subst(3tcl)
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, variable substitution
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