Linux Terminal Commands

bash 5.x — Ubuntu 24.04 LTS

# Case-sensitive! ls ≠ LS. Pipe | chains commands. && runs next only if previous succeeds

logan@ubuntu:~$ ls -lah /var/log | grep "\.log" | sort -k5 -rh | head -5

-rw-r--r-- 1 root adm 42M Apr 28 syslog

-rw-r----- 1 syslog adm 18M Apr 28 auth.log

Critical: Linux is always case-sensitive — commands, filenames, and paths. ls works; LS → "command not found". This is the most common source of confusion when switching from Windows or macOS.

// Navigation & File Operations

ls -lah --color=auto

-l=long -a=all hidden -h=human sizes

List directory. -a shows dot files, -S sort by size, -t sort by time. --sort=extension for ext grouping.

cd /etc | cd .. | cd ~ | cd -

absolute · parent · home · previous

Change directory. cd - returns to previous directory. cd ~user goes to that user's home.

cp -rv /src /dst

-r=recursive -v=verbose

Copy files/dirs. -p preserves timestamps and permissions. -u only copies if source is newer.

rm -rf /path/to/dir

⚠ PERMANENT — no Recycle Bin

Delete permanently. -r for dirs, -f skips confirmation. No undo. Never run rm -rf / as root.

cat f | less f | head -20 | tail -f

print · page · top N · live follow

View file content. less: q=quit /=search space=next page. tail -f follows live log writes.

mkdir -p parent/child/sub

-p creates full path tree

Create directory. -p creates all parent directories — no error if already exists.

// Search & Text Processing

grep -rni "pattern" /path/

-r=recurse -n=line# -i=case insensitive

Search text. -l for filenames only. -c for count. --include="*.log" to scope. -E for extended regex.

find /etc -name "*.conf" -type f

-name -type -mtime -size -user -perm

Find files. -mtime -1 for last day. -size +100M for large files. -exec rm {} \; to act on results.

sed -i 's/old/new/g' file.txt

stream editor — in-place replace

Replace text in files. /g = all occurrences. Without -i, outputs to stdout. -E for extended regex.

awk '{print $1, $3}' file.txt

$0=line $1=first field $NF=last

Extract columns. -F: sets field separator. Powerful for parsing logs, CSVs, /etc/passwd.

sort -k2 -rn | uniq -c

-k=key -r=reverse -n=numeric

Sort lines. uniq -c counts duplicates. Classic combo: sort | uniq -c | sort -rn for frequency.

wc -l file | cut -d: -f1 /etc/passwd

count · field extract

wc counts lines/words/bytes. cut extracts specific fields from delimited text. Essential for log parsing.

// Process & System Management

ps aux | grep nginx

a=all users u=format x=no-tty

List processes. Pipe to grep to filter. top/htop/btop for interactive live monitoring.

kill -9 PID | killall nginx

-9=SIGKILL (force) -15=SIGTERM

SIGTERM (15) requests graceful shutdown; SIGKILL (9) forces immediate termination.

systemctl status/start/stop nginx

systemd service control

Manage systemd services. enable/disable controls auto-start. journalctl -u nginx -f for logs.

df -h | du -sh /var/log/*

disk free · disk usage

df shows filesystem usage. du shows directory sizes. -h for human-readable MB/GB output.

ss -tlnp

replaces netstat — faster

Show listening sockets. -t=TCP -l=listening -n=numeric -p=process. Filter: ss -anp | grep :80

ssh user@host -p 22 -i ~/.ssh/key

-p=port -i=identity file

Secure shell. -L 8080:localhost:80 for port forward. -N for tunnel only. scp for file transfers.

// Linux Permissions — Read, Set, Understand

- rwx r-x r-- root staff 4096 script.sh

■ Type: - file | d dir | l symlink

■ Owner: rwx = 7 (read+write+exec)

■ Group: r-x = 5 (read+exec)

■ Other: r-- = 4 (read only)

Symbolic

chmod u+x script.sh

chmod a+r public.html

u=user g=group o=other a=all

Octal

chmod 755 script.sh # rwxr-xr-x

chmod 644 config.txt # rw-r--r--

chmod 600 id_rsa # rw-------

// Power Patterns — Pipes & Chaining

grep -i "fail" /var/log/auth.log | awk '{print $NF}' | sort | uniq -c | sort -rn | head

Top failed login IPs

find / -perm -4000 -type f 2>/dev/null

Find all SUID files (security audit)

cat /etc/passwd | cut -d: -f1 | sort | uniq

List all unique usernames

ps aux | awk '{sum+=$3} END {print "Total CPU%:", sum}'

Sum CPU across all processes