GNU bug report logs - #47859
Additional seq outlandish example: seq 0 dangers

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>
To: bug-coreutils <at> gnu.org
Subject: Additional seq outlandish example: seq 0 dangers
Date: Sun, 18 Apr 2021 09:26:28 +0800

On (info "(coreutils) seq invocation") we read
   Be careful when using ‘seq’ with outlandish values: otherwise you
   may...

Here's another 'fun/sad/DDOS yourself' example you might add:

One day I wrote a Makefile,
m:
	seq 0 9|sed s/$$/號.html/|xargs make
but before using it, I though I'll just test with one item,
m:
	seq 0  |sed s/$$/號.html/|xargs make
well of course... as seq prints nothing here,
this triggered a massive ever growing recursive loop...

Yes, all my fault for picking 0. I'll pick 1 next time.

P.S., perhaps document how to get seq to cough up just "0". One way I
found was:
$ seq 0 1 0
0

Message #8 received at 47859 <at> debbugs.gnu.org (full text, mbox):

From: Erik Auerswald <auerswal <at> unix-ag.uni-kl.de>
To: 47859 <at> debbugs.gnu.org
Subject: Re: bug#47859: Additional seq outlandish example: seq 0 dangers
Date: Sun, 18 Apr 2021 08:29:59 +0200

Hi,

On Sun, Apr 18, 2021 at 09:26:28AM +0800, 積丹尼 Dan Jacobson wrote:
> On (info "(coreutils) seq invocation") we read
>    Be careful when using ‘seq’ with outlandish values: otherwise you
>    may...
> 
> Here's another 'fun/sad/DDOS yourself' example you might add:
> 
> One day I wrote a Makefile,
> m:
> 	seq 0 9|sed s/$$/號.html/|xargs make
> but before using it, I though I'll just test with one item,
> m:
> 	seq 0  |sed s/$$/號.html/|xargs make
> well of course... as seq prints nothing here,
> this triggered a massive ever growing recursive loop...
> 
> Yes, all my fault for picking 0. I'll pick 1 next time.
> 
> P.S., perhaps document how to get seq to cough up just "0". One way I
> found was:
> $ seq 0 1 0
> 0

I would like to add more information to this bug report with the intent of
helping everybody involved now or in the future.

A slighly simpler method to make 'seq' print just '0' is:

    $ seq 0 0
    0

This is documented, but more generally, e.g., in 'seq --help':

    $ seq --help
    Usage: seq [OPTION]... LAST
      or:  seq [OPTION]... FIRST LAST
      or:  seq [OPTION]... FIRST INCREMENT LAST
    Print numbers from FIRST to LAST, in steps of INCREMENT.
    [...]
    If FIRST or INCREMENT is omitted, it defaults to 1.  [...]
    [...]

Thus, 'seq 0' is the same as 'seq 1 1 0' and 'seq 0 0' is the same as
'seq 0 1 0'.

The default value of '1' for omitted parameters affects other values, too,
not just '0':

    $ seq -1
    $ seq -1 -1
    -1
    $ seq -100000
    $ seq -100000 -100000
    -100000

When "FIRST" and "LAST" are the same, any valid "INCREMENT" value results
in 'seq' printing just one value, not just the default of '1':

    $ seq 0 200 0
    0
    $ seq 0 -200 0
    0
    $ seq 0 0 0
    seq: invalid Zero increment value: ‘0’
    Try 'seq --help' for more information.

Thus IMHO a possible addition to the documentation should probably not
just single out 'seq 0', but mention any number smaller than the default
value for "FIRST" of '1'.

HTH, HAND
Erik
-- 
Inside every large problem is a small problem struggling to get out.
                        -- Hoare's Law of Large Problems

Message #11 received at 47859 <at> debbugs.gnu.org (full text, mbox):

From: Bernhard Voelker <mail <at> bernhard-voelker.de>
To: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>,
 47859 <at> debbugs.gnu.org
Subject: Re: bug#47859: Additional seq outlandish example: seq 0 dangers
Date: Mon, 19 Apr 2021 08:23:41 +0200

On 4/18/21 3:26 AM, 積丹尼 Dan Jacobson wrote:
> Here's another 'fun/sad/DDOS yourself' example you might add:
> 
> One day I wrote a Makefile,
> m:
> 	seq 0 9|sed s/$$/號.html/|xargs make
> but before using it, I though I'll just test with one item,
> m:
> 	seq 0  |sed s/$$/號.html/|xargs make
> well of course... as seq prints nothing here,
> this triggered a massive ever growing recursive loop...
> 
> Yes, all my fault for picking 0. I'll pick 1 next time.

Besides the main discussion about seq(1), I want to add that xargs(1) [0]
is often run most usefully with:

  -r, --no-run-if-empty        if there are no arguments, then do not run COMMAND;
                                 if this option is not given, COMMAND will be
                                 run at least once

Portability hint:
Unfortunately, the -r option is not required by POSIX.  At least, the BSD family
(FreeBSD [1], NetBSD [2], OpenBSD [3]) has it; on Solaris [4], only the '1g'
variant [5] has it.  Not sure about other implementations.

[0] https://www.gnu.org/software/findutils/manual/html_node/find_html/xargs-options.html
[1] https://www.freebsd.org/cgi/man.cgi?xargs
[2] https://man.netbsd.org/xargs.1
[3] https://man.openbsd.org/xargs.1
[4] https://docs.oracle.com/cd/E88353_01/html/E37839/xargs-1.html
[5] https://docs.oracle.com/cd/E88353_01/html/E37839/xargs-1g.html

Have a nice day,
Berny

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