Re: [Help-bash] New conditional evaluation with a regex

[Eric Blake]

On 12/08/2011 03:49 AM, Jesse Molina wrote:
> 
> Duh.
> 
> I could use case.

Yep.  But while we're on the topic:

> 
> case "$TEST1" in

Here, you don't need "" around $TEST1, since no shell performs word
splitting or filename expansion here.

>         2|3|4|5|6|7) 

And if you're going for conciseness, it is portable to write:

case $TEST1 in
  [2-7]) ...

Note that ranges inside [] are only portable if you use LC_ALL=C, or if
the range is strictly numeric and not alphabetic; as alphabetic ranges
are undefined in other locales.

>> The first way to do this uses single [] brackets and I'll break it up
>> for readability;
>>
>> if [ \
>> "$TEST1" == 2 \
>> -o "$TEST1" == 3 \
>> -o "$TEST1" == 4 \
>> -o "$TEST1" == 5 \
>> -o "$TEST1" == 6 \
>> -o "$TEST1" == 7 \
>> ] ; then

Not portable.  Use of -o and -a inside [ ] (or test) is a disaster
waiting to happen in code trying to be portable.  Likewise, with [ ],
the portable spelling is '=', not '=='.

>> Then, I remembered that you could use =~ to do something like that, but
>> I had to google up to remember that you can only do it in double [[]]
>> brackets (new style);
>>
>> if [[ "$TEST1" =~ 2|3|4|5|6|7 ]] ; then

Again, non-portable if your script is ever run on just a POSIX /bin/sh
(although there is an effort underway to add [[ ]] to the next version
of POSIX, several years down the road).

>> Any other clever ways to do complex conditional tests, or neat tricks on
>> the subject?

For ranges, you could have used numeric comparison operators in [ ]:

if [ "$TEST1" -ge 2 ] && [ "$TEST1" -le 7 ]; then
...

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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