bash string-operator problem

bash string-operator problem

[Rockefeller, Harry]

CYGWIN_NT-6.1 HARRYR-PC 3.0.0(0.336/5/3) 2019-02-16 13:21 x86_64 Cygwin
GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)

#!/bin/bash
A="A"
B="A"
if [ $A!=$B ]; then
    echo -e "not identical"
fi
if [ $A==$B ]; then
    echo -e "identical"
fi
exit 0

Running this script gives
not identical
identical

Both tests are true.

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Re: bash string-operator problem

[Eric Blake]

On 2/21/19 10:00 AM, Rockefeller, Harry wrote:
> CYGWIN_NT-6.1 HARRYR-PC 3.0.0(0.336/5/3) 2019-02-16 13:21 x86_64 Cygwin
> GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)
> 
> #!/bin/bash
> A="A"
> B="A"
> if [ $A!=$B ]; then
>     echo -e "not identical"
> fi
> if [ $A==$B ]; then
>     echo -e "identical"
> fi
> exit 0
> 
> Running this script gives
> not identical
> identical
> 
> Both tests are true.

Well, yeah, but it's not a bug in bash, but in your script. Whitespace
for argument separation is essential in [.  You've provided exactly one
argument, and in one-argument mode, you are testing for non-empty
strings.  Since both strings "A!=B" and "A==B" are non-empty strings,
the if clause is taken both times.  You meant to provide three
arguments, and with proper quoting, and using the portable spelling of
equality (if you want to port your script to more than just bash):

[ "$A" != "$B" ]
[ "$A" = "$B" ]

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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Re: bash string-operator problem

[Lee]

On 2/21/19, Rockefeller, Harry  wrote:
> CYGWIN_NT-6.1 HARRYR-PC 3.0.0(0.336/5/3) 2019-02-16 13:21 x86_64 Cygwin
> GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)
>
> #!/bin/bash
> A="A"
> B="A"
> if [ $A!=$B ]; then          <----- needs spaces around the !=
>     echo -e "not identical"
> fi
> if [ $A==$B ]; then         <------ needs spaces around the ==
>     echo -e "identical"
> fi
> exit 0
>
> Running this script gives
> not identical
> identical
>
> Both tests are true.

because you're testing "A==B"
you need to give 3 parameters  "A"  "=="  "B"

$ cat x
#!/bin/bash
A="A"
B="A"
if [ $A != $B ]; then
    echo  "not identical"
fi
if [ $A == $B ]; then
    echo  "identical"
fi
if [ A!=B ]; then
    echo  "not identical"
fi
if [ A==B ]; then
    echo  "identical"
fi

Lee

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Re: bash string-operator problem

[john doe]

On 2/21/2019 5:18 PM, Lee wrote:
> On 2/21/19, Rockefeller, Harry  wrote:
>> CYGWIN_NT-6.1 HARRYR-PC 3.0.0(0.336/5/3) 2019-02-16 13:21 x86_64 Cygwin
>> GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)
>>
>> #!/bin/bash
>> A="A"
>> B="A"
>> if [ $A!=$B ]; then          <----- needs spaces around the !=
>>     echo -e "not identical"
>> fi
>> if [ $A==$B ]; then         <------ needs spaces around the ==
>>     echo -e "identical"
>> fi
>> exit 0
>>
>> Running this script gives
>> not identical
>> identical
>>
>> Both tests are true.
>
> because you're testing "A==B"
> you need to give 3 parameters  "A"  "=="  "B"
>
> $ cat x
> #!/bin/bash
> A="A"
> B="A"
> if [ $A != $B ]; then
>     echo  "not identical"
> fi
> if [ $A == $B ]; then
>     echo  "identical"
> fi
> if [ A!=B ]; then
>     echo  "not identical"
> fi
> if [ A==B ]; then
>     echo  "identical"
> fi
>

One equal sign ('=') should be used instead of two equal signs ('==').

--
John Doe

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Re: bash string-operator problem

[Lee]

On 2/21/19, john doe wrote:
> On 2/21/2019 5:18 PM, Lee wrote:
>> On 2/21/19, Rockefeller, Harry  wrote:
>>> CYGWIN_NT-6.1 HARRYR-PC 3.0.0(0.336/5/3) 2019-02-16 13:21 x86_64 Cygwin
>>> GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)
>>>
>>> #!/bin/bash
>>> A="A"
>>> B="A"
>>> if [ $A!=$B ]; then          <----- needs spaces around the !=
>>>     echo -e "not identical"
>>> fi
>>> if [ $A==$B ]; then         <------ needs spaces around the ==
>>>     echo -e "identical"
>>> fi
  <.. snip ..>
>
> One equal sign ('=') should be used instead of two equal signs ('==').

The man page for test says you're correct, but just out of curiosity
-- why do two equal signs work?

$ cat /tmp/x
#!/bin/bash
A="A"
B="A"
if [ $A == $B ]; then
    echo  "identical"
fi
if [ $A = $B ]; then
    echo  "identical"
fi

$ bash /tmp/x
identical
identical

Thanks,
Lee

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Re: bash string-operator problem

[Eric Blake]

On 2/21/19 2:17 PM, Lee wrote:

>>
>> One equal sign ('=') should be used instead of two equal signs ('==').
> 
> The man page for test says you're correct, but just out of curiosity
> -- why do two equal signs work?

Use of [ ... == ... ] is a bash extension. It works in bash, but is not
portable to other /bin/sh (notoriously == fails in dash, and is not
required by POSIX).

Oddly enough, POSIX is considering standardizing a common subset of [[;
in that case, [[ ... = ... ]] is non-portable, and the proposal only
documents [[ ... == ... ]] as being valid.  For more than you ever
wanted to know, read http://austingroupbugs.net/view.php?id=375

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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Re: bash string-operator problem

[Eliot Moss]

On 2/21/2019 3:17 PM, Lee wrote:
> On 2/21/19, john doe wrote:
>> On 2/21/2019 5:18 PM, Lee wrote:
>>> On 2/21/19, Rockefeller, Harry  wrote:
>>>> CYGWIN_NT-6.1 HARRYR-PC 3.0.0(0.336/5/3) 2019-02-16 13:21 x86_64 Cygwin

> $ cat /tmp/x
> #!/bin/bash
> A="A"
> B="A"
> if [ $A == $B ]; then
>      echo  "identical"
> fi
> if [ $A = $B ]; then
>      echo  "identical"
> fi
> 
> $ bash /tmp/x
> identical
> identical

My reading of the bash man page says that = and == in this usage
do the same thing.  However, = is Posix compliant, while (presumably)
== is not ...   Eliot Moss

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