Linux Commands

Practical reference for everyday Linux command-line work. Prefer the safer/more explicit form of a command when one is listed.

Files and directories

• ls -la -- list all files (including hidden) with permissions, owner, size.
• cp -a src dst -- copy preserving permissions/timestamps/symlinks.
• mv src dst -- move/rename.
• rm -i file -- remove with a confirmation prompt. Prefer -i over a bare rm for anything destructive; never run rm -rf on a path built from an unchecked variable (rm -rf "$dir"/ when $dir is empty deletes /'s contents relative to cwd).
• mkdir -p a/b/c -- create nested directories, no error if they exist.
• find . -name "*.log" -mtime +7 -delete -- find and delete files matching a pattern older than 7 days. Run without -delete first to see what would match.
• du -sh * -- size of each item in the current directory, human-readable.
• df -h -- disk space per mounted filesystem, human-readable.

Text and files content

• grep -rn "pattern" . -- recursive search with line numbers.
• grep -riE "pattern1|pattern2" -- case-insensitive, extended regex, OR.
• sed -i.bak 's/old/new/g' file -- in-place replace, keeping a .bak backup (drop .bak only once you've verified the result).
• awk -F, '{print $2}' file.csv -- print the 2nd comma-separated field.
• head -n 50 file / tail -n 50 file / tail -f file -- first/last lines, or follow a growing file (logs).
• wc -l file -- count lines.
• diff -u a b -- unified diff between two files.
• sort -u file -- sort and remove duplicate lines.

Processes

• ps aux -- every process, full listing.
• ps aux | grep -i name -- find a process by name (matches its own grep too; add | grep -v grep or use pgrep -f name instead).
• top / htop -- live resource usage (htop is nicer if installed).
• kill -TERM pid -- ask a process to exit gracefully. Only use kill -KILL (-9) if -TERM didn't work after a few seconds -- it skips cleanup.
• pkill -f pattern -- kill by command-line pattern match. Double-check the pattern against pgrep -f pattern first; it's easy to match more than intended.
• nohup cmd & or setsid cmd & -- run a command that survives the terminal closing.
• jobs, fg, bg -- manage jobs in the current shell session.

Permissions and ownership

• chmod 644 file -- owner read/write, group/other read-only (typical for regular files). 755 is the equivalent for directories/executables.
• chmod +x script.sh -- make a file executable without touching other bits.
• chown user:group file -- change owner and group together.
• sudo -n true -- test whether passwordless sudo is available without actually running a privileged command (useful before scripting sudo into something that might otherwise hang on a password prompt).

Networking

• curl -sf URL -- fetch quietly, fail (non-zero exit) on an HTTP error instead of printing the error body as if it were the response.
• curl -I URL -- headers only, no body (quick status/content-type check).
• ss -tlnp -- listening TCP sockets with the owning process (modern replacement for netstat -tlnp).
• dig +short name / nslookup name -- resolve a hostname.
• scp file host:path / rsync -avz src/ host:dst/ -- copy files over SSH; prefer rsync for anything resumable or repeated (only transfers changed data).

System info

• uname -a -- kernel/arch summary.
• uptime -- how long the system has been running, load averages.
• free -h -- memory usage, human-readable.
• systemctl status name / journalctl -u name -f -- a systemd service's status, or follow its live logs.

Gotchas worth remembering

• Always quote variables that hold paths: "$var", not $var -- an unquoted variable containing a space or being empty changes what a command actually operates on.
• cd failing silently in a script (e.g. a typo'd path) means every following command runs in the wrong directory, not that nothing happens -- chain it as cd "$dir" && rest-of-command, or set -e in scripts.
• Redirecting into a file you're also reading from in the same pipeline (sort file > file) truncates the file before sort finishes reading it, losing the data. Use a temp file or sponge instead.
• sudo re-reads PATH from the invoking user's environment by default in most distros, but a script relying on sudo some-alias won't work -- aliases aren't inherited by non-interactive shells.