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So, I've got a command that I'm issuing that is essentially 'ssh'ing into various hosts:

 command 2&>1 | grep "desired output" 

Every once in a while, in the midst of the command, it'll run into a host where I don't have my keys setup. When that happens, I get this output:

 Password: 

Since I'm redirecting STDERR into STDOUT, then filtering STDOUT to only give me "desired output" (which doesn't include "Password:"), how is the password prompt being presented to me?

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1 Answer 1

up vote 4 down vote accepted

ssh opens /dev/tty for read and write to prompt for the password.

I guess this a security feature, the input has to be from the tty rather than stdin.

If you run strace ssh <host> strace will show you the system calls the command is making.

I get:

open("/dev/tty", O_RDWR|O_LARGEFILE)    = 4
...
write(4, "dave@host"..., 16dave's password: ) = 16
read(4,
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1  
From my understanding that's also the reason, why you can't simply pass the password to ssh from a script with a redirect. You'll need expect (expect.sourceforge.net and see also stackoverflow.com/questions/459182/…) –  mpy Mar 21 '13 at 15:04
    
Terrific. I didn't think to dig in and strace it, since it was a passing curiosity, but thanks for that. I'm sure the knowledge will come in useful! –  Matt Simmons Mar 21 '13 at 15:08
    
@terdon - the command you want (I think) is in the middle of the answer posted by parkydr: strace ssh <host> –  Kevin Fegan Mar 21 '13 at 18:22
    
@KevinFegan thanks, I know. The post has been edited since I posted my comment. Check the editing log. –  terdon Mar 21 '13 at 18:24

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