> ## Documentation Index > Fetch the complete documentation index at: https://notes.vvkhash.com/llms.txt > Use this file to discover all available pages before exploring further. # Linux Essentials: Commands Every Engineer Must Know > Core Linux commands for filesystem navigation, text processing, process management, and system inspection — a daily-use reference for working engineers. Linux mastery is built on muscle memory. After 25+ years working with RHEL, CentOS, Fedora, Ubuntu, Debian, and FreeBSD systems, the commands below are the ones I reach for every single day. This page is deliberately dense — it is a reference, not a tutorial. If a flag or idiom looks unfamiliar, run `man ` to dig deeper. *** ## Filesystem Navigation ```bash ls variants theme={null} ls -la # long listing including hidden files ls -lh # human-readable sizes (K, M, G) ls -lt # sort by modification time, newest first ls -lS # sort by file size, largest first ls --color=auto -F # colorized + type indicator (/, *, @) ``` ```bash find essentials theme={null} # Find by name (case-insensitive) find /etc -iname "*.conf" -type f # Find files modified in the last 24 hours find /var/log -mtime -1 -type f # Find files larger than 100 MB find / -size +100M -type f 2>/dev/null # Find and execute on results find /tmp -name "*.tmp" -exec rm -f {} \; # Find SUID binaries (security audit) find / -perm -4000 -type f 2>/dev/null ``` ```bash locate / pwd / cd theme={null} pwd # print current working directory cd - # jump back to previous directory cd ~username # jump to another user's home # locate uses a prebuilt index — fast but not real-time locate nginx.conf sudo updatedb # refresh the locate database ``` Use `find` when you need real-time results or complex filters. Use `locate` when you want speed and the file hasn't changed recently. Run `sudo updatedb` in a cron job nightly to keep the index fresh. *** ## File Operations ```bash copy / move / remove theme={null} cp -av /src /dst # archive mode + verbose (preserves permissions/timestamps) cp -p file1 file2 # preserve mode, ownership, timestamps cp --backup=numbered f1 f2 # keep numbered backups of the destination mv -iv oldname newname # interactive + verbose rename/move rm -rf /path/to/dir # force-remove recursively (no confirmation — be careful) rm -i file # prompt before each removal ``` ```bash mkdir / chmod / chown theme={null} mkdir -p /opt/app/{bin,conf,logs,tmp} # create nested dirs in one shot chmod 755 /opt/app/bin/start.sh # rwxr-xr-x chmod -R 644 /var/www/html # recursive on files chmod u+x,g-w script.sh # symbolic notation chown -R app:app /opt/app # owner:group recursive chown --reference=ref.txt target.txt # copy ownership from reference file ``` ```bash symlinks theme={null} ln -s /opt/app/current/bin/app /usr/local/bin/app # soft link ln /data/file.dat /backup/file.dat # hard link # Update an existing symlink atomically ln -sfn /opt/app/v2.1 /opt/app/current ``` `rm -rf` with a misplaced space or variable is catastrophic. Always double-check the path. Consider `trash-cli` on workstations, or at minimum `alias rm='rm -i'` in your `.bashrc`. *** ## Text Processing This is where Linux really shines. Master these tools and you rarely need a dedicated log-analysis GUI. ```bash theme={null} grep -rn "ERROR" /var/log/app/ # recursive + line numbers grep -i "timeout" access.log # case-insensitive grep -v "DEBUG" app.log # invert match (exclude) grep -E "WARN|ERROR|CRIT" syslog # extended regex (alternation) grep -A3 -B3 "OOM" /var/log/messages # 3 lines of context around match grep -c "404" access.log # count matching lines grep -l "pattern" /etc/**/*.conf # list filenames only ``` ```bash theme={null} # Print specific columns (tab/space-delimited) awk '{print $1, $4}' access.log # Sum a column awk '{sum += $5} END {print "Total:", sum}' report.txt # Filter rows by field value awk '$9 == "500"' access.log # Use custom field separator awk -F: '{print $1, $3}' /etc/passwd # Print lines between two patterns awk '/START/,/END/' logfile.txt ``` ```bash theme={null} sed -i 's/oldstring/newstring/g' file.conf # in-place replace sed -i.bak 's/foo/bar/g' file.conf # with .bak backup sed -n '10,20p' bigfile.txt # print lines 10–20 sed '/^#/d' config.conf # delete comment lines sed 's/[[:space:]]*$//' file.txt # strip trailing whitespace ``` ```bash theme={null} # cut — slice fields from delimited input cut -d: -f1,3 /etc/passwd # fields 1 and 3 cut -c1-80 wide.txt # first 80 characters per line # sort sort -k2 -n file.txt # numeric sort on column 2 sort -t, -k3 -rn data.csv # CSV, col 3, reverse numeric sort -u names.txt # sort + deduplicate # uniq (input must be sorted first) sort ips.txt | uniq -c | sort -rn # count occurrences, rank by frequency # wc wc -l file.txt # line count wc -w file.txt # word count ls /var/log/*.log | wc -l # count log files # head / tail head -n 50 app.log # first 50 lines tail -n 100 app.log # last 100 lines tail -f /var/log/syslog # follow in real time tail -F /var/log/app.log # follow + reopen if rotated ``` *** ## Process Management ```bash Inspect processes theme={null} ps aux # all processes, BSD style ps -ef # all processes, POSIX style ps aux | grep nginx # filter by name ps -o pid,ppid,cmd,%mem,%cpu --sort=-%cpu | head -20 # custom columns, sort by CPU top # live view (press M to sort by memory, P by CPU) htop # nicer UI; F5 for tree view, F6 to sort ``` ```bash Control processes theme={null} kill -15 # SIGTERM — graceful shutdown (default) kill -9 # SIGKILL — force kill (last resort) killall -HUP nginx # send SIGHUP (reload) to all nginx processes pkill -u www-data # kill all processes owned by www-data nice -n 10 ./heavy-job.sh # start with lower priority (range: -20 to 19) renice -n 15 -p # change priority of running process ``` ```bash Background jobs theme={null} nohup ./long-task.sh > /tmp/task.log 2>&1 & # survive terminal logout disown -h %1 # detach job from shell # screen screen -S mysession # start named session screen -ls # list sessions screen -r mysession # reattach # Inside screen: Ctrl+A D to detach, Ctrl+A K to kill # tmux tmux new -s deploy # new named session tmux ls # list sessions tmux attach -t deploy # reattach # Inside tmux: Ctrl+B D to detach, Ctrl+B [ to scroll ``` Prefer `kill -15` (SIGTERM) first — it gives the process a chance to flush buffers and clean up. Only escalate to `kill -9` (SIGKILL) if the process doesn't respond after a few seconds. *** ## System Information ```bash OS / hardware identity theme={null} uname -r # kernel version uname -a # full kernel info (arch, hostname, date) hostnamectl # systemd-based hostname + OS info cat /etc/os-release # distribution name and version lscpu # CPU topology, cores, threads, cache lsblk -f # block devices + filesystem types + mount points lspci # PCI devices (NICs, GPUs, controllers) lsusb # USB devices dmidecode -t system # hardware/BIOS info (requires root) ``` ```bash Uptime / load theme={null} uptime # uptime + 1/5/15-min load averages w # who is logged in + their activity last reboot # reboot history ``` ```bash Disk and memory theme={null} df -hT # disk usage, human-readable + filesystem type df -i # inode usage (critical for many small files) du -sh /var/* # size of each item under /var du -h --max-depth=2 /opt | sort -rh | head -20 # top disk consumers free -m # memory in MB (total/used/free/cache/available) vmstat 2 5 # virtual memory stats, 5 samples every 2 seconds cat /proc/meminfo # full memory details ``` *** ## Package Management Quick Reference ```bash theme={null} apt update # refresh package index apt upgrade # upgrade all packages apt install nginx # install a package apt remove nginx # remove (keep config) apt purge nginx # remove + purge config apt autoremove # remove orphaned dependencies apt search "web server" # search by keyword apt show nginx # package details dpkg -l | grep nginx # check if installed dpkg -L nginx # list installed files dpkg -S /usr/sbin/nginx # which package owns a file ``` ```bash theme={null} dnf check-update # list available updates dnf upgrade # apply all updates dnf install httpd # install a package dnf remove httpd # uninstall dnf search "web server" # search dnf info httpd # package details dnf history # transaction history dnf history undo last # rollback last transaction rpm -qa | grep httpd # list installed RPMs matching name rpm -ql httpd # files owned by package rpm -qf /usr/sbin/httpd # which RPM owns a file ``` ```bash theme={null} pkg update # refresh index pkg upgrade # upgrade all pkg install nginx # install pkg delete nginx # remove pkg search nginx # search pkg info nginx # details pkg which /usr/local/sbin/nginx # which package owns a file ``` *** ## User and Group Management ```bash Users theme={null} useradd -m -s /bin/bash -G wheel appuser # create user with home + shell + group usermod -aG docker appuser # add to additional group usermod -L appuser # lock account usermod -U appuser # unlock account userdel -r appuser # delete user + home directory passwd appuser # set/change password chage -l appuser # show password expiry info chage -M 90 appuser # password expires in 90 days id appuser # show UID, GID, groups ``` ```bash Groups theme={null} groupadd developers # create group groupmod -n devs developers # rename group groupdel devs # delete group gpasswd -d appuser devs # remove user from group getent group docker # list members of a group ``` ```bash sudo / su theme={null} su - appuser # switch user (full login shell) sudo -u appuser /opt/app/start.sh # run as another user sudo -l # list your sudo privileges visudo # safely edit /etc/sudoers # Add line in sudoers for passwordless sudo: # appuser ALL=(ALL) NOPASSWD: /bin/systemctl restart app ``` Never edit `/etc/sudoers` directly. Always use `visudo`, which validates syntax before saving. A broken sudoers file can lock you out of root access. *** ## Related Pages Turn these one-liners into reusable, robust shell scripts. Network diagnostics, SSH, firewalls, and DNS troubleshooting. Systematic workflows for CPU, memory, disk, and service issues. CI/CD, containers, and orchestration context for these commands.