I am trying to use awk to cut with multi-character delimiter

echo 'f1##f2' |awk -F '##' '{print $2}'

It prints f2. Whereas

echo 'f1||f2' |awk -F '||' '{print $1}'

prints f1||f2.

Can anyone explain what is going on?

  • I'm surprised that GNU awk allows that at all. BSD awk throws an error, awk: illegal primary in regular expression || at |, as does mawk, mawk: line 0: regular expression compile failed (missing operand). – Kusalananda Sep 12 '17 at 15:48
  • @Kusalananda, the behaviour is unspecified by POSIX. The GNU behaviour is more useful and makes more sense IMO. It makes more sense to me to have (foo|bar|) be like (foo|bar)? rather than returning an error. That's also what perl REs do. ? in zsh globs is done that (? meaning something different in globs) – Stéphane Chazelas Sep 12 '17 at 18:51

The vertical bar char | is treated as special character in your case and should be escaped:

echo "f1||f2" |awk -F'\\|\\|' '{print $1}'

As an alternative you may put | into the character class:

echo "f1||f2||f3" |awk -F'[|]{2}' '{print $3}'
  • Why escape it twice? – pfnuesel Sep 12 '17 at 15:59
  • 1
    @pfnuesel, because with this -F'\\||' you'll get empty {print $2} (second field) – RomanPerekhrest Sep 12 '17 at 16:02
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    @ctrl-alt-delor, FYI: linguistically | IS "vertical bar" char (not OR character), semantically - in some context it's logical OR operator, in some other context - regex alternation operator – RomanPerekhrest Sep 12 '17 at 16:34
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    @ctrl-alt-delor Adding to the comment of RomanPerekhrest: Calling | an OR character is confusing because there is an actual OR character which is different: "∨" (Logical Or, U+2228, in Unicode block Mathematical Operators ). (In some fonts, it may look almost like the alphabet letter V in uppercase. It should be larger and have a larger angle.) – Volker Siegel Sep 13 '17 at 1:27
  • @VolkerSiegel I was talking about its meaning in the language not the character name. In a regexp it is apparently called alternation, though I call it OR (not logical OR). – ctrl-alt-delor Sep 13 '17 at 8:01

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