Add basic linux troubleshooting tips (#1885)

Author: samfreund

docs/source/docs/troubleshooting/index.md

@@ -7,4 +7,5 @@ common-errors
 logging
 camera-troubleshooting
 networking-troubleshooting
+unix-commands
 ```

docs/source/docs/troubleshooting/unix-commands.md

@@ -0,0 +1,132 @@
+# Useful Unix Commands
+
+## Networking
+
+### SSH
+
+[SSH (Secure Shell)](https://www.mankier.com/1/ssh) is used to securely connect from a local to a remote system (ex. from a laptop to a coprocessor). Unlike other commands on this page, ssh is not Unix specific and can be done on Windows and MacOS from their respective terminals.
+
+:::{note}
+You may see a warning similar to `The authenticity of host 'xxx' can't be established...` or `WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!`, in most cases this can be safely ignored if you have confirmed that you are connecting to the correct host over a secure connection, and the fingerprint will change when your operating system is reinstalled or PhotonVision's coprocessor image is re-flashed. This can also occur if you have multiple coprocessors with the same hostname on your network. You can read more about it [here](https://superuser.com/questions/421997/what-is-a-ssh-key-fingerprint-and-how-is-it-generated)
+:::
+
+Example:
+
+```
+ssh pi@hostname
+```
+
+For PhotonVision, the username will be `pi` and the password will be `raspberry`.
+
+### ip
+
+Run [ip address](https://www.mankier.com/8/ip) with your coprocessor connected to a monitor in order to see its IP address and other network configuration information.
+
+Your output might look something like this:
+
+```
+2: end1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
+    link/ether de:9a:8f:7d:31:aa brd ff:ff:ff:ff:ff:ff
+    inet 10.88.47.12/24 brd 10.88.47.255 scope global dynamic noprefixroute end1
+        valid_lft 27367sec preferred_lft 27367sec
+```
+
+In this example, the numbers following `inet` (10.88.47.12) are your IP address.
+
+### ping
+
+[ping](https://www.mankier.com/8/ping) is a command-line utility used to test the reachability of a host on an IP network. It also measures the round-trip time for messages sent from the originating host to a destination computer. It can be used to determine if a network interface is available, which can be helpful when debugging.
+
+## File Transfer
+
+All files under `/opt/photonvision` are owned by the root user. This means that if you want to modify them, the commands to do so must be ran as sudo.
+
+### SCP
+
+[SCP (Secure Copy)](https://www.mankier.com/1/scp) is used to securely transfer files between local and remote systems.
+
+Example:
+
+```
+scp [file] pi@hostname:/path/to/destination
+```
+
+### SFTP
+
+[SFTP (SSH File Transfer Protocol)](https://www.mankier.com/1/sftp#) is another option for transferring files between local and remote systems.
+
+### Filezilla
+
+[Filezilla](https://filezilla-project.org/) is a GUI alternative to SCP and SFTP. It is available for Windows, MacOS, and Linux.
+
+## Miscellaneous
+
+### v4l2-ctl
+
+[v4l2-ctl](https://www.mankier.com/1/v4l2-ctl) is a command-line tool for controlling video devices.
+
+List available video devices (used to verify the device recognized a connected camera):
+
+```
+v4l2-ctl --list-devices
+```
+
+List supported formats and resolutions for a specific video device:
+
+```
+v4l2-ctl --list-formats-ext --device /path/to/video_device
+```
+
+List all video device's controls and their values:
+
+```
+v4l2-ctl --list-ctrls --device path/to/video_device
+```
+
+:::{note}
+This command is especially useful in helping to debug when certain camera controls, like exposure, aren't behaving as expected. If you see an error in the logs similar to `WARNING 30: failed to set property [property name] (UsbCameraImpl.cpp:646)`, that means that PhotonVision is trying to use a control that doesn't exist or has a different name on your hardware. If you encounter this issue, please [file an issue](https://github.com/PhotonVision/photonvision/issues) with the necessary logs and output of the `v4l2-ctl --list-ctrls` command.
+:::
+
+### systemctl
+
+[systemctl](https://www.mankier.com/1/systemctl) is a command that controls the `systemd` system and service manager.
+
+Start PhotonVision:
+
+```
+systemctl start photonvision
+```
+
+Stop PhotonVision:
+
+```
+systemctl stop photonvision
+```
+
+Restart PhotonVision:
+
+```
+systemctl restart photonvision
+```
+
+Check the status of PhotonVision:
+
+```
+systemctl status photonvision
+```
+
+### journalctl
+
+[journalctl](https://www.mankier.com/1/journalctl) is a command that queries the systemd journal, which is a logging system used by many Linux distributions.
+
+View the PhotonVision logs:
+
+```
+journalctl -u photonvision
+```
+
+View the PhotonVision logs in real-time:
+
+```
+journalctl -u photonvision -f
+```