On 2020-05-14, loic.tregouet wrote:
> Hi,
>
> I've reproduced a strange behaviour of grep command when a file
> name of a single letter is present in current directory .
>
> $ echo a | grep [a-z]
> a
> $ touch t
> $ echo a | grep [a-z]
> $ rm t
> $ echo a | grep [a-z]
> a
> $
>
>
> Any idea ?
You are not quoting the pattern so it is being expanded by the
shell. The shell sees the argument as a pathname pattern to be
expanded. If there is no matching file name, the shell leaves the
argument unchanged and passes [a-z] to grep. If there is a matching
file name, as there is after you touch t, then the shell expands
that pattern to the matching file t and passes t to grep.
The solution is to enclose the pattern in quotes, either single or
double, like this:
$ echo a | grep "[a-z]"
or to escape one or both of the brackets, like this:
$ echo a | grep \[a-z\]
Regards,
Gary
