Am I right that 'if ! command; then ' form is not posix (the ! part?) ?

If so, then what are nice ways to reverse the condition in if ? Let's
assume the command after if is not 'test' otherwise it's easy
to reverse (test ! ...). I know several ways but none of them satisfy
me:

1. mytestcmd; if test $? != 0; then body; fi
sometimes i don't like that it adds extra line
2. if mytestcmd; then : nothing; else body; fi
adds 2-3 extra lines
3. mytestcmd || body
sometimes fine, sometimes I just prefer the 'if' form
4. if ! mytestcmd; then body; fi
i suspect this is not posix ????
5. if test `mytestcmd; echo $?` != 0; then body; fi
invokes extra subshell

Yakov

2006-11-1, 03:50(-08), Yakov:
> Am I right that 'if ! command; then ' form is not posix (the ! part?) ?

Yes it is POSIX. It is not Bourne, and early versions of ksh
didn't have it. I'm not sure about initial versions of ash.

All the standard shs or modern Unices have it. (note that
Solaris standard sh is not /bin/sh).

> If so, then what are nice ways to reverse the condition in if ? Let's
> assume the command after if is not 'test' otherwise it's easy
> to reverse (test ! ...). I know several ways but none of them satisfy
> me:
>
> 1. mytestcmd; if test $? != 0; then body; fi
> sometimes i don't like that it adds extra line
> 2. if mytestcmd; then : nothing; else body; fi
> adds 2-3 extra lines
> 3. mytestcmd || body
> sometimes fine, sometimes I just prefer the 'if' form
> 4. if ! mytestcmd; then body; fi
> i suspect this is not posix ????
> 5. if test `mytestcmd; echo $?` != 0; then body; fi
> invokes extra subshell
[...]



not() {
"$@"
[ "$?" -ne 0 ]
}

But beware that it works diffently than "!" in the sense that it
only works for simple commands.

not foo | bar

only reverses foo status.

! foo | bar

reverses the "foo | bar" status.

--
Stéphane

Stephane CHAZELAS wrote:

> I'm not sure about initial versions of ash.

It came with 44BSDalpha, that is,
when ash started aiming at POSIX.

> ! foo | bar
> reverses the "foo | bar" status.

That's why calling it "negation of pipeline"
(a command is only a special case of a pipeline)
is the both correct and to the point.