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Accepted
Kyle Smith
Asked: 2011-01-20 09:43:39 +0800 CST

ssh fails to execute remote command when run from cron bash script - works from CLI

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I have a script, written in bash, which runs from cron. One of the first things it does is SSH to a remote host and retrieve a list of files in a directory. Everything works fine when run from the command line, but not from cron.

The section of the script originally looked like this:

FILES=$($SSH_BINARY -i $SSH_KEY $SSH_USER@$REMOTE_HOST "ls $REMOTE_DIRECTORY")
echo "Got files = $FILES"

I added an echo statement above that line (shown below) to prove it wasn't a path or variable issue:

echo "$SSH_BINARY -i $SSH_KEY $SSH_USER@$REMOTE_HOST \"ls $REMOTE_DIRECTORY\""

If I take the resultant output line and run it as the same user cron will (root), it works without issue.

Thinking it may have something to do with assignment to a variable, I modified the FILES= line to read (thus, putting the output directly into my last_run_output file):

$SSH_BINARY -vv -i $SSH_KEY $SSH_USER@$REMOTE_HOST "ls $REMOTE_DIRECTORY"

The cron entry looks like this:

34 * * * * /root/path/to/my/script/get_logs.sh > /root/path/to/last_run_output 2>&1

So, PATH, variable assignment, and permissions issues aside, I started using debug flags in ssh. I ran once from the command line, then from cron, and compared the outputs. Here are some highlights from the diff:

The - side is the unsuccessful attempt, the + side is the successful attempt, run outside of cron.

@@ -87,9 +77,7 @@
 debug1: Remote: X11 forwarding disabled.
 debug1: Remote: Forced command: /home/sshacs/acssshsink netstorageuser
 debug1: Authentication succeeded (publickey).
-debug2: fd 4 setting O_NONBLOCK
 debug2: fd 5 setting O_NONBLOCK
-debug2: fd 6 setting O_NONBLOCK
 debug1: channel 0: new [client-session]
 debug2: channel 0: send open
 debug1: Entering interactive session.

I can't explain why these extra file descriptors are mentioned in debug2 when run from cron, but it appears to be related (notice read<=0 rfd 4 len 0 line below):

@@ -100,20 +88,672 @@
 debug2: callback done
 debug2: channel 0: open confirm rwindow 0 rmax 32768
 debug2: channel 0: rcvd adjust 131072
-debug2: channel 0: read<=0 rfd 4 len 0
-debug2: channel 0: read failed
-debug2: channel 0: close_read
-debug2: channel 0: input open -> drain
-debug2: channel 0: ibuf empty
-debug2: channel 0: send eof
-debug2: channel 0: input drain -> closed
+  [[ Very large output of the ls command ]]
+debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
 debug2: channel 0: rcvd eof
 debug2: channel 0: output open -> drain
 debug2: channel 0: obuf empty
 debug2: channel 0: close_write
 debug2: channel 0: output drain -> closed
-debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
 debug2: channel 0: rcvd close
+debug2: channel 0: close_read
+debug2: channel 0: input open -> closed
 debug2: channel 0: almost dead
 debug2: channel 0: gc: notify user
 debug2: channel 0: gc: user detached

Any ideas are greatly appreciated.

linux bash ssh shell-scripting

2 Answers

  1. Best Answer

    Try adding -t -t to your SSH connection options. This will; force a pseudo-terminal to be allocated.

    Multiple -t options force tty allocation, even if ssh has no local tty.

    • 8

  2. You should debug your script further. Two things to try:

    Rwrite it to do file output directly instead of through output redirection. That is, in the script do this:

    $SSH_BINARY -i $SSH_KEY $SSH_USER@$REMOTE_HOST "ls $REMOTE_DIRECTORY" > savefile

    Run your script with bash -x inside cron and save result. Change first line of script to #!/bin/bash -x and try something like

    34 * * * * /root/path/to/my/script/get_logs.sh > script_debug 2>&1

    that should help you narrow down exactly what your script is doing. I suspect redirection of output in cron is not working correctly in this case.

    Also, dumb question: have you verified that root can write to /root/path/to/last_run_output? Have you verified that you don't have noclobber set somehow when the script runs in cron?

    EDIT: further troubleshooting ideas based on comments from OP.

    So none of the above ideas seem to be working. What about saving the fielist in a file on the remote machine and scp'ing it back? That would eliminate any weird quoting or input redirection problems.

    • 1