Linux Commands Every Developer Must Know — Complete DevOps Reference
Sanjeev SharmaAdvertisement
Introduction
Why This Matters
Linux powers over 96% of the world's top web servers and virtually every cloud computing platform. Effective Linux command-line proficiency separates engineers who can investigate and resolve production incidents from those who are stuck waiting for help. Whether you are debugging a memory leak, investigating disk exhaustion, or analyzing network connectivity — these commands are your primary diagnostic tools.
File System Navigation and Operations
# Navigation
pwd # print working directory
ls -la # list with permissions, hidden files, sizes
ls -lh --sort=size # sort by size, human-readable
tree -L 2 # tree view, 2 levels deep
cd - # go to previous directory
# File creation and editing
touch file.txt # create empty file or update timestamp
mkdir -p a/b/c # create nested directories
cp -r source/ dest/ # recursive copy
mv old-name new-name # rename or move
rm -rf directory/ # delete directory (no confirmation)
ln -s /path/to/file link # create symbolic link
# File content
cat file.txt # print entire file
less file.txt # paginated view (q to quit)
head -n 20 file.txt # first 20 lines
tail -n 50 file.txt # last 50 lines
tail -f /var/log/app.log # follow live log output
wc -l file.txt # count lines
# Search
grep -r "pattern" /var/log/ # recursive search
grep -n "ERROR" app.log # show line numbers
grep -i "error" app.log # case-insensitive
grep -A 3 -B 3 "exception" app.log # 3 lines context before/after
grep -v "healthcheck" access.log # exclude matching lines
find /var/log -name "*.log" -mtime -1 # logs modified in last day
find /opt -name "*.jar" -size +100M # files larger than 100MB
find /tmp -type f -mtime +7 -delete # delete files older than 7 days
# Archives
tar -czf archive.tar.gz directory/ # compress directory
tar -xzf archive.tar.gz -C /tmp/ # extract to /tmp
tar -tzf archive.tar.gz # list contents
zip -r archive.zip directory/
unzip archive.zip -d /tmp/Permissions and Ownership
# View permissions
ls -la
stat file.txt
# Octal permissions
# 7 = rwx (4+2+1)
# 6 = rw- (4+2)
# 5 = r-x (4+1)
# 4 = r-- (4)
chmod 755 script.sh # rwxr-xr-x — executable script
chmod 644 config.txt # rw-r--r-- — regular file
chmod 600 ~/.ssh/id_rsa # rw------- — private key
chmod +x script.sh # add execute bit
chmod -R 755 /var/www/ # recursive
chown user:group file.txt # change owner and group
chown -R www-data /var/www/ # recursive ownership
sudo chown root:root /etc/cron.d/myjob # root-owned cron
# ACLs (Access Control Lists)
getfacl file.txt
setfacl -m u:deploy:rw file.txt # give deploy user rw accessProcess Management
# View processes
ps aux # all processes with CPU/memory
ps aux | grep node # filter by process name
top # live process monitor (q to quit)
htop # enhanced top (if installed)
pgrep -l nginx # find process IDs by name
# Kill processes
kill PID # graceful SIGTERM
kill -9 PID # force SIGKILL
killall nginx # kill all processes with name
pkill -f "python script.py" # kill by pattern
# Background jobs
command & # run in background
nohup command & # run immune to hangup
jobs # list background jobs
fg %1 # bring job 1 to foreground
bg %1 # send stopped job to background
# System resource usage
free -h # RAM usage
vmstat 1 10 # VM stats, 1s intervals, 10 samples
iostat -x 1 # disk I/O stats
lsof -i :3000 # processes using port 3000
lsof -u username # files opened by userDisk and Storage
# Disk usage
df -h # filesystem disk usage
df -h /var # specific path
du -sh /var/log/ # directory total size
du -sh /* 2>/dev/null | sort -rh | head # find large directories
du -sh /var/log/* | sort -rh | head -20 # top 20 by size
# Find disk hogs
find / -size +1G -type f 2>/dev/null # files larger than 1GB
find /var/log -name "*.gz" -delete # delete compressed logs
# Disk performance
dd if=/dev/zero of=/tmp/test bs=1M count=1024 # write speed test
hdparm -tT /dev/sda # disk read speed
# Mount and partitions
lsblk # list block devices
fdisk -l # list disk partitions (root)
mount | column -t # show mounted filesystems
df -iT # inodes usageNetworking
# Network interfaces and IP
ip addr show # list all interfaces and IPs
ip addr show eth0 # specific interface
ip route show # routing table
hostname -I # local IP addresses
# Connectivity testing
ping -c 4 google.com # ICMP ping (4 packets)
traceroute example.com # trace route to host
mtr example.com # continuous traceroute
curl -I https://example.com # HTTP headers only
curl -v https://example.com # verbose HTTP request
wget -O /dev/null http://...# download speed test
# DNS
nslookup example.com # DNS lookup
dig example.com A # DNS A record
dig +short example.com # short output
dig @8.8.8.8 example.com # query specific DNS server
# Open ports and connections
ss -tlnp # listening TCP ports and processes
ss -s # socket statistics summary
netstat -tlnp # older alternative to ss
lsof -i TCP:443 # processes on port 443
# Firewall
sudo ufw status verbose # Ubuntu firewall status
sudo ufw allow 443/tcp # allow HTTPS
iptables -L -n # list iptables rulesSystem Information and Services
# System info
uname -a # kernel and OS info
cat /etc/os-release # OS distribution info
uptime # system uptime and load average
who # logged-in users
last | head -20 # recent logins
# Systemd services
systemctl status nginx # service status
systemctl start nginx # start service
systemctl stop nginx # stop service
systemctl restart nginx # restart
systemctl reload nginx # reload config (no downtime)
systemctl enable nginx # auto-start on boot
systemctl disable nginx # remove auto-start
journalctl -u nginx -f # follow service logs
journalctl -u nginx --since "1 hour ago" # logs from last hour
# Cron jobs
crontab -l # list cron jobs for current user
crontab -e # edit cron jobs
crontab -l -u www-data # list cron for specific user
cat /etc/cron.d/* # system-wide cron jobsCommon Mistakes
- Using
rm -rf /orrm -rf *without double-checking the current directory — always verifypwdbefore recursive deletions - Running production debug commands without understanding side effects —
strace,tcpdump, andlsofadd overhead - Killing processes with
kill -9before tryingkill—-9does not allow processes to clean up, potentially leaving lock files or corrupt state - Not using
sudo -lto check sudo permissions before escalating — always know what you can do before doing it - Ignoring
ulimitsettings — open file limits and process limits cause mysterious failures at scale
Best Practices
- Use
screenortmuxfor long-running commands on remote servers — prevents losing work when SSH disconnects - Pipe commands to
teewhen you need both terminal output and a file:command | tee output.txt - Use
history | grepto find previously executed commands rather than retyping complex one-liners - Add
set -euo pipefailto scripts — errors exit immediately rather than silently continuing - Use
watch -n 2 'command'to refresh command output every 2 seconds for monitoring
Key Takeaways
findwith-execor-deleteis the standard tool for bulk file operations based on name, age, or sizess -tlnpreplaces the deprecatednetstatfor listing listening ports and associated processes- Process load average in
uptimeshows 1/5/15 minute averages — values above CPU count indicate saturation journalctl -u service --since "1 hour ago"is the modern way to view systemd service logslsof -i :PORTreveals which process owns a port — essential when a port appears already in usedf -hshows disk space by filesystem;du -sh /pathshows how much a directory actually consumes- Permissions
755(rwxr-xr-x) for directories and executables;644(rw-r--r--) for regular files is the standard baseline tail -ffollows log files in real time — combine withgrep --line-bufferedto filter live output
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