I can't understand why using alias prompts an error, but using the same syntax without alias does not...

alias grep='egrep -iIs '
19:47:24 ~
cat /etc/services | \grep ssh
ssh     22/tcp              # SSH Remote Login Protocol
19:47:26 ~
cat /etc/services | grep ssh
grep: option requires an argument -- 'X'
Usage: grep [OPTION]... PATTERN [FILE]...
Try 'grep --help' for more information.
19:47:33 ~
cat /etc/services | egrep -iIs ssh
ssh     22/tcp              # SSH Remote Login Protocol

Can you explain this?

  • Cannot reproduce it - works fine here on Fedora24.
    – NickD
    Sep 14 '17 at 18:02
  • Works fine for me with GNU grep but note that -I is not POSIX. What OS are you using?
    – John1024
    Sep 14 '17 at 18:02
  • 3
    Do you by any chance also have ssh aliased to ssh -X? if so, I suspect the trailing space in your alias grep='egrep -iIs ' is allowing that to be expanded to ssh -X - resulting in the additional -X argument being passed to grep Sep 14 '17 at 20:16
  • 1
    @steeldriver That's the only possible reason, you should make this an answer.
    – xhienne
    Sep 14 '17 at 23:42
  • 2
    I bet I'm not the only one to wonder what may be that unknown -X option to grep. So here is the answer... along with further questions.
    – xhienne
    Sep 15 '17 at 0:33

As noted in the ALIASES section of man bash

If the last character of the alias value is a blank, then the next command word following the alias is also checked for alias expansion.

Since you have defined your grep alias with such a blank last character, the shell will also expand any alias that you have defined for ssh - if that is alias ssh='ssh -X' for example, then the command will become

cat /etc/services | egrep -iIs ssh -X

which passes the -X as an additional argument to egrep.

Unless there is a particular reason to include the trailing blank, simply define the alias without it, i.e.

alias grep='egrep -iIs'

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