Your complete reference guide to essential Linux commands — from file operations and process management to networking, scripting, and beyond.
Disclaimer and Credit: GenAI created contents, prompt-engineered by Jay for FIT3184 students. Any issues or errors please email me (jay dot zhao at monash dot edu).
cd — Change Directory
Description
The cd command changes the current working directory. It is one of the most frequently used commands in any Linux session.
Examples
cd /home/user— navigate to absolute path
cd ..— go up one directory level
cd ~— go to home directory
cd -— return to previous directory
mkdir & rmdir — Create and Remove Directories
mkdir
Creates a new directory. Use -p to create nested directories in one command.
mkdir mydir— create a directory
mkdir -p a/b/c— create nested dirs
rmdir
Removes an empty directory. For non-empty directories, use rm -r instead.
rmdir mydir— remove empty dir
rmdir -p a/b/c— remove nested empty dirs
pwd, ls & tree — View Directory Contents
pwd
Print Working Directory — shows the full path of your current location.
pwd → /home/user/projects
ls / dir
List directory contents. Use ls -la for detailed listing including hidden files.
ls -la /etc
tree
Display directory structure as a visual tree. Use -L 2 to limit depth.
tree -L 2 /var
find & locate — Search for Files
find
Search for files in a directory hierarchy in real time.
find / -name "*.log"— find all .log files
find . -type d— find all directories
find /tmp -mtime +7— files older than 7 days
locate
Quickly find files using a pre-built database. Run updatedb to refresh the index.
locate nginx.conf— find file by name
locate -i readme— case-insensitive search
cp, mv & rm — Copy, Move, Delete Files
cp — Copy
Copy files or directories. Use -r for recursive copy of directories.
cp file.txt /backup/ | cp -r dir/ /backup/
mv — Move/Rename
Move files or rename them. Works across directories.
mv old.txt new.txt | mv file.txt /tmp/
rm — Remove
Delete files or directories. Use -rf with caution — it is irreversible.
rm file.txt | rm -rf /tmp/olddir
cat, head & tail — View File Contents
cat
Concatenate and display file contents. Also used to combine files.
cat file.txt— display file
cat a.txt b.txt > c.txt— combine files
head & tail
View the beginning or end of a file. Use -n to specify line count. tail -f follows live log output.
head -n 20 file.txt
tail -f /var/log/syslog
less & more — Paginate File Output
less
View file contents one page at a time with forward and backward navigation. Preferred over more.
less /var/log/auth.log
- Press
qto quit,/to search
- Press
Gto jump to end
more
Similar to less but only allows forward navigation through a file.
more file.txt
- Press
Spaceto advance a page
- Press
qto quit
touch, ln & shred — Create, Link & Destroy Files
touch
Create an empty file or update a file's timestamp.
touch newfile.txt
ln
Create hard or symbolic links between files.
ln -s /etc/nginx nginx-link
shred
Securely overwrite a file to make recovery difficult before deletion.
shred -u secret.txt
echo, expand & fold — Text Output Utilities
echo
Print text or variable values to the terminal.
echo "Hello World" | echo $HOME
expand
Convert tabs to spaces in a file or input stream.
expand -t 4 file.txt
fold
Wrap long lines of text to a specified width.
fold -w 80 longfile.txt
grep, egrep & fgrep — Search Text Patterns
grep
Search for patterns in files using regular expressions. One of the most powerful Linux tools.
grep "error" /var/log/syslog
grep -r "TODO" ./src/— recursive search
grep -i "warning" file.log— case-insensitive
egrep & fgrep
egrep supports extended regex (same as grep -E). fgrep treats the pattern as a fixed string, not regex.
egrep "error|warn" file.log
fgrep "192.168.1.1" access.log
awk & sed — Stream Editors and Text Processors
awk
A powerful pattern-scanning and processing language. Ideal for column-based data.
awk '{print $1}' file.txt— print first column
awk -F: '{print $1}' /etc/passwd
awk '/error/ {print}' log.txt
sed
Stream editor for filtering and transforming text. Commonly used for find-and-replace.
sed 's/old/new/g' file.txt
sed -n '5,10p' file.txt— print lines 5–10
sed '/^#/d' config.txt— delete comment lines
sort, cut & tr — Transform Text Data
sort
Sort lines of text files alphabetically or numerically.
sort file.txt | sort -n numbers.txt | sort -r file.txt
cut
Extract specific columns or fields from each line of a file.
cut -d: -f1 /etc/passwd — extract usernames
tr
Translate or delete characters from input.
echo "hello" | tr 'a-z' 'A-Z' → HELLO
diff, cmp & split — Compare and Divide Files
diff
Compare two files line by line and show differences. Use -u for unified format (used in patches).
diff file1.txt file2.txt | diff -u old.c new.c
cmp
Compare two files byte by byte. Returns the position of the first difference.
cmp file1.bin file2.bin
split
Split a large file into smaller pieces by line count or byte size.
split -l 1000 bigfile.txt part_
tac, paste & join — Combine and Reverse Text
tac
Print file contents in reverse line order (opposite of cat).
tac file.txt
paste
Merge lines of files side by side, separated by a delimiter.
paste file1.txt file2.txt
join
Join lines of two files on a common field, like a database JOIN.
join file1.txt file2.txt
wc, fmt & column — Count and Format Text
wc — Word Count
Count lines, words, and characters in a file.
wc -l file.txt— count lines
wc -w file.txt— count words
wc -c file.txt— count bytes
fmt & column
fmt reformats paragraph text to a specified width. column formats input into aligned columns for readable output.
fmt -w 72 essay.txt
column -t data.txt
ps & top — Monitor Running Processes
ps
Display a snapshot of currently running processes. Use ps aux for a full listing of all processes.
ps aux— all processes
ps -ef | grep nginx— find specific process
top / htop
top provides a real-time dynamic view of running processes. htop is an enhanced interactive version with color and mouse support.
top— live process monitor
htop— interactive process viewer
kill, bg & fg — Control Processes
kill
Send a signal to a process, typically to terminate it. Use kill -9 to force-kill.
kill 1234 | kill -9 5678 | killall nginx
bg
Resume a suspended job in the background so the terminal remains free.
bg %1 — resume job 1 in background
fg
Bring a background job to the foreground of the current terminal.
fg %1 — bring job 1 to foreground
strace, pmap & pidof — Inspect Processes
strace
Trace system calls and signals made by a process. Invaluable for debugging.
strace ls— trace ls command
strace -p 1234— attach to running process
pmap & pidof
pmap reports the memory map of a process. pidof finds the PID of a running program by name.
pmap 1234— memory map of PID 1234
pidof nginx— get nginx PID
time, watch & mpstat — Timing and CPU Stats
time
Measure the execution time of a command.
time ./myscript.sh — shows real, user, and sys time
watch
Execute a command repeatedly at a set interval and display the output.
watch -n 2 df -h — refresh disk usage every 2s
mpstat
Report CPU statistics per processor. Part of the sysstat package.
mpstat -P ALL 1 — per-CPU stats every second
useradd, userdel & usermod — Manage Users
useradd
Create a new user account. Use -m to create a home directory.
useradd -m -s /bin/bash alice
userdel
Delete a user account. Use -r to also remove the home directory.
userdel -r alice
usermod
Modify an existing user account — change shell, group, home directory, etc.
usermod -aG sudo alice
passwd, chpasswd & chage — Manage Passwords
passwd
Change a user's password. Run as root to change any user's password.
passwd— change your own password
passwd alice— change alice's password
chpasswd & chage
chpasswd updates passwords in batch from a file. chage manages password expiry and aging policies.
chpasswd < users.txt
chage -l alice— view expiry info
chage -M 90 alice— set 90-day expiry
who, whoami & id — Identify Users
who
Show who is currently logged into the system, including login time and terminal.
who | who -a — all details
whoami
Print the username of the currently logged-in user.
whoami → alice
id
Display user and group IDs for the current or specified user.
id | id alice — show UID, GID, groups
finger, chfn & chsh — User Info and Shell
finger
Display information about a user including login name, real name, terminal, and idle time.
finger alice— info about alice
finger— list all logged-in users
chfn & chsh
chfn changes a user's full name and contact info. chsh changes the default login shell.
chfn alice— update finger info
chsh -s /bin/zsh alice— set zsh as shell
chmod — Change File Permissions
Description
Change the read, write, and execute permissions of files and directories. Permissions can be set using symbolic or octal notation.
Examples
chmod 755 script.sh— rwxr-xr-x
chmod +x script.sh— add execute
chmod -R 644 /var/www— recursive
chmod u+w,g-r file.txt— symbolic mode
chown, chattr & mount — Ownership and Attributes
chown
Change the owner and/or group of a file or directory.
chown alice:staff file.txt | chown -R www-data /var/www
chattr
Change special file attributes on Linux filesystems (e.g., make a file immutable).
chattr +i file.txt — make immutable
mount
Mount a filesystem or device to a directory in the file tree.
mount /dev/sdb1 /mnt/usb
groupadd, groupdel & groupmod — Manage Groups
groupadd & groupdel
Create or delete a user group on the system.
groupadd developers— create group
groupdel developers— delete group
groupmod & groups
groupmod modifies a group's name or GID. groups lists all groups a user belongs to.
groupmod -n devs developers
groups alice— show alice's groups
gpasswd & grpck — Group Passwords and Verification
gpasswd
Administer /etc/group and /etc/gshadow. Add or remove users from groups and set group passwords.
gpasswd -a alice developers — add alice to group
gpasswd -d alice developers — remove alice
grpck
Verify the integrity of group files. Checks for invalid entries in /etc/group and /etc/gshadow.
grpck — run group file integrity check
ping & traceroute — Test Network Connectivity
ping
Send ICMP echo requests to test if a host is reachable and measure round-trip time.
ping google.com
ping -c 4 192.168.1.1— send 4 packets
traceroute / tracepath
Trace the route packets take to reach a destination, showing each hop along the way.
traceroute google.com
tracepath 8.8.8.8— no root required
ifconfig & ip — Network Interface Configuration
ifconfig
Display or configure network interfaces. Older tool, largely replaced by ip on modern systems.
ifconfig— show all interfaces
ifconfig eth0 up— bring interface up
ip
Modern replacement for ifconfig, route, and arp. Manages addresses, routes, and links.
ip addr show— show IP addresses
ip route show— show routing table
ip link set eth0 up
ssh & scp — Secure Remote Access and Transfer
ssh
Securely connect to a remote machine over an encrypted channel.
ssh alice@192.168.1.10
ssh -p 2222 alice@server.com
scp
Securely copy files between hosts over SSH.
scp file.txt alice@server:/tmp/
scp -r /local/dir alice@server:/remote/
rsync
Efficiently sync files locally or over SSH, transferring only changed data.
rsync -avz /src/ alice@server:/dst/
curl & wget — Download Files from the Web
curl
Transfer data from or to a server using various protocols. Highly versatile for API testing and downloads.
curl https://example.com
curl -O https://example.com/file.zip
curl -X POST -d '{"key":"val"}' api.com
wget
Non-interactive downloader supporting HTTP, HTTPS, and FTP. Great for recursive downloads.
wget https://example.com/file.tar.gz
wget -r https://example.com— recursive
wget -c file.zip— resume download
netstat, iptables & nmcli — Network Status and Firewall
netstat
Display network connections, routing tables, and interface statistics.
netstat -tuln — show listening ports
iptables
Configure Linux kernel firewall rules for packet filtering and NAT.
iptables -L — list rules | iptables-save > rules.txt
nmcli
Command-line tool for NetworkManager — manage connections, Wi-Fi, and VPNs.
nmcli con show | nmcli dev wifi connect SSID
hostname, nslookup & host — DNS and Host Info
hostname
Display or set the system's hostname. hostnamectl provides more control on systemd systems.
hostname | hostnamectl set-hostname myserver
nslookup
Query DNS servers to resolve domain names to IP addresses and vice versa.
nslookup google.com | nslookup 8.8.8.8
host
Simple DNS lookup utility. Converts names to IPs and IPs to names.
host google.com | host 8.8.8.8
nc (netcat), arp & vnstat — Advanced Network Tools
nc (netcat)
The "Swiss Army knife" of networking — read/write data across TCP/UDP connections. Used for port scanning, file transfer, and debugging.
nc -zv host 80— test port 80
nc -l 1234 > file.txt— receive file
arp & vnstat
arp displays and modifies the ARP cache (IP-to-MAC mappings). vnstat monitors network traffic usage over time.
arp -a— show ARP table
vnstat -i eth0— traffic stats
apt, apt-get & aptitude — Debian Package Management
apt
Modern, user-friendly package manager for Debian/Ubuntu. Combines the best of apt-get and apt-cache.
apt update && apt upgrade | apt install nginx
apt-get
Classic package management tool. Preferred in scripts for its stable interface.
apt-get install vim | apt-get remove vim
aptitude
Advanced package manager with a text-based UI and better dependency resolution.
aptitude install nginx | aptitude search vim
cron & crontab — Schedule Recurring Jobs
cron / crontab
Schedule commands to run automatically at specified times. Edit your crontab with crontab -e. The cron syntax is: minute hour day month weekday command.
Examples
crontab -e— edit cron jobs
crontab -l— list cron jobs
0 2 * * * /backup.sh— run at 2am daily
*/5 * * * * /check.sh— every 5 minutes
at, atq & atrm — One-Time Job Scheduling
at
Schedule a command to run once at a specific time in the future.
echo "reboot" | at 02:00
at now + 1 hour — then type commands
atq
List pending jobs in the at queue.
atq — show all queued jobs
atrm
Remove a job from the at queue by its job number.
atrm 3 — remove job number 3
df & du — Disk Space Usage
df — Disk Free
Report the amount of disk space used and available on filesystems.
df -h— human-readable sizes
df -T— show filesystem type
df /home— specific mount point
du — Disk Usage
Estimate file and directory space usage.
du -sh /var/log— total size of dir
du -ah /home— all files with sizes
du -sh * | sort -h— sorted by size
fdisk, cfdisk & mkfs — Partition and Format Disks
fdisk
Partition table manipulator for MBR disks. Use gdisk for GPT.
fdisk -l — list partitions | fdisk /dev/sdb
cfdisk
Curses-based interactive partition editor — easier to use than fdisk.
cfdisk /dev/sdb
dosfsck
Check and repair FAT filesystems (MS-DOS file system checker).
dosfsck -a /dev/sdb1 — auto-repair
dump, restore & sync — Backup and Sync
dump & restore
dump creates backups of ext2/3/4 filesystems. restore recovers files from those backups.
dump -0uf /backup/root.dump /
restore -rf /backup/root.dump
sync
Flush filesystem buffers, writing all pending data to disk. Run before unmounting a device.
sync— flush all buffers to disk
- Always run before
umounton removable media
tar — Archive Files
Description
tar creates and extracts archive files. Combine with compression tools like gzip or bzip2 for compressed archives.
Examples
tar -czf archive.tar.gz /dir— create gzip archive
tar -xzf archive.tar.gz— extract gzip archive
tar -cjf archive.tar.bz2 /dir— bzip2 archive
tar -tf archive.tar.gz— list contents
gzip, bzip2 & zip — Compress Files
gzip / gunzip
Compress or decompress files using the GNU zip algorithm.
gzip file.txt → file.txt.gz
gunzip file.txt.gz
bzip2 / bunzip2
Higher compression ratio than gzip, but slower. Produces .bz2 files.
bzip2 file.txt → file.txt.bz2
zip / unzip
Create and extract ZIP archives, compatible with Windows.
zip archive.zip file1 file2
unzip archive.zip
uname, lsblk & lsusb — System and Device Info
uname
Print system information including kernel name, version, and architecture.
uname -a — all info | uname -r — kernel version
lsblk
List block devices (disks, partitions, and their mount points) in a tree format.
lsblk | lsblk -f — show filesystems
lsusb / lshw
lsusb lists USB devices. lshw provides detailed hardware configuration.
lsusb | lshw -short
free, iostat & hdparm — Memory and Disk Performance
free
Display the amount of free and used memory in the system, including swap.
free -h— human-readable output
free -m— output in megabytes
iostat & hdparm
iostat reports CPU and I/O statistics for devices. hdparm gets or sets hard disk parameters.
iostat -x 1— extended stats every 1s
hdparm -I /dev/sda— disk info
hdparm -tT /dev/sda— benchmark disk
dmesg, dmidecode & acpi — Kernel and Hardware Diagnostics
dmesg
Print or control the kernel ring buffer — useful for diagnosing hardware and boot issues.
dmesg | tail -20 | dmesg | grep error
dmidecode
Dump DMI/SMBIOS data — shows hardware info like CPU, RAM, BIOS, and motherboard details.
dmidecode -t memory | dmidecode -t bios
acpi
Show battery status, AC adapter state, and thermal information on laptops.
acpi -b — battery | acpi -t — temperature
systemctl — Control System Services
Description
systemctl is the primary tool for managing systemd services and the system state. Start, stop, enable, and inspect services.
Examples
systemctl start nginx— start service
systemctl stop nginx— stop service
systemctl enable nginx— start on boot
systemctl status nginx— check status
systemctl list-units— list all units
halt, reboot, poweroff & shutdown — Power Management
halt
Halt the system immediately, stopping all processes without powering off.
halt | halt -f — force halt
reboot
Restart the system gracefully, shutting down all services first.
reboot | reboot -f — force reboot
poweroff & shutdown
poweroff halts and powers off the machine. shutdown schedules a shutdown with an optional message.
shutdown -h now | shutdown -r +5 "Rebooting"
journalctl & sar — System Logs and Performance
journalctl
Query and display logs from the systemd journal. The primary logging tool on modern Linux systems.
journalctl -xe— recent errors
journalctl -u nginx— service logs
journalctl --since "1 hour ago"
sar
Collect, report, and save system activity information (CPU, memory, I/O). Part of the sysstat package.
sar -u 1 5— CPU usage, 5 samples
sar -r— memory stats
sar -b— I/O stats
vmstat, uptime & dstat — System Performance Overview
vmstat
Report virtual memory, processes, I/O, and CPU activity statistics.
vmstat 1 5 — 5 reports, 1 second apart
uptime
Show how long the system has been running, plus load averages for 1, 5, and 15 minutes.
uptime → up 3 days, load average: 0.5, 0.3, 0.2
dstat
Versatile resource statistics tool combining vmstat, iostat, and netstat output.
dstat -cdngy — CPU, disk, net, page, sys
script & last — Session Recording and Login History
script
Record everything printed to the terminal into a file. Useful for auditing and documentation.
script session.log— start recording
exit— stop recording
scriptreplay timing.log session.log
last
Show a listing of last logged-in users, reading from /var/log/wtmp.
last— all recent logins
last alice— logins for alice
last reboot— system reboots
md5sum, cksum & sum — Verify File Integrity
md5sum
Compute or verify MD5 checksums to confirm file integrity after download or transfer.
md5sum file.iso | md5sum -c checksums.md5
cksum
Calculate a CRC checksum and byte count for a file.
cksum file.txt — outputs checksum and size
sum
Compute a simple checksum and block count for a file.
sum file.txt — legacy checksum tool
date, cal & hwclock — Date and Time Commands
date
Display or set the system date and time. Supports custom format strings.
date | date "+%Y-%m-%d %H:%M" | date -s "2024-01-01 12:00"
cal
Display a calendar for the current or specified month and year.
cal | cal 12 2024 — December 2024
hwclock
Read or set the hardware (BIOS) clock. Sync it with the system clock.
hwclock --show | hwclock --systohc
alias & export — Shell Customization
alias
Create shortcuts for long or frequently used commands. Add to ~/.bashrc for persistence.
alias ll='ls -la'
alias gs='git status'
unalias ll— remove alias
export
Set environment variables and make them available to child processes.
export PATH=$PATH:/usr/local/bin
export EDITOR=vim
export -p— list all exported vars
if, for & while — Shell Control Flow
if / else
Conditional execution based on exit status or test expressions.
if [ -f file.txt ]; then echo "exists"; fi
for
Iterate over a list of items or a range of numbers.
for i in 1 2 3; do echo $i; done
while / until
Execute commands repeatedly while or until a condition is true.
while true; do sleep 1; done
read, printf & eval — Input and Output in Scripts
read
Read a line of input from the user or a file into a variable.
read -p "Enter name: " name
echo $name
printf
Format and print data, similar to C's printf. More powerful than echo.
printf "Hello, %s!\n" "$name"
eval
Evaluate and execute a string as a shell command. Use with caution.
cmd="ls -la"; eval $cmd
declare, source & type — Variable and Script Management
declare
Declare variables and give them attributes (integer, array, read-only, etc.).
declare -i num=5— integer variable
declare -r PI=3.14— read-only
declare -a arr=(1 2 3)— array
source & type
source (or .) executes a script in the current shell, loading its variables. type shows how a command name is interpreted.
source ~/.bashrc
type ls— is it alias, function, or binary?
expr, factor & seq — Math and Sequences
expr
Evaluate arithmetic and string expressions in shell scripts.
expr 5 + 3 → 8
expr length "hello" → 5
factor
Print the prime factorization of a given number.
factor 60 → 60: 2 2 3 5
seq
Generate a sequence of numbers with optional step and format.
seq 1 10 | seq 0 2 10 — even numbers
expect & setsid — Automation and Session Control
expect
Automate interactive command-line programs by scripting expected prompts and responses. Useful for automating SSH logins and FTP sessions.
expect script.exp— run expect script
- Uses
spawn,expect,sendcommands
setsid
Run a program in a new session, detached from the controlling terminal. Useful for daemonizing processes.
setsid ./myscript.sh
- Process continues after terminal closes
lsmod, insmod & rmmod — Kernel Module Management
lsmod
List currently loaded kernel modules and their dependencies.
lsmod | lsmod | grep bluetooth
insmod
Insert a module into the Linux kernel. Requires the full path to the .ko file.
insmod /lib/modules/mymod.ko
rmmod
Remove a loaded module from the kernel. Use modprobe -r for safer removal with dependency handling.
rmmod mymod
depmod & modinfo — Module Dependencies and Info
depmod
Generate a list of module dependencies and map files. Run after installing a new kernel module.
depmod -a— update all module dependencies
- Required before
modprobecan find new modules
modinfo
Display information about a kernel module — author, description, parameters, and license.
modinfo bluetooth
modinfo -F description e1000
gcc, g++ & gdb — Compile and Debug Code
gcc
GNU C Compiler — compile C source files into executables.
gcc -o myapp main.c | gcc -Wall -g main.c
g++
GNU C++ Compiler — compile C++ source files.
g++ -o myapp main.cpp | g++ -std=c++17 main.cpp
gdb
GNU Debugger — step through code, set breakpoints, and inspect variables at runtime.
gdb ./myapp | gdb -p 1234 — attach to process
autoconf, automake & make — Build Automation
autoconf & automake
Generate portable configure scripts and Makefile.in templates for building software across different platforms.
autoconf— generate configure script
automake --add-missing
autoreconf -i— run all auto tools
ctags & readelf
ctags generates tag files for source code navigation in editors. readelf displays information about ELF binary files.
ctags -R .— tag all source files
readelf -h myapp— ELF header info
cpp, addr2line & ranlib — Preprocessor and Binary Tools
cpp
The C preprocessor — expands macros, includes, and conditional compilation directives before compilation.
cpp main.c | cpp -DDEBUG main.c
addr2line
Convert program addresses into file names and line numbers for debugging crash reports.
addr2line -e myapp 0x4005f0
ranlib
Generate an index for a static library archive, making linking faster.
ranlib libmylib.a
man, info & apropos — Get Help
man
Display the manual page for a command. The most important help tool in Linux.
man ls— manual for ls
man 5 passwd— section 5 of passwd
man -k keyword— search manuals
info, apropos & whatis
info provides more detailed GNU documentation. apropos searches man page descriptions. whatis gives a one-line description.
info bash
apropos network
whatis ls|which python3
screen & tmux — Terminal Multiplexers
screen
Create multiple terminal sessions within one window. Sessions persist after disconnection — ideal for remote work.
screen— start new session
screen -r— reattach to session
Ctrl+A D— detach from session
stty, tty & reset
stty configures terminal settings. tty prints the terminal device name. reset reinitializes a garbled terminal.
stty -a— show all settings
tty→/dev/pts/0
reset— fix broken terminal
agetty, chvt & showkey — Console Management
agetty
Open a terminal line and invite a login. Used by init/systemd to manage virtual console logins.
agetty tty1 38400 — open login on tty1
chvt
Change the active virtual terminal (console). Equivalent to pressing Ctrl+Alt+F1.
chvt 2 — switch to virtual terminal 2
showkey
Display the keycodes or scancodes of keys pressed on the keyboard. Useful for keyboard mapping.
showkey — show keycodes | showkey -s — scancodes
cupsd, aplay & amixer — Printing and Audio
cupsd
The CUPS print server daemon. Manages print queues and printer drivers on Linux.
systemctl start cups | Access at http://localhost:631
aplay
Play audio files from the command line using ALSA.
aplay sound.wav | aplay -l — list sound cards
amixer
Control ALSA sound card mixer settings from the command line.
amixer set Master 80% | amixer get Master
mail, write & wall — User Communication Commands
mail & mailq
mail sends and reads email from the command line. mailq displays the mail transfer agent's queue of pending messages.
echo "Hello" | mail -s "Subject" user@host
mailq— show pending mail queue
write & wall
write sends a message to a specific logged-in user. wall broadcasts a message to all logged-in users.
write alice pts/1— message to alice
wall "System going down in 5 min"
biff y— notify on new mail
Output Redirection — >, >>, 2>, &>
> (Overwrite)
Redirect stdout to a file, overwriting existing content.
ls > files.txt — save listing to file
>> (Append)
Redirect stdout to a file, appending to existing content.
echo "log entry" >> app.log
2> and &>
Redirect stderr (2>) or both stdout and stderr (&>) to a file.
cmd 2> err.log | cmd &> all.log
Pipes and Input Redirection — |, <, <<
Pipe — |
Send the output of one command as input to another. The backbone of Linux command chaining.
ps aux | grep nginx
cat file.txt | sort | uniq
ls -la | less
Input — < and <<
< redirects a file as stdin to a command. << (here-doc) provides inline multi-line input.
sort < unsorted.txt
cat << EOF— start here-doc
mysql db < script.sql
Vim — Essential Navigation and Editing
Vim is a powerful terminal text editor. Understanding its modal nature is key — press i to enter Insert mode, Esc to return to Normal mode, and :wq to save and quit.
Vim — Navigation, Search & Replace
Navigation
gg— go to beginning of file
G— go to end of file
0— go to beginning of line
$— go to end of line
Ctrl+F— scroll forward one page
Search
/pattern— search forward
?pattern— search backward
n— next match |N— previous match
*— search word under cursor
Replace
:s/old/new/— replace first on line
:s/old/new/g— replace all on line
:%s/old/new/g— replace all in file
:g/pattern/d— delete matching lines
Vim — Editing, Deletion & Buffers
Editing Commands
dd— delete current line
D— delete to end of line
dw— delete word forward
db— delete word backward
yy— copy (yank) current line
p— paste after cursor
u— undo |Ctrl+R— redo
Buffers and Files
:w— save file
:q!— quit without saving
:wq— save and quit
:e file.txt— open another file
:split file.txt— horizontal split
Ctrl+W W— switch between splits
:bn/:bp— next/prev buffer
Linux Commands Cheat Sheet