Replace spaces characters by a hard tab in a 6 GB file on Linux

I have a vi command that replaces spaces characters by a hard tab:

     vi myfile.txt
       : # To go to the last line 
          1,$s/ /\t/g   # Then I type in this to开发者_JS百科 replace spaces by a tab

How can I do it with AWK or sed?

Unless you strictly want sed/awk, tr is the best option for it:

tr ' ' '\t'  < inputfile > outputfile

Almost the same thing works with sed, just use a real tab instead of \t:

$ sed 's/ /  /g' < input_file > output_file
# a tab ---^^

If you're doing this from the bash command line then you'll need to do a Ctrl-VTab pair to get a tab.