A bug on cat command for zsh

A bug on cat command for zsh

[Wang, Yichao]

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Hi there,

I am using zsh 5.9 (x86_64-apple-darwin23.0) on MacOS.

The following command in zsh will cause severe damage by running forever into a dead loop.
- when you accidentally mess up the redirection flows

```bash
touch 1.txt
echo "anything" > 1.txt
cat < 1.txt >> 1.txt
```

However, on Linux bash, this behavior is prohibited and hinted.

```bash
cat < 1.txt >> 1.txt
cat: -: input file is output file
```

Best,
Unknown name

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Re: A bug on cat command for zsh

[Andreas Kähäri]

On Mon, Dec 04, 2023 at 07:01:42AM +0000, Wang, Yichao wrote:
> Hi there,
> 
> I am using zsh 5.9 (x86_64-apple-darwin23.0) on MacOS.
> 
> The following command in zsh will cause severe damage by running forever into a dead loop.
> - when you accidentally mess up the redirection flows
> 
> ```bash
> touch 1.txt
> echo "anything" > 1.txt
> cat < 1.txt >> 1.txt
> ```
> 
> However, on Linux bash, this behavior is prohibited and hinted.
> 
> ```bash
> cat < 1.txt >> 1.txt
> cat: -: input file is output file
> ```
> 
> Best,
> Unknown name

Allowing the programmer to shoot themselves in the foot does not
seem like a bug to me.  Note that bash release 5.2.15 does not
seem to babysit the user in the way that you describe by default.
Rather, it's the coreutils implementation of cat that does this.

You may have two implementaions of cat on your system (it's possibly
a BSD system, or at least non-GNU system, with coreutils installed
alongside the native utilities).  Check with "type cat" in both shells
to see what cat is being picked up.


-- 
Andreas (Kusalananda) Kähäri
Uppsala, Sweden

.

Re: A bug on cat command for zsh

[Wang, Yichao]

Hi Andreas,

Thanks for pointing that out. 

Yep, I see such different behavior (just by chance when playing around with command line) from MacOS(which is BSD system) and Linux (which follows GNU). I think you’re right they have different implementations of `cat`.

Have a nice day!


> On Dec 4, 2023, at 12:01 AM, Andreas Kähäri <andreas.kahari@abc.se> wrote:
> 
> On Mon, Dec 04, 2023 at 07:01:42AM +0000, Wang, Yichao wrote:
>> Hi there,
>> 
>> I am using zsh 5.9 (x86_64-apple-darwin23.0) on MacOS.
>> 
>> The following command in zsh will cause severe damage by running forever into a dead loop.
>> - when you accidentally mess up the redirection flows
>> 
>> ```bash
>> touch 1.txt
>> echo "anything" > 1.txt
>> cat < 1.txt >> 1.txt
>> ```
>> 
>> However, on Linux bash, this behavior is prohibited and hinted.
>> 
>> ```bash
>> cat < 1.txt >> 1.txt
>> cat: -: input file is output file
>> ```
>> 
>> Best,
>> Unknown name
> 
> Allowing the programmer to shoot themselves in the foot does not
> seem like a bug to me.  Note that bash release 5.2.15 does not
> seem to babysit the user in the way that you describe by default.
> Rather, it's the coreutils implementation of cat that does this.
> 
> You may have two implementaions of cat on your system (it's possibly
> a BSD system, or at least non-GNU system, with coreutils installed
> alongside the native utilities).  Check with "type cat" in both shells
> to see what cat is being picked up.
> 
> 
> -- 
> Andreas (Kusalananda) Kähäri
> Uppsala, Sweden
> 
> .