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Every contemporary AWK supports -v. Real AWK from UNIX®️ supported -v since at least the '80's.

True. But there are differences when -v is used, as opposed to FS. Try this, where "nawk" is Lucent awk used by BSD

     cat > 1.awk << eof

     { print $ARGC }

     eof

     echo|nawk -f 1.awk FS=":"
     
     echo|gawk -f 1.awk FS=":"     
     
     echo|nawk -f 1.awk -v FS=":"

     echo|gawk -f 1.awk -v FS=":"

That is not how FS is set; It's set with -F. And there is actually no need to use -v, passing variables at the end works consistently across all AWK's and always has:

  echo "" | awk '{print Bla;}' Bla="Hello."

What if you set FS with -F but then later in the script want to change FS to something else.

The results will be unpredictable at best; either set it with -F, or use 'BEGIN {FS = "...";}', but not both.

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