gflohr:

Unless you explicitly close it, you do have an STDIN available, and the script is hanging because it's waiting for input that'll never arrive.

If the script is yours or you can change it, I'd probably try:

• Looking for an environment variable that can indicate whether you're in the Docker container and use it to bail out of your program, or
• Use alarm() to add a timeout for user input so it can't hang indefinitely.

If the script isn't yours, but you're using fork() to invoke the script, then you could try duplicating the STDIN stream and closing it before forking, so the child script won't have an open STDIN. That should make it immediately return with undef in $input. (That may require some jiggery-pokery with IO::Handle or maybe a double-fork, where the child of the initial fork can close its STDIN and then fork to the script you want to run.)

...roboticus

When your only tool is a hammer, all problems look like your thumb.

my $input = <STDIN>; It is clear that this will not work in my container because there is no standard input. I want it to fail and bail out. The problem is that it never returns. How can I detect that situation in a clean way?

If you want to detect whether the user can enter input (not directly whether it's running inside a Docker container), I'd suggest IO::Interactive, or at the very least -t STDIN. Or, you could use a prompting module instead, depending on what you're actually trying to implement.

this is relevant Detecting interactive/non-interactive shell.

$ echo "yes" | docker run --rm -i debian perl -e 'print(<STDIN>)'
yes
[download]

$ docker run --rm debian perl -e 'print(<STDIN> or "(EOF)")'
(EOF)
[download]

G'day gflohr,

I'm a bit late to the party on this one; however, I wondered if fileno might help you:

$input = <STDIN> if defined fileno(STDIN);
[download]

I don't have docker to test but it seems to work equally well on both UNIX-like and MSWin OSes.

Cygwin:

ken@titan ~/tmp
$ perl -E 'my $input = "none"; $input = <STDIN> if defined fileno(STDI
+N); say $input'
qwerty
qwerty


ken@titan ~/tmp
$ perl -E 'my $input = "none"; close STDIN; $input = <STDIN> if define
+d fileno(STDIN); say $input'
none

ken@titan ~/tmp
[download]

Win10 - cmd.exe:

C:\Users\ken\tmp>perl -E  "my $input = 'none'; $input = <STDIN> if def
+ined fileno(STDIN); say $input"
qwerty
qwerty


C:\Users\ken\tmp>perl -E  "my $input = 'none'; close STDIN; $input = <
+STDIN> if defined fileno(STDIN); say $input"
none

C:\Users\ken\tmp>
[download]

Win10 - PowerShell:

PS C:\Users\ken\tmp> perl -E 'my $input = "none"; close STDIN; eval { 
+defined fileno(STDIN) } and do { $input = <STDIN> }; say $input'
none
PS C:\Users\ken\tmp> perl -E 'my $input = "none"; $input = <STDIN> if 
+defined fileno(STDIN); say $input'
qwerty
qwerty

PS C:\Users\ken\tmp> perl -E 'my $input = "none"; close STDIN; $input 
+= <STDIN> if defined fileno(STDIN); say $input'
none
PS C:\Users\ken\tmp>
[download]

And in case you were wondering, those are using different Perl installations for Cygwin and Win10:

This is perl 5, version 30, subversion 0 (v5.30.0) built for cygwin-th
+read-multi
This is perl 5, version 30, subversion 0 (v5.30.0) built for MSWin32-x
+64-multi-thread
[download]

You don't need v5.30; that's just what I have installed. I suspect the guts of that code would work on pretty much any Perl 5.