#1: "or" in bash if statements
Posted on 2006-02-02 17:16:49 by didds
feel really stoooopid asking this but I've gogled etc to no avail...
how does one put an "or" in a bash shell if statement?
I am trying the following
if [ /etc/passwd -nt timestamp || /etc/shadow -nt timestamp ]; then
echo yes
fi
but receive the error
: [: missing `]'
: /etc/shadow: Permission denied
the if construct works as a single file check...
TIA
ian
Report this message |
Mr Ad
Google
|
#2: Re: "or" in bash if statements
Posted on 2006-02-02 17:24:50 by Icarus Sparry
On Thu, 02 Feb 2006 08:16:49 -0800, didds wrote:
> feel really stoooopid asking this but I've gogled etc to no avail...
>
> how does one put an "or" in a bash shell if statement?
>
> I am trying the following
>
> if [ /etc/passwd -nt timestamp || /etc/shadow -nt timestamp ]; then
> echo yes
> fi
>
>
> but receive the error
>
> : [: missing `]'
> : /etc/shadow: Permission denied
>
> the if construct works as a single file check...
"if" takes a pipeline of commands, so you can do
if [ /etc/passwd -nt timestamp ] || [ /etc/shadow -nt timestamp ]; then
echo yes
fi
This is portable.
Or you can use the "[["/"]]" instead of "["/"]". This is less portable.
Report this message |
#3: Re: "or" in bash if statements
Posted on 2006-02-02 17:33:08 by Stephane CHAZELAS
On Thu, 02 Feb 2006 08:24:50 -0800, Icarus Sparry wrote:
[...]
> "if" takes a pipeline of commands, so you can do
>
> if [ /etc/passwd -nt timestamp ] || [ /etc/shadow -nt timestamp ]; then
> echo yes
> fi
It takes a *list* of commands which might be pipelines.
cmd1 && cmd2 || cmd3
may be refered to as a OR-AND list.
cmd1 | cmd2 | cmd3
is a pipeline
>
> This is portable.
Except for "-nt" which is not POSIX nor Bourne.
> Or you can use the "[["/"]]" instead of "["/"]". This is less portable.
Probably as portable as "-nt": ksh/bash/zsh specific.
One can also use:
[ condition1 -o condition2 ]
(-o is then a "[" operator while "||" is a shell operator)
except that it is less reliable.
--
Stephane
Report this message |
#4: Re: "or" in bash if statements
Posted on 2006-02-02 18:12:37 by merlyn
>>>>> "Stephane" == Stephane Chazelas <stephane_chazelas@yahoo.fr> writes:
Stephane> It takes a *list* of commands which might be pipelines.
Stephane> cmd1 && cmd2 || cmd3
Eeek. Not that crappy meme again. Please stop spreading that.
It gets used as *if* you're saying:
if cmd1; then cmd2; else cmd3; fi
But consider what happens when cmd2 fails after being selected. cmd3 runs!
Eeek! So that's really not a if/then/else... it's an
if-then-and-then-if-that-works-ok-do-nothing-else-the-other :)
If you want an if/then/else, you know where it is. Stop using the
abbreviated form that breaks when people least expect it.
Maybe I need a "Dangerous Use Of And-Or" award to go with my Useless Use of
Cat awards... :)
Just another Unix hacker (since 1977, yes *77*),
--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095
<merlyn@stonehenge.com> <URL:http://www.stonehenge.com/merlyn/>
Perl/Unix/security consulting, Technical writing, Comedy, etc. etc.
See PerlTraining.Stonehenge.com for onsite and open-enrollment Perl training!
Report this message |
#5: Re: "or" in bash if statements
Posted on 2006-02-02 20:56:21 by Stephane CHAZELAS
2006-02-02, 09:12(-08), Randal L. Schwartz:
>>>>>> "Stephane" == Stephane Chazelas <stephane_chazelas@yahoo.fr> writes:
>
>Stephane> It takes a *list* of commands which might be pipelines.
>
>Stephane> cmd1 && cmd2 || cmd3
>
> Eeek. Not that crappy meme again. Please stop spreading that.
> It gets used as *if* you're saying:
>
> if cmd1; then cmd2; else cmd3; fi
>
> But consider what happens when cmd2 fails after being selected. cmd3 runs!
> Eeek! So that's really not a if/then/else... it's an
> if-then-and-then-if-that-works-ok-do-nothing-else-the-other :)
>
> If you want an if/then/else, you know where it is. Stop using the
> abbreviated form that breaks when people least expect it.
[...]
That's very true, but was not the point. The point was about
terminology.
cmd1 || cmd2
is not a pipeline, but an AND-OR list.
And we were discussing the
if cmd1 || cmd2 && cmd3; then
thing, i.e. the syntax of the if/then/else/fi statement, where
using an AND-OR list makes a lot of sense.
--
Stéphane
Report this message |
