Linux disk full or no space left on device
Prove which filesystem is full, check inodes, then rank directories and files before deleting anything.
df -h
problem hubs
Start from the Linux problem, then choose the command.
These hubs collapse common search wording into repair paths: first safe command, expected output, when not to use it, supporting commands, guides, and drills.
9 problem hubs
Problem library
SSH Permission denied publickey
Inspect auth logs, authorized_keys, file modes, and effective sshd policy before changing access.
grep -E 'Failed publickey|Accepted publickey|Authentication refused' /var/log/auth.log | tail -50
Linux permission denied
Trace owner, mode, ACL, symlink, and parent-directory permissions before recursive chmod or chown.
namei -l /var/www/example/index.html
Nginx 502 Bad Gateway
Separate Nginx config, upstream listener, proxy target, and application failure before reloading services.
sudo nginx -t
DNS not resolving on Linux
Compare resolver answers, authoritative answers, TTL, direct HTTP checks, and server reachability before changing DNS again.
dig +short example.com A
Port already in use on Linux
Find the listener, process, bind address, and firewall context before killing or restarting anything.
ss -ltnp
APT or dpkg repair needed
Inspect package state, held packages, candidates, and simulated changes before running repair commands.
apt-get -s upgrade
High CPU process on Linux
Rank CPU consumers and capture process context before killing or restarting a service.
ps -eo pid,ppid,stat,pcpu,pmem,comm,args --sort=-pcpu | head
High memory process on Linux
Check memory, swap, top memory processes, containers, and OOM logs before restarting workloads.
free -h