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 sub-stitutions substitutions
stitutions 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 interpreta-tion. interpretation.
tion.
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 substitu- |
tion, 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 {$a}} |
return ``xyz {p} q {r}'', not ``xyz {p\} q \{r}''. |
When command substitution is performed, it includes any variable substitution necessary to evaluate |
the script. |
set a 44 |
subst -novariables {$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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