set existing_enc [encoding system]
#Any "puts" before setting 'encoding system' will set the existing system encoding on the channel (others too, but depending if console vs piped) 
#e.g
### puts stderr test
#Uncommenting the above will mean that both stdout and stderr (when in a piped-situation, ie no console) are initialised to existing_enc - not the one we set below!
set arg_setencoding [lindex $::argv 0]
if {$arg_setencoding ne ""} {
    if {$arg_setencoding ni [encoding names]} {
        puts stderr "Usage: encoding.tcl ?tcl_encoding?"
        puts stderr "(Note difference in stdout/stderr encodings when piped: e.g encoding.tcl cp437 | cat)"
        puts stderr "encoding names:\n"
        puts stderr "[encoding names]"
        exit 1
    }
    encoding system $arg_setencoding
} else {
    encoding system utf-8
}
puts "original encoding system  : $existing_enc"
puts "configured encoding system: [encoding system]"
puts "stdout: [chan conf stdout]"
puts "stderr: [chan conf stderr]"
puts "[lindex $::argv 0]"

#compare:
#1) both stderr and stdout are to console - not affected by changed system encoding
#>tclsh encoding.tcl
# original encoding system  : utf-8
# configured encoding system: utf-8
# stdout: -blocking 1 -buffering line -buffersize 4096 -encoding utf-16 -eofchar {} -profile tcl8 -translation crlf -winsize {224 57}
# stderr: -blocking 1 -buffering none -buffersize 4096 -encoding utf-16 -eofchar {} -profile tcl8 -translation crlf -winsize {224 57}

#2) stdout not going to console
#>tclsh encoding.tcl | cat
# original encoding system  : utf-8
# configured encoding system: utf-8
# stdout: -blocking 1 -buffering line -buffersize 4096 -encoding utf-8 -eofchar {} -profile tcl8 -translation crlf
# stderr: -blocking 1 -buffering none -buffersize 4096 -encoding utf-16 -eofchar {} -profile tcl8 -translation crlf -winsize {224 57}

#3) neither channel to console
#>tclsh encoding.tcl |& cat
# original encoding system  : utf-8
# configured encoding system: utf-8
# stdout: -blocking 1 -buffering line -buffersize 4096 -encoding utf-8 -eofchar {} -profile tcl8 -translation crlf
# stderr: -blocking 1 -buffering none -buffersize 4096 -encoding utf-8 -eofchar {} -profile tcl8 -translation crlf