Linux - Command Quick Reference

Essential Linux command reference - filesystem, permissions, process management, disk usage, networking, and systemd for daily server work.

Linux — Command Quick Reference

File System Navigation

Move around and locate files quickly.

BASH

pwd                                    ## print working directory
ls -lah /var/log/nginx/                ## list with size, hidden files
cd /var/log/nginx/ && cd -             ## jump, then return to previous dir
find /var/log -name "*.log" -mtime -1  ## files modified in last 1 day
find / -size +100M 2>/dev/null         ## files larger than 100MB
locate nginx.conf                      ## fast indexed search
which python3                          ## show full path of a command

Parameter Breakdown:

ls -lah: Long format, human-readable sizes, includes hidden files
find -mtime -1: Negative number means "within" the last N days
2>/dev/null: Suppresses permission-denied errors when searching from /

File Operations

Copy, move, link, and remove files safely.

BASH

cp -r ./config /etc/orders-api/config   ## copy directory recursively
mv old-report.csv archive/report.csv
rm -i sensitive-file.txt                ## confirm before deleting
rm -rf /tmp/build-cache/                ## force recursive delete, no prompt
mkdir -p /opt/orders-api/{logs,config}  ## create nested dirs in one shot
ln -s /opt/orders-api/current /opt/orders-api/latest

Parameter Breakdown:

-i: Interactive — prompts before overwrite or delete, good default for prod boxes
mkdir -p: Creates parent directories as needed, no error if they exist
ln -s: Symbolic link — points to a path, breaks if the target moves

Permissions and Ownership

Control who can read, write, and execute.

BASH

chmod 644 app.conf                      ## rw-r--r--
chmod 755 deploy.sh                     ## rwxr-xr-x
chmod -R 750 /opt/orders-api/           ## recursive, owner+group only
chown deploy:deploy /opt/orders-api/    ## change owner and group
chgrp devops /var/log/orders-api/
umask 022                               ## default permission mask for new files

Command Parameter Table:

Value	Meaning
`644`	Owner read/write, group and others read-only
`755`	Owner full access, group and others read/execute
`750`	Owner full access, group read/execute, others nothing

Process Management

Find, monitor, and control running processes.

BASH

ps aux | grep orders-api                ## find a process by name
top                                     ## live process viewer
htop                                    ## improved interactive viewer
kill 4521                               ## graceful terminate (SIGTERM)
kill -9 4521                            ## force kill (SIGKILL)
pkill -f orders-api                     ## kill by name match
nice -n 10 ./batch-export.sh            ## run with lower priority
jobs                                    ## list background jobs in this shell
fg %1                                   ## bring job 1 to foreground

Parameter Breakdown:

kill -9: Last resort — bypasses cleanup, the process cannot trap or ignore it
nice -n: Range is -20 (highest priority) to 19 (lowest); only root can go negative
ps aux: a = all users, u = user-readable format, x = include no-tty processes

Disk and Storage

Check usage and manage mounted filesystems.

BASH

df -h                                   ## disk space per filesystem
du -sh /var/log/*                       ## size per subdirectory
du -sh /opt/orders-api/ --max-depth=1
lsblk                                   ## list block devices
mount | grep /data                      ## show mount details for /data
fdisk -l                                ## list partitions (needs root)

Parameter Breakdown:

df -h: Human-readable totals — first command when you get "no space left on device"
du -sh: Summary, human-readable — use to find which directory is eating disk
lsblk: Tree view of disks and partitions, no root required

Networking

Diagnose connectivity from the command line.

BASH

ip addr show                            ## list interfaces and IPs
ss -tulnp                               ## listening ports and owning process
curl -I https://api.razorpay.com/health ## headers only, fast health check
curl -o output.json https://api.example.com/data
wget https://example.com/release.tar.gz
ping -c 4 db.internal.razorpay.net      ## 4 packets, then stop
traceroute api.razorpay.com             ## hop-by-hop path
dig orders-api.payments.svc.cluster.local

Command Parameter Table:

Flag	Description
`ss -tulnp`	TCP/UDP listening sockets with process name and PID
`curl -I`	HEAD request — checks status without downloading the body
`dig`	DNS lookup — shows resolved IP and which server answered

Text Processing

Search, transform, and filter file content.

BASH

grep -r "ERROR" /var/log/orders-api/    ## recursive search
grep -i "timeout" app.log               ## case-insensitive
grep -c "WARN" app.log                  ## count matching lines
sed 's/staging/production/g' config.yaml > config-prod.yaml
awk '{print $1, $4}' access.log         ## print columns 1 and 4
cut -d',' -f2 report.csv                ## extract field 2 from CSV
sort access.log | uniq -c | sort -rn    ## frequency count, descending

Parameter Breakdown:

grep -c: Returns a count instead of the matching lines themselves
sed 's/old/new/g': Global find-replace, prints to stdout unless -i is added
sort | uniq -c | sort -rn: Classic pattern for "most frequent line" analysis

Archiving and Compression

Package files for transfer or backup.

BASH

tar -czvf backup-2026-06-19.tar.gz /opt/orders-api/   ## create gzipped archive
tar -xzvf backup-2026-06-19.tar.gz -C /restore/       ## extract to a directory
tar -tzvf backup-2026-06-19.tar.gz                    ## list contents, no extract
zip -r config-backup.zip ./config/
unzip config-backup.zip -d ./restored/
gzip large-export.csv                                  ## compress in place
gunzip large-export.csv.gz

Parameter Breakdown:

-czvf: Create, gzip, verbose, file — the standard combo for backups
-xzvf: Extract, gzip, verbose, file — the matching restore combo
-C: Extracts into a target directory instead of the current one

System Monitoring

Check load and resource health.

BASH

uptime                                  ## load average + how long up
free -h                                 ## memory usage, human-readable
vmstat 2 5                              ## 5 samples, 2 seconds apart
iostat -x 2                             ## disk I/O stats every 2 seconds
dmesg -T | tail -50                     ## recent kernel messages, timestamped
journalctl -p err -since "1 hour ago"   ## recent error-level logs

Parameter Breakdown:

uptime: The three load numbers are 1, 5, and 15-minute averages
free -h: Watch the available column, not just free — it accounts for reclaimable cache
dmesg -T: -T converts raw timestamps into human-readable dates

systemd Service Management

Control and inspect services on modern Linux distros.

BASH

systemctl status orders-api
systemctl start orders-api
systemctl stop orders-api
systemctl restart orders-api
systemctl enable orders-api             ## start automatically on boot
systemctl disable orders-api
journalctl -u orders-api -f             ## stream logs for one service
journalctl -u orders-api --since today

Parameter Breakdown:

enable vs start: enable controls boot behavior, start controls right now — they're independent
journalctl -u: Filters the systemd journal to one unit's logs only
-f: Follows the log in real time, same idea as tail -f

Useful Aliases and One-Liners

Shortcuts worth adding to .bashrc.

BASH

alias ll='ls -lah'
alias ports='ss -tulnp'
alias df='df -h'
alias du='du -h'

## Find and kill whatever is listening on a port
lsof -i :8080 | awk 'NR==2{print $2}' | xargs kill -9

## Top 10 largest directories under current path
du -ah . | sort -rh | head -10

## Watch a log file with color-highlighted errors
tail -f app.log | grep --color=always -E "ERROR|$"