Troubleshooting

Named after the error you were given: what it means, and what to do about it.

apt update failed

apt update reports a failure, exits 0 and carries on with an index fetched days ago. How to read the transcript and find the repository that broke.
command not found

The shell says this for a command that is missing, one that is not on PATH, one it remembers in the wrong place, and a script whose interpreter is gone.
Could not get lock /var/lib/dpkg/lock-frontend

apt refuses to run because something else is already installing. Modern apt tells you exactly which process, and deleting the lock file is the wrong fix.
No space left on device

A write failed for want of space. Find which filesystem is full, whether it ran out of blocks or inodes, and why deleting the big file often frees no space.
Package installation failed

apt install stops before dpkg runs. Telling a package apt has never heard of from one it cannot download, and why --fix-missing is rarely the answer.
The following packages have been kept back

apt-get upgrade refuses to install new packages, so an upgrade that needs one is held back instead. What the message means and the three things that cause it.
Permission denied

The message names the path and never the reason. Work out which check failed, which account was refused, and when the mode is not the thing doing the refusing.
The repository does not have a Release file

apt asked a repository for dists/<suite>/Release and got a 404. Telling an archived release from a typo and from a flat repository, and what each one needs.
The repository is not signed

apt cannot verify a repository's signature: the NO_PUBKEY and Missing key errors. What Debian 13 changed, and why apt-key adv is no longer the fix.
Works in the shell, fails in cron

cron hands a job five variables and a PATH of /usr/bin:/bin, runs it from your home directory under dash, and sends the failure somewhere you are not looking.
A service won't start

A service will not come up, and systemctl either reported a job failure or stayed silent. Read the status, read the journal, then branch on the code.
sudo: command not found

On Debian, sudo is often simply not installed, and your user is not in the sudo group. Why the installer does that, and how to fix it from a root shell.
Text file busy

A deploy failed because the program is running. What ETXTBSY protects, why rm then copy works, and the rename that avoids the window between them.
Unmet dependencies

apt refuses to install and lists unmet dependencies. The three ways it words them, what each rules out, and finding the hold or conflict behind the refusal.
Why is this process using so much CPU?

The machine is slow and one process is at the top of top. Find it, check whether the figure means what it seems to, see what it is doing, and make it give way.
Why is this process using so much memory?

A process looks huge in top, or free shows almost nothing free. Check whether memory is really short, which of RSS, VSZ and PSS to believe, and how to limit it.