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. Choose the safest first command.
Repair paths for common Linux failures: first safe command, expected output, what to read next, and what not to change yet.
32 problem hubs
Problem library
Docker disk full on Linux
Measure Docker image, container, volume, and build-cache usage before pruning anything that may hold data.
docker system df -v
Journal logs too large on Linux
Measure systemd journal usage and retention before vacuuming logs or deleting journal directories.
journalctl --disk-usage
Read-only filesystem on Linux
Check mount options and kernel storage errors before remounting read-write or editing fstab.
findmnt -no TARGET,OPTIONS /
Too many open files on Linux
Check system file-handle pressure, process open files, and service limits before raising limits or restarting services.
cat /proc/sys/fs/file-nr
Cron job not running on Linux
Check the active crontab, cron service, logs, user, environment, and script assumptions before rewriting the schedule.
crontab -l
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
systemd service failed
Read service state, result fields, current-boot logs, restart loops, and unit drop-ins before restarting or editing a service.
systemctl status app-worker --no-pager --lines=50
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
Package lock held on Linux
Find the package manager process, lock owner, and timer before deleting lock files or forcing package repair.
ps -ef | grep -E 'apt|dpkg|dnf|yum|pacman' | grep -v grep
Bad interpreter or CRLF script error
Inspect the shebang and line endings before rewriting scripts or changing execute permissions.
head -1 script.sh | cat -v
Firewall port blocked on Linux
Prove the service is listening, then read host firewall rules before opening ports.
ss -ltnp
Mount failed from fstab
Verify fstab entries and boot-time mount failures before rebooting or editing storage paths.
findmnt --verify
OOM killer logs on Linux
Confirm whether the kernel killed a process for memory pressure before restarting workloads or changing limits.
journalctl -k --since '24 hours ago' --no-pager | grep -iE 'out of memory|oom-killer|killed process'
User is not in sudoers
Check identity, groups, and sudo policy before editing sudoers or adding broad admin access.
id username
Nginx 403 forbidden
Diagnose path permissions, Nginx root/alias config, access rules, and error logs before changing modes or ownership.
sudo nginx -t
Certbot renewal failed
Use dry-run renewal, certificate inventory, Certbot logs, and DNS checks before editing live TLS config.
sudo certbot renew --dry-run
Default route missing on Linux
Inspect route table, interface addresses, NetworkManager state, and resolver state before restarting networking.
ip route
Logrotate not running
Debug logrotate config and timer state before forcing rotation or truncating logs.
logrotate -d /etc/logrotate.conf
Boot partition full on Linux
Check /boot usage, installed kernels, and the running kernel before cleanup.
df -h /boot
SSH known_hosts changed warning
Verify stored and presented host keys out of band before editing known_hosts.
ssh-keygen -F hostname
Nginx serving the wrong SSL certificate
Inspect the live certificate, Nginx server block, Certbot inventory, and DNS before reissuing or reloading TLS config.
openssl s_client -connect example.com:443 -servername example.com </dev/null 2>/dev/null | openssl x509 -noout -subject -issuer -dates
Apache 403 forbidden
Check Apache syntax, enabled sites, document root, Directory rules, path permissions, and error logs before chmod or ownership changes.
sudo apachectl configtest
Docker container restart loop
Inspect container status, logs, restart policy, exit code, and recent events before rebuilding or deleting containers.
docker ps -a --format 'table {{.Names}}\t{{.Image}}\t{{.Status}}\t{{.RestartCount}}'
systemd timer not running
Inspect timer schedule, status, unit file, and recent journal entries before enabling, starting, or editing timers.
systemctl list-timers --all --no-pager
SSH too many authentication failures
Check auth logs, loaded agent keys, explicit identity selection, and verbose client output before changing sshd or deleting keys.
grep -i 'Too many authentication failures' /var/log/auth.log /var/log/secure 2>/dev/null | tail -20