Linux Commands — quick reference

Processes and load

Command	Purpose
`uptime`	Load averages (1/5/15 min)
`top -o %MEM` / `htop`	Live process view
`ps -eo pid,ppid,user,%cpu,%mem,etime,cmd --sort=-%cpu \| head`	Top CPU consumers
`pgrep -af nginx`	Find processes by name
`strace -f -p <pid> -e trace=network`	Trace syscalls of a running process
`lsof -p <pid>`	Open files/sockets of a process
`kill -TERM <pid>` then `kill -KILL <pid>`	Graceful, then forced

Command	Purpose
`free -h`	Memory incl. cache/buffers
`vmstat 1 5`	Paging, IO wait, context switches
`df -hT`	Filesystem usage
`du -xsh /var/* 2>/dev/null \| sort -h \| tail`	What is filling the disk
`lsof +L1`	Deleted-but-open files holding space
`iostat -xz 1 3`	Disk latency and utilisation
`dmesg -T \| tail -50`	Kernel messages (OOM killer, disk errors)

Command	Purpose
`find /var/log -name '*.log' -mtime -1 -size +50M`	Recent large logs
`grep -rn --include='*.conf' 'listen' /etc/nginx`	Recursive search
`tail -F /var/log/app.log`	Follow, survive rotation
`journalctl -u nginx -f --since "10 min ago"`	Service logs
`sed -n '120,140p' file`	Print a line range
`awk -F: '$3 >= 1000 {print $1}' /etc/passwd`	Human users
`stat -c '%U:%G %a %n' /etc/ssh/sshd_config`	Owner and mode