Linux Commands Reference Guide

December 1, 2024

Last updated: December 24, 2024

What is a Command?

A command is a binary file kept under a specific directory that performs a particular function when executed.

Command Categories

File and File System Management

cat | cd | chmod | chown | chgrp | cp | du | df | file | fsck | ln | ls | lsof | mkdir | mount | mv | pwd | rm | rmdir | split | touch

Process Management

User Management/Environment

Text Processing

Communications & Networking

inetd | netstat | ping | rlogin | traceroute

Searching

find | grep | strings

Printing

lp

Miscellaneous

banner | bc | cal | man | size | yes

Filesystem Utilities

cd - Change to another directory location
ls - List directory contents
cp - Copy a file or directory to another location
pwd - Print the current working directory
mkdir - Make a directory
mv - Move or rename a file or directory
rmdir - Delete an empty directory
rm - Delete a file or directory tree
touch - Create a new file or update its modification time
which - Locate a command
locate - Find files by name
ln - Link one file/directory to another
df - Report disk space
du - Calculate used disk space
find - Search for files through a directory hierarchy
chmod - Change the permissions of a file or directory
chown - Change the owner of a file or directory
chgrp - Change the group of a file or directory
quota - Display disk usage and limits
strings - Print the strings of printable characters in files
info - The GNU alternative to man
man - The standard Unix documentation system
less - Opposite of more ;)
wc - Print newline, word, and byte counts for each file

Hands-On Examples

# Find files larger than 10MB in /usr
find /usr -size +10M

# Find files modified more than X days ago
find /home -mtime +7

# Find files accessed within the last X days
find /var -atime -7

# Find and remove core files
find / -name core -exec rm {} \;

Text Processing Commands

echo - Display line of text
cat - Concatenate files to standard output
less - Improved more-like text pager
more - Text pager
head - Output the first parts of a file
tail - Output the last parts of a file
cut - Remove sections from each line of a file or standard input
paste - Merge lines of files
diff - Compare two text files line by line
cmp - Compare two files byte for byte
sort - Sort lines of text files
join - Join lines of two files on a common field
awk - A pattern scanning and processing language
grep - Print lines matching a pattern
egrep - Extended pattern matching (synonym for "grep -E")
fgrep - Simplified pattern matching (synonym for "grep -F")
sed - Stream editor for filtering and transforming text
split - Split a file into pieces
tee - Read from standard input, write to standard output and files
tr - Translate or delete characters
uniq - Report or omit repeated lines
wc - Word/line/byte count
fold - Wrap each input line to fit within the given width
iconv - Convert the encoding of the specified files
uuencode - Encode a binary file for transmission using electronic mail
uudecode - Decode a binary file that was used for transmission
cksum - Print the CRC checksum and bytecount of a file
banner - Create ASCII art version of an input string

Grep Commands - Hands-On

Basic Pattern Searching

# Search for pattern "july" (case-insensitive) in all files
grep -i july *

# Print file names that do NOT contain "july"
grep -L july *

# Select lines containing "july" as a whole word
grep -w july filename

# Search recursively in all files under a directory
grep -r july .

Context and Line Control

# Print 10 lines after the match
grep -A 10 july filename

# Print 1 line before the match
grep -B 1 july filename

# Print 4 lines before and after the match (context)
grep -C 4 july filename

# Print line numbers of matches
grep -n "july" filename

# Print lines excluding the pattern
grep -v july filename

General User Commands

exit - Cause normal process termination
logout - Terminate login shell
dd - Convert and copy a file (Disk Dump)
dirname - Strip non-directory suffixes from a path
echo - Print to standard output
env - Show environment variables; run a program with altered environment
file / stat - Determine the type of a file
nohup - Run a command with immunity to hangups outputting to non-tty
sh - The Bourne shell, the standard Unix shell
uptime - Print how long the system has been running
history - GNU History Library
source - Execute commands from filename in current shell environment
seq - Print a sequence of numbers

Archives and Compression

tar - Tape ARchiver, concatenates files
gzip - The gzip file compressor
bzip2 - Block-sorting file compressor
ar - Maintain, modify, and extract from archives (largely obsoleted by tar)
cpio - A traditional archiving tool/format
zcat - Print files to stdout from gzip archives without unpacking
afio - Compatible superset of cpio with added functionality
p7zip - 7zip for Unix/Linux
pax - POSIX archive tool that handles multiple formats
pack / pcat / unpack - Old ATT Unix tools using Huffman coding (obsoleted by compress)

Process and Task Management

top - Display Linux processes
htop - Interactive ncurses-based process viewer with scrolling
ps - Report process status
kill - Send a signal to process, or terminate a process (by PID)
killall - Terminate all processes (in GNU/Linux, kill by name)
pkill - Look up or signal processes based on name and other attributes
pgrep - Find PIDs of processes by name
pidof - GNU/Linux equivalent of pgrep
watch - Execute a program periodically, showing output fullscreen
nohup - Run a command immune to hangups, with output to a non-tty
nice - Alter priorities for processes
renice - Alter the priorities of an already running process
sleep - Delay for specified time
time - Time a command
wait - Wait for the specified process's exit status

Background Processing

& - Run process in background
fg - Send job to foreground (interactive)
bg - Send job to background, as if started with &
| - Pipe standard output of first program to standard input of second

User Management and Support

chsh - Change user shell
finger - Get details about user
id - Print real/effective UIDs/GIDs
last - Show listing of last logged in users
lastlog - Show last log in information for users
locale - Get locale specific information
localedef - Compile locale definitions
logname - Print user's login name
man - Manual browser
mesg - Control write access to your terminal
passwd - Change user password
su - Start a new process as a different user (defaults to root)
sudo - Execute a command as a different user
users - Show who is logged on (only user names)
w - Show logged-in users and their current tasks
whatis - Command description from whatis database
whereis - Locate the command's binary and manual pages
which - Locate where a command is executed from
who - Show who is logged on (with some details)
write - Send a message to another user

Development and Compilation Tools

as - GNU assembler tool
c99 - C programming language compiler
cc - C compiler
gcc - GNU Compiler Collection C frontend
f77 - Fortran 77 compiler
dbx - (System V and BSD) Symbolic debugger
gdb - GNU symbolic debugger
ld - Program linker
lex - Lexical scanner generator
m4 - Macro language
make - Automate builds
nm - List symbols from object files
size - Return the size of the sections of an ELF file
ltrace - (Linux) Trace dynamic library calls
strace - (Linux) or truss (Solaris) Trace system calls with arguments and signals

This reference guide will be continuously updated as new commands and examples are discovered.