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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 substi-
tutions on its string argument and returns the fully-substituted result. The substitu-
tions 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 corre-
sponding substitutions are not performed. For example, if -nocommands is specified, com-
mand substitution is not performed: open and close brackets are treated as ordinary char-
acters with no special interpretation.
Note that the substitution of one kind can include substitution of other kinds. For exam-
ple, 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 substi-
tution 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 vari-
able 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(n), eval(n), break(n), continue(n)
KEYWORDS
backslash substitution, command substitution, variable substitution
Tcl 7.4 subst(n)
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Under GNU General Public License
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