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03 — Linux Command Line Basics

← File System | Index | Windows CLI →

The Shell

A shell is a command-line interpreter — it reads commands from the user and executes them. It is both an interactive tool and a scripting language.

Common Linux Shells

Shell Path Notes
bash /bin/bash Default on most distros, Bourne Again Shell
zsh /bin/zsh Extended bash, used by macOS default, Arch Linux
fish /usr/bin/fish Friendly interactive, autosuggestions
sh /bin/sh POSIX shell (may be dash/bash symlink)
dash /bin/dash Lightweight POSIX, used in scripts 
echo $SHELL          # Show current shell
cat /etc/shells      # List installed shells
chsh -s /bin/zsh     # Change default shell

Terminal Anatomy

alice@myserver:~$  ls -la /home
  ↑       ↑      ↑  ↑   ↑   ↑
  │       │      │  │   │   └── argument
  │       │      │  │   └────── flag/option
  │       │      │  └────────── command
  │       │      └───────────── prompt symbol ($ = user, # = root)
  │       └──────────────────── current directory (~ = home)
  └──────────────────────────── username@hostname

Navigation Commands

pwd — Print Working Directory

pwd
# Output: /home/alice/projects

ls — List Directory Contents

ls                    # Basic listing
ls -l                 # Long format (permissions, size, date)
ls -a                 # Show hidden files (starting with .)
ls -la                # Long format + hidden
ls -lh                # Human-readable sizes (KB, MB)
ls -lt                # Sort by modification time
ls -R                 # Recursive listing
ls /etc               # List specific directory
ls *.txt              # Glob: all .txt files

Long format breakdown:

-rw-r--r-- 1 alice staff 1024 Apr 22 10:00 file.txt
↑           ↑ ↑     ↑     ↑    ↑            ↑
│           │ │     │     │    timestamp     filename
│           │ │     │     size (bytes)
│           │ │     group
│           │ owner
│           hard link count
permissions

cd — Change Directory

cd /etc               # Absolute path
cd documents          # Relative path
cd ..                 # Go up one level
cd ../..              # Go up two levels
cd ~                  # Go to home directory
cd -                  # Go to previous directory
cd                    # Go to home (same as cd ~)

File Viewing Commands

cat — Concatenate and Print

cat file.txt                  # Print entire file
cat file1.txt file2.txt       # Concatenate two files
cat -n file.txt               # Show line numbers
cat -A file.txt               # Show non-printing characters

less — Paginated Viewer

less file.txt                 # Open file in pager
# Inside less:
#   Space / PgDn = next page
#   b / PgUp     = previous page
#   /pattern     = search forward
#   ?pattern     = search backward
#   n            = next match
#   g            = go to start
#   G            = go to end
#   q            = quit

more — Simple Pager (older)

more file.txt

head — Show Beginning

head file.txt           # First 10 lines (default)
head -n 20 file.txt     # First 20 lines
head -c 100 file.txt    # First 100 bytes

tail — Show End

tail file.txt           # Last 10 lines
tail -n 50 file.txt     # Last 50 lines
tail -f /var/log/syslog # Follow log in real time (stream)
tail -F logfile         # Follow + reopen if rotated

💡 tail -f is essential for live log monitoring → see Monitoring & Logging

File and Directory Operations

cp — Copy

cp source dest              # Copy file
cp -r dir/ newdir/          # Copy directory recursively
cp -p file dest             # Preserve permissions, timestamps
cp -u source dest           # Copy only if source is newer
cp -v source dest           # Verbose output
cp *.txt backup/            # Copy all .txt files

mv — Move/Rename

mv oldname newname          # Rename file
mv file /path/to/dest       # Move file
mv dir/ /new/location/      # Move directory
mv -i file dest             # Interactive (ask before overwrite)
mv -v file dest             # Verbose

rm — Remove

rm file.txt                 # Delete file
rm -i file.txt              # Interactive confirmation
rm -r directory/            # Remove directory recursively
rm -rf directory/           # Force remove (no prompts) ⚠️ DANGEROUS
rm -v file.txt              # Verbose
rm *.log                    # Remove all .log files

⚠️ WARNING: rm -rf is irreversible. There is no Recycle Bin in CLI. Double-check paths!

mkdir — Make Directory

mkdir newdir                # Create directory
mkdir -p path/to/new/dir    # Create nested directories
mkdir -m 755 secured        # Create with specific permissions

rmdir — Remove Empty Directory

rmdir emptydir              # Only works if empty
rm -r nonemptydir/          # Use rm -r for non-empty

touch — Create/Update File

touch newfile.txt           # Create empty file or update timestamp
touch -t 202401010000 f     # Set specific timestamp

ln — Create Links

ln file hardlink            # Hard link
ln -s /path/to/target link  # Symbolic link

Finding Files

find — Search File System

find /home -name "*.txt"             # Find all .txt in /home
find . -type f                       # Only regular files
find . -type d                       # Only directories
find . -name "*.log" -mtime -7       # Modified last 7 days
find /var -size +100M                # Files larger than 100MB
find . -perm 644                     # Files with exact permissions
find . -user alice                   # Files owned by alice
find . -empty                        # Empty files/directories
find / -name "passwd" 2>/dev/null    # Suppress permission errors

# Execute command on found files
find . -name "*.tmp" -exec rm {} \;
find . -name "*.txt" -exec ls -l {} +

locate — Fast Search (uses database)

locate filename              # Fast but may be outdated
sudo updatedb                # Update the database
locate -i filename           # Case-insensitive

which / whereis — Find Executables

which python3                # Location of command
whereis ls                   # Binary, source, man page locations

Text Processing Commands

grep — Search Text

grep "pattern" file.txt              # Basic search
grep -i "pattern" file.txt           # Case-insensitive
grep -r "pattern" /etc/              # Recursive
grep -n "pattern" file.txt           # Show line numbers
grep -v "pattern" file.txt           # Invert match (lines NOT matching)
grep -c "pattern" file.txt           # Count matching lines
grep -l "pattern" *.txt              # Only filenames
grep -w "word" file.txt              # Whole word match
grep -A 3 "pattern" file.txt         # 3 lines After match
grep -B 2 "pattern" file.txt         # 2 lines Before match
grep "^start" file.txt               # Lines starting with "start"
grep "end$" file.txt                 # Lines ending with "end"
grep -E "regex+" file.txt            # Extended regex

sort — Sort Lines

sort file.txt                        # Alphabetical
sort -r file.txt                     # Reverse
sort -n numbers.txt                  # Numeric sort
sort -u file.txt                     # Unique (remove duplicates)
sort -k2 file.txt                    # Sort by column 2

uniq — Remove Duplicates

uniq file.txt                        # Remove consecutive duplicates
sort file.txt | uniq                 # All duplicates (sort first)
uniq -c file.txt                     # Count occurrences
uniq -d file.txt                     # Only duplicated lines

wc — Word Count

wc file.txt             # Lines, words, bytes
wc -l file.txt          # Only line count
wc -w file.txt          # Only word count
wc -c file.txt          # Only byte count

cut — Extract Columns

cut -d: -f1 /etc/passwd              # Field 1, delimiter ":"
cut -c1-10 file.txt                  # Characters 1-10

sed — Stream Editor

sed 's/old/new/' file.txt            # Replace first occurrence per line
sed 's/old/new/g' file.txt           # Replace all occurrences
sed -i 's/old/new/g' file.txt        # In-place edit
sed '/pattern/d' file.txt            # Delete matching lines
sed -n '5,10p' file.txt              # Print lines 5-10

awk — Field Processing

awk '{print $1}' file.txt            # Print first field
awk -F: '{print $1, $3}' /etc/passwd # Custom delimiter
awk '{sum+=$1} END{print sum}' f     # Sum first column
awk 'NR==5' file.txt                 # Print line 5

Pipes and Redirection

graph LR
    CMD1["command 1"] -->|"stdout"| PIPE["|"]
    PIPE --> CMD2["command 2"]

    CMD["command"] -->|"> file"| FILE["file (overwrite)"]
    CMD -->|">> file"| FILE2["file (append)"]
    FILE3["file"] -->|"< file"| CMD2["command"]
    CMD -->|"2> file"| ERR["stderr file"]
    CMD -->|"2>&1"| BOTH["stdout + stderr"]
Loading 
# Pipe: pass stdout of one command to stdin of next
ls -la | grep ".txt"
cat /etc/passwd | grep alice | cut -d: -f1,3

# Redirect stdout to file (overwrite)
ls -la > filelist.txt

# Redirect stdout to file (append)
echo "new line" >> log.txt

# Redirect stdin from file
sort < unsorted.txt

# Redirect stderr
command 2> errors.txt

# Redirect both stdout and stderr
command > output.txt 2>&1
command &> output.txt        # bash shorthand

# Discard output
command > /dev/null 2>&1

Permissions Quick Reference

Full detail: User Permissions →

chmod 755 file          # rwxr-xr-x
chmod +x script.sh      # Add execute bit
chown alice:staff file  # Change owner:group
sudo command            # Run as root
su - alice              # Switch to user alice

Process Commands

Full detail: Services & Processes →

ps aux                  # All running processes
top                     # Interactive process monitor
htop                    # Better interactive monitor
kill PID                # Send SIGTERM to process
kill -9 PID             # Force kill (SIGKILL)
jobs                    # List background jobs
bg                      # Resume job in background
fg                      # Bring job to foreground
command &               # Run command in background
nohup command &         # Run immune to hangup

Useful Shortcuts

Shortcut Action
Ctrl+C Interrupt (kill) current process
Ctrl+Z Suspend current process
Ctrl+D EOF / logout
Ctrl+L Clear terminal
Ctrl+A Go to beginning of line
Ctrl+E Go to end of line
Ctrl+U Delete from cursor to start
Ctrl+K Delete from cursor to end
Ctrl+R Reverse history search
Tab Autocomplete
↑ / ↓ Navigate command history
!! Repeat last command
!n Run command number n from history
sudo !! Re-run last command with sudo

Environment Variables

echo $HOME              # Print HOME variable
echo $PATH              # Print PATH (where shell looks for commands)
echo $USER              # Current username
echo $SHELL             # Current shell
env                     # List all environment variables
export VAR=value        # Set variable for current session + children
VAR=value command       # Set variable only for one command
unset VAR               # Remove variable

# Permanent: add to ~/.bashrc or ~/.zshrc
echo 'export MY_VAR=hello' >> ~/.bashrc
source ~/.bashrc         # Reload config

Shell Scripting Basics

#!/bin/bash
# This is a comment

# Variables
NAME="Alice"
echo "Hello, $NAME"

# Conditionals
if [ -f /etc/hosts ]; then
    echo "hosts file exists"
elif [ -d /etc ]; then
    echo "/etc is a directory"
else
    echo "not found"
fi

# Loops
for i in 1 2 3; do
    echo "Number: $i"
done

for file in *.txt; do
    echo "Processing: $file"
done

while [ condition ]; do
    command
done

# Functions
greet() {
    echo "Hello, $1"
}
greet "World"

# Exit codes
command && echo "success"    # Run if previous succeeded
command || echo "failed"     # Run if previous failed

Related Topics

← File System | Index | Windows CLI →