Bash alias problem

Question

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?

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
type -a grep egrep
grep is aliased to `egrep -iIs '
grep is /bin/grep
egrep is /bin/egrep — johnmuir, Sep 14 '17 at 18:06
Can not reproduce on Ubuntu 17.04 with the information given. No error. — Kusalananda, Sep 14 '17 at 18:19
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` — steeldriver, Sep 14 '17 at 20:16
@steeldriver That's the only possible reason, you should make this an answer. — xhienne, Sep 14 '17 at 23:42
I bet I'm not the only one to wonder what may be that unknown `-X` option to `grep`. So [here is the answer](https://unix.stackexchange.com/q/392341/203203)... along with further questions. — xhienne, Sep 15 '17 at 00:33

score 10 · Accepted Answer · answered Sep 14 '17 at 23:52

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'

Bash alias problem

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