CODEWITHSUNDEEP
pythonterminalsubprocessttystty
blindsnowmobile
blindsnowmobile

Reputation: 4308

Terminal messed up (not displaying new lines) after running Python script

I have a Python script I use to execute commands in parallel across multiple hosts using the Python subprocess module. It wraps SSH, and basically makes a call like this:

output = subprocess.Popen(["/bin/env", env, "/usr/bin/ssh", "-t", "%s@%s" % (user, host), "--", command], stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()

The effective command gets executed like this:

/bin/env TERM=$TERM:password /usr/bin/ssh -t "%s@%s" % (user, host), "--", command

It works fine, except I get an intermittent error where my terminal gets messed up (loses newlines) after running the script. A "reset" from the command line fixes it, but I'm not sure how this is happening, exactly. I noticed that sometimes there's a "\r\n" at the end of the the first item in the tuple's output, and sometimes it's not there. See the following, specifically "Permission denied\r\n":

**** Okay output ****
[user@/home/user]# ./command.py hosts.lists "grep root /etc/shadow"
Running command "grep root /etc/shadow" on hosts in file "hosts.test"
('grep: /etc/shadow: Permission denied\r\n', 'Connection to server1.example.com closed.\r\n')
('grep: /etc/shadow: Permission denied\r\n', 'Connection to server2.example.com closed.\r\n')
[user@/home/user]#


**** Output causes terminal to not display newlines ****
[user@/home/user]# ./command.py hosts.list "grep root /etc/shadow"
('grep: /etc/shadow: Permission denied\r\n', 'Connection to server1.example.com closed.\r\n')
('grep: /etc/shadow: Permission denied\n', 'Connection to server2.example.com closed.\r\n')
                                 [user@/home/user]# [user@/home/user]# [user@/home/user]

The second output has been slightly modified, but shows the missing "\r", and how my prompt gets "wacked" after running the script.

I think this is related to using the "-t" option in my subprocess command. Somehow I'm losing the \r. If I remove the "-t" option, this issue goes away, but long story short, I need it for passing through environmental variables for use on the remote machine (I'm hackishly using the TERM variable to pass through the user's password for sudo purposes, because I can't assume AcceptEnv is allowing arbitrary variable passing on the remote sshd server; I'm doing this to avoid passing the password on the command line, which will show up in the process list on the remote machine).

Just wondering if anyone knows a way to get around this, without removing the "-t" option?

UPDATE: It looks like my tty settings get altered after running the subprocess.Popen(...).communicate() command within my script, regardless of whether or not I actually print the output to screen. I find that really strange. Here are the before/after differences in my tty config (from stty -a):

-ignbrk brkint ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff
-ignbrk brkint ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff


opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0

isig icanon -iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt

I'm wondering how to stop communicate() from altering my terminal settings? Is it possible, or is this a bug?

Upvotes: 11

Views: 5213

Answers (2)

jhylands
jhylands

Reputation: 1014

I found that 

stty sane

restores the console to how it was before. I didn't really understand the other answer here so I this helps someone.

Found the answer here.

Upvotes: 11

Lennon Machado
Lennon Machado

Reputation: 51

I had the same issue in a Perl script. To solve the problem I had to save the current settings of the local terminal (in order to restore it at the end of the script) and prepending "stty -raw" before executing the remote command.

So in Perl:

#Save current terminal settings (you may add the PID in the filename)

`stty -g > ~/tmp/.currentTtySettings`;

#Execute remote command prepending "stty -raw"

my @out=`ssh -t -q [email protected] "stty -raw ; grep root /etc/shadow"`;

#Restore terminal settings

`stty \`cat ~/tmp/.currentTtySettings\``;

Hope it helps you!

Other very useful links:

-Detailed explanation of ssh and tty (-t option) https://unix.stackexchange.com/questions/151916/why-is-this-binary-file-being-changed

-Some Perl and ssh inspiration http://search.cpan.org/~bnegrao/Net-SSH-Expect-1.09/lib/Net/SSH/Expect.pod

-How to avoid "-t" for sudo https://unix.stackexchange.com/questions/122616/why-do-i-need-a-tty-to-run-sudo-if-i-can-sudo-without-a-password

Upvotes: 5

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