Why does autossh spawn dozens of instances of itself and ssh?

Question

I'm running autossh on a Mac (Mavericks) from a bash shell script controlled by launchd. Unfortunately, they way I've set this up causes autossh to spawn dozens of instances of both itself and ssh. Eventually the shell seems to grind to a halt and remote connects become impossible. If I killall both autossh and ssh things get back to normal.

This on the host:

Axe:~ mnewman$ ps -A | grep -c -w ssh
41
Axe:~ mnewman$ ps -A | grep -c -w autossh
152

This on the client:

MrMuscle:~ mnewman$ ssh -p19990 mnewman@localhost
Last login: Sat Jul  6 16:19:50 2019 from localhost
-bash: fork: Resource temporarily unavailable
-bash-3.2$

It took me a long time to get this running. So far, I haven't a clue how to figure out what's going on here.

The shell script:

#!/bin/bash

/opt/local/bin/autossh -f -M 0 -N -o ExitOnForwardFailure=yes -o ServerAliveInterval=30 -o ServerAliveCountMax=2 -R 19990:localhost:22 [email protected] -p 10000 

The launchd plist file which is in ~/Library/LaunchAgents:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Debug</key>
    <false/>
    <key>Disabled</key>
    <false/>
    <key>EnvironmentVariables</key>
    <dict>
        <key>PATH</key>
        <string>/opt/local/bin:/opt/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin
</string>
    </dict>
    <key>KeepAlive</key>
    <true/>
    <key>Label</key>
    <string>com.mgnewman.autossh</string>
    <key>ProgramArguments</key>
    <array>
        <string>/Users/mnewman/bin/autossh.sh</string>
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>StandardErrorPath</key>
    <string>/Users/mnewman/Desktop/autossh.txt</string>
    <key>StandardOutPath</key>
    <string>/Users/mnewman/Desktop/autossh.txt</string>
    <key>WorkingDirectory</key>
    <string>/Users/mnewman/Documents</string>
</dict>
</plist>

What have I done here to spawn so many instances of autossh and ssh?

Answer 1

Clearly it doesn't require to keep script alive, but ssh.
Again, this is what autossh is used for (to automatically reconnect ssh connection when dropped).

Set KeepAlive to false
or, if still require you may fine tune it like below:

<key>KeepAlive</key>
<dict>
     <key>SuccessfulExit</key>
     <false/>
</dict>

Also, it's good to add line exit 0 at end of script.

Finally, if this is the only purpose of script, to run autossh, consider to add autossh in launchd.

---

plist file with autossh
(I don't have an environment to test but it should work.)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Debug</key>
    <false/>
    <key>Disabled</key>
    <false/>
    <key>KeepAlive</key>
    <true/>
    <key>Label</key>
    <string>com.mgnewman.autossh</string>
    <key>ProgramArguments</key>
    <array>
        <string>/opt/local/bin/autossh</string>
        <string>-M</string>
        <string>0</string>
        <string>-N</string>
        <string>-o</string>
        <string>ExitOnForwardFailure=yes</string>
        <string>-o</string>
        <string>ServerAliveInterval=30</string>
        <string>-o</string>
        <string>ServerAliveCountMax=2</string>        
        <string>-R</string>
        <string>19990:localhost:22</string>
        <string>[email protected]</string>
        <string>-p</string>        
        <string>10000</string>        
    </array>
    <key>RunAtLoad</key>
    <true/>
    <key>StandardErrorPath</key>
    <string>/Users/mnewman/Desktop/autossh.txt</string>
    <key>StandardOutPath</key>
    <string>/Users/mnewman/Desktop/autossh.txt</string>
    <key>WorkingDirectory</key>
    <string>/Users/mnewman/Documents</string>
</dict>
</plist>  

Changes:
Placed autossh as program argument with each piece of argument, except -f (to send in background) which is no longer needed.
Omitted environment variable PATH as we have autossh with full path.

Welcome.