Linux – Why can I see password prompts through redirecting output

linuxredirectionsshstderrstdout

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?

Best Answer

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,

Related Solutions

Different output when redirecting

Edit: I cannot be certain this is how mysql does it, but it could be using isatty(3) to determine whether STDOUT is a terminal or not, and modifying the output accordingly.

Edit 2: The mysql command line tool definitely uses isatty(). You can read the source code.

There are some good examples of this (although not in C) over at Stack Overflow:

To answer your question, "Why?": Because that is what the documentation says. See the reference manual:

When used interactively, query results are presented in an ASCII-table format. When used noninteractively (for example, as a filter), the result is presented in tab-separated format.

I suspect this decision was made for readability. When used interactively, mysql can assume a human is reading the output, and it is generally easier for us humans to read data that is delimited by those lines. When used non-interactively, the assumption is that another program is going to consume the output, and that tab-delimited output is easier to programmatically consume.

If you are interested in overriding this default and getting ASCII-table format when redirecting the output, you can use the --table (-t) command line option:

mysql -t -e 'select a,b from tablefoo' databasename > file

Best Answer

Related Solutions

Different output when redirecting

Related Question