The Tcl interpreter does only one substitution pass during command evaluation. Some situations, such as placing the name of a variable in a variable, require two passes through the substitution phase. In this case, the subst command is useful.

Subst performs a substitution pass without performing any execution of commands except those required for the substitution to occur, ie: commands within [] will be executed, and the results placed in the return string.

In the example code:

puts "[subst $$c]\n"

shows an example of placing a variable name in a variable, and evaluating through the indirection.

The format command can also be used to force some levels of substitution to occur.

subst ?-nobackslashes? ?-nocommands? ?-novariables? string

Passes string through the Tcl substitution phase, and returns the original string with the backslash sequences, commands and variables replaced by their equivalents.

If any of the -no... arguments are present, then that set of substitutions will not be done.

NOTE: subst does not honor braces or quotes.

Example

set a "alpha"
set b a

puts {a and b with no substitution: $a $$b}
puts "a and b with one pass of substitution: $a $$b"
puts "a and b with subst in braces: [subst {$a $$b}]"
puts "a and b with subst in quotes: [subst "$a $$b"]\n"

puts "format with no subst [format {$%s} $b]"
puts "format with subst: [subst [format {$%s} $b]]"
eval "puts \"eval after format: [format {$%s} $b]\""

set num 0;
set cmd "proc tempFileName {} "
set cmd [format "%s {global num; incr num;" $cmd]
set cmd [format {%s return "/tmp/TMP.%s.$num"} $cmd [pid] ]
set cmd [format "%s }" $cmd ]
eval $cmd

puts "[info body tempFileName]"

set a arrayname
set b index
set c newvalue
eval [format "set %s(%s) %s" $a $b $c]

puts "Index: $b of $a was set to: $arrayname(index)"

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