Can not evoke watch command with non-integer time option

This is a locale problem. watch uses strtod(3), which is locale-dependent, to convert the argument to -n to a double.

To fix the problem, you need to either specify the argument to -n with a different separator:

watch -n 0,1 w

Or change your locale to a setting where the period character is used for the decimal point:

export LC_NUMERIC=en_US.UTF-8
watch -n 0.1 w

A couple of references:

1. A relevant portion of the Linux manpage for strtod:

A decimal number consists of a nonempty sequence of decimal digits possibly containing a radix character (decimal point, locale-dependent, usually '.')

2. You can review your current settings by running locale in your terminal:

   locale
   LANG=en_US.UTF-8
   LC_CTYPE="en_US.UTF-8"
   LC_NUMERIC="en_US.UTF-8"
   LC_TIME="en_US.UTF-8"
   LC_COLLATE="en_US.UTF-8"
   LC_MONETARY="en_US.UTF-8"
   ...
   

3. The source code in question can be reviewed at gitlab:

   https://gitlab.com/procps-ng/procps/blob/85fff468fa263cdd2ff1c0144579527c32333695/watch.c#L625

   https://gitlab.com/procps-ng/procps/blob/85fff468fa263cdd2ff1c0144579527c32333695/lib/strutils.c#L49

(edit 2017-09-07): updated gitlab links

Just a complement to zackse's fine answer.

There are two problems with:

LC_NUMERIC=en_US.UTF-8 watch -n 0.1 w

as a work around for the fact that watch expect numbers formatted in the user's convention while you expect it to be in the English format.

1. That doesn't work if LC_ALL is set. LC_ALL overrides all other locale settings including LC_NUMERIC. The work around would be to use:

   LC_ALL=en_US.UTF-8 watch -n 0.1 w
   

   but then it would make the second point below even worse

2. the command started by watch (in this case w) inherits that LC_NUMERIC. So instead of outputting its numbers in a format expected by the user, it will output it in the US English format.

Ideally here, we'd want to tell watch to run w every tenth of a second (regardless of the user's locale) without affecting the behaviour of the w command (which should always give an output understandable by the user in his own locale).

With the yash shell, you can do it with:

 watch -n "$((0.1))" w

yash is one of the 3 Bourne-like shells that support floating point arithmetics (the other ones being zsh and ksh93). It's the only one that does internationalisation properly though. zsh always uses . as the decimal mark, and ksh93 honours the one from the locale event in its internal syntax.

For yash, . is the decimal mark for its arithmetic syntax, but it honours the locale's one upon input/output.

Another trick you can use here is to avoid inputting the decimal mark altogether by using scientific notation:

watch -n 1e-1 w

Or you can query the decimal mark from the locale:

m=$(locale decimal_point)
watch -n "0${m}1" w