return ``xyz {{p}} q {{r}}'', not ``xyz {{p\\}} q \\{{r}}''.
When command substitution is performed, it includes any variable sub-
stitution necessary to evaluate the script.
set a 44
subst -novariables {{$$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 100 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)
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