Re: Why do we need "eval"? (Expect question)

Am 12.09.2024 um 17:07 schrieb Kenny McCormack:

Consider this (Unix/Linux/bash) command line:
 $ expect -- /dev/fd/3 3<<< 'eval spawn -noecho printf {{\t%s\n}} $argv;interact' $(seq 1 10)
 This correctly generates the output (each line starts with a tab):
  1
2
3
4
5
6
7
8
9
10
 But notice how we have to use "eval" in order to split up the args in $argv.
And, since we are using "eval", we have to "double quote" (with {}) the
"format" arg to "printf".  It'd be nice if neither of these things were
necessary.
 I've always believed that "eval" was "evil" and to be avoided if at all
possible - both in shell and in Tcl.  It has strange side effects, such as
we see here (the need to "double quote").  Is there any way to get the
above effect w/o using "eval" ?
 

Kenny,
thanks for the question. Here is the answer by a TCL'er without expect knowledge.
"eval" does an additional round of substitutions. This happens for all elements and is intended, if a whole command is included in a variable.
To only expand some arguments, the list expansion operator may be used:
In your case:
eval spawn -noecho printf {{\t%s\n}} $argv
equal to:
spawn -noecho printf {\t%s\n} {*}$argv
eventually, this works to:
spawn -noecho printf \t%s\n {*}$argv
Harald