Re: <<expr 100 * 100>> returns syntax error

[Adam Dinwoodie <adam at dinwoodie dot org>]

On Thu, Jul 28, 2016 at 12:55:43PM +0000, 45ownu+e60iporpd8lec@guerrillamail.com wrote:
> Steps to reproduce (I cannot update cygwin on this server, but it also happen on 2.5.2(0.297/5/3)):
> Main window:
>   $ uname -r
>   1.7.25(0.270/5/3)
>   $ expr 100 * 100
>   10000
> 
> ---------------
> Other window:
>   $ $ uname -r
>   1.7.25(0.270/5/3)
>   $ HOST=myhostname
>   $ echo 'whatever' > $HOST.txt
> 
> ---------------
> Main window:
>   $set -xv 
>   $expr 100 * 100
>   + expr 100 myhostname.txt 100
>   syntax error
> 
> ------------------------------------------------
> 
> When this happen I can only fix by rebooting the server, which is obviously unideal!
> Here my questions:
>    1) How can I reset cygwin to the initial behaviour without rebooting?
>    2) why does this happen? how to prevent this?

This is working as intended: '*' is a shell glob character, so Bash will
attempt to expand it.

In the first case, you're evidently in a directory that contains no
files or directories (excluding ones that start with a '.'), so the glob
doesn't expand and is left as-is.  This means the `expr` command sees
the arguments as you typed them: `100 * 100`.

In the second case, you've created a file called myhostname.txt in this
directory, so the '*' glob will expand to the list of matching file
names, i.e. myhostname.txt.  This means the `expr` command will see the
arguments after the glob expansion, i.e. `100 myhostname.txt 100`.

Run the following commands in a shell and you might see what's
happening:

    mkdir tmp; cd tmp
    echo *
    touch file1
    echo *
    touch file2
    echo *
    touch file3
    echo *

See also https://en.wikipedia.org/wiki/Glob_(programming)

You can disable this behaviour with `set -f`, or by escaping or quoting
the '*' character (i.e. `expr 100 \* 100` or `expr 100 '*' 100`).

HTH

Adam

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