/home/alex>perl -Mstrict -w -E 'say fileno(STDERR) ? "Filehandle" : "N
+A"'
Filehandle

/home/alex>perl -Mstrict -w -E 'say fileno(STDOUT) ? "Filehandle" : "N
+A"'
Filehandle

/home/alex>perl -Mstrict -w -E 'say fileno(STDIN) ? "Filehandle" : "NA
+"'
NA

/home/alex>

[download]

Oooops ...

In a program running in background, you might have closed STDIN, this will very likely make your next opened file have a fileno of 0.

/home/alex>perl -Mstrict -w -E 'open my $f,"<","/dev/null"; say fileno
+($f)'
3

/home/alex>perl -Mstrict -w -E 'close STDIN; open my $f,"<","/dev/null
+"; say fileno($f)'
0

/home/alex>

[download]

So better use defined:

/home/alex>perl -Mstrict -w -E 'my $f="foo"; say defined(fileno($f)) ?
+ "filehandle" : "not a filehandle"'
not a filehandle

/home/alex>perl -Mstrict -w -E 'open my $f,"<","/dev/null"; say define
+d(fileno($f)) ? "filehandle" : "not a filehandle"'
filehandle

/home/alex>perl -Mstrict -w -E 'open my $f,"<",\my $mem; say defined(f
+ileno($f)) ? "filehandle" : "not a filehandle"'
filehandle

/home/alex>perl -Mstrict -w -E 'close STDIN; open my $f,"<","/dev/null
+"; say defined(fileno($f)) ? "filehandle" : "not a filehandle"'
filehandle

/home/alex>

[download]

Update:

fileno called on a string has some nasty surprises:

/home/alex>perl -Mstrict -w -E 'say fileno(STDERR); say fileno("STDERR
+"); say fileno("stderr"); say fileno("foobar")//"undef"; say fileno($
+_)//"undef" for (qw( STDERR stderr foo ))'
Name "main::foobar" used only once: possible typo at -e line 1.
2
2
2
undef
2
2
undef


/home/alex>

[download]

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

In reply to Re^3: check scalar holds filehandle by afoken
in thread check scalar holds filehandle by bagyi

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