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vobelic
Asked: 2014-05-25 09:00:29 +0800 CST

Capturing output from a remotely executed command via SSH fails in CRON

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This question is in a direct relation to this one: ssh fails to execute remote command when run from cron bash script - works from CLI

I'm not able to comment on the accepted answer as I don't have enough rep so please bear with me.

I'm running a script on a linux PC machine and the host i'm trying to get the output from is a router with its OS so it's nothing I can influence in terms of configuring the console. Basically executing this under cron: OUT=$(ssh -tt -vv [email protected] "remote command") gets me an empty variable.

debug1: Sending command: remote command
debug2: channel 0: request exec confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 2621440 rmax 262144
debug2: channel 0: read<=0 rfd 4 len 0
debug2: channel 0: read failed

If I'm executing this outside cron, i.e. in the CLI i'm getting output as expected. As you can see the -tt option to force pseudo-tty allocation doesn't help.

Any solutions for this to help cron overcome the buggy remote console?

linux

3 Answers

  1. Your command print results stderr or stdout?

    You can reroute stderr to stdout using

    OUT=$(ssh -tt -vv [email protected] "remote command" 2>&1 )
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  2. Did you consider to use fully qualified paths for the cronjob?

    Use /usr/bin/ssh user@host instead of ssh user@host

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  3. Some routers or embedded devices have very bad SSH implementations. I've seen cases where they only know how to read commands from the console, and cannot deal with commands passed on the command line. If this is the case you have, you may want to try piping the remote command into the remote ssh.

    echo remote command | ssh user@host

    If this helps, you may have better luck trying to code something with expect.

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