Tcl update alacritty config

opacity.tcl

#!/usr/bin/env tclsh
set timestamp [clock format [clock seconds] -format {%Y%m%d%H%M%S}]
set newOpacity [lindex $argv 0]
puts "got arg: $newOpacity" # check args
if {[string is integer $newOpacity] == 0} {
    puts "You must pass an integer value for opacity"    
    exit 1
}
set filename "~/.config/alacritty/alacritty.yml"
set temp     $filename.new.$timestamp
set backup   $filename.bak.$timestamp
set in [open $filename r]
set out [open $temp w]
while {[gets $in line] != -1} {    
    if {[string match background_opacity* $line] == 1} {        
        set line "background_opacity: 0.$newOpacity"        
        puts $line    
    }    
    puts $out $line
}
close $in
close $out
file rename -force $temp $filename