in tcl, how do I replace a line in a file?

Question

let\'s say I opened a file, then parsed it into lines. Then I use a loop:

foreach line $lines {}

inside the loop, for some lines, I want to

Answer 1

for plain text files, it's safest to move the original file to a "backup" name then rewrite it using the original filename:

Update: edited based on Donal's feedback

set timestamp [clock format [clock seconds] -format {%Y%m%d%H%M%S}]

set filename "filename.txt"
set temp     $filename.new.$timestamp
set backup   $filename.bak.$timestamp

set in  [open $filename r]
set out [open $temp     w]

# line-by-line, read the original file
while {[gets $in line] != -1} {
    #transform $line somehow
    set line [string tolower $line]

    # then write the transformed line
    puts $out $line
}

close $in
close $out

# move the new data to the proper filename
file link -hard $filename $backup
file rename -force $temp $filename