Phase 2: Command Line - mishraxharshit/harshitxmishra.github.io GitHub Wiki

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2.1 How the Shell Processes a Command

Before learning individual commands, understand what the shell does when you press Enter.

When you type ls -la /home, the shell performs these steps in order:

Tokenisation: Splits the line into words: ls, -la, /home
Alias expansion: Checks if ls is an alias (e.g., ls --color=auto)
Variable expansion: Replaces $HOME, $USER, etc. with their values
Glob expansion: Expands wildcards: *.txt becomes file1.txt file2.txt
Command lookup: Searches $PATH directories to find the ls binary
Execution: Forks a child process, runs the command, returns the exit code

Understanding this sequence explains many confusing behaviours, like why echo $HOME prints your home directory instead of the literal string $HOME.

2.2 Navigating the Filesystem

# Print Working Directory — where you currently are
pwd
# /home/alice
 
# Change Directory
cd /var/log        # go to absolute path
cd Documents       # go to relative path (from current location)
cd ..              # go up one level
cd ../..           # go up two levels
cd ~               # go to your home directory
cd -               # go to the previous directory (very useful!)
 
# List files
ls                 # simple list
ls -l              # long format (permissions, owner, size, date)
ls -a              # show hidden files (those starting with .)
ls -la             # combine both flags
ls -lh             # human-readable file sizes (KB, MB, GB)
ls -lt             # sort by modification time, newest first
ls -lS             # sort by file size, largest first
ls /etc            # list a specific directory without going there

Understanding ls -l output:

-rw-r--r-- 1 alice alice 4096 Jan 15 14:22 notes.txt
^          ^ ^     ^     ^    ^             ^
|          | |     |     |    |             filename
|          | |     |     |    last modified date
|          | |     |     file size in bytes
|          | |     group owner
|          | user owner
|          number of hard links
file type and permissions

The first character is the file type:

- regular file
d directory
l symbolic link
b block device (disk)
c character device (terminal, serial port)

2.3 Working with Files and Directories

# Create a directory
mkdir projects
mkdir -p projects/web/css    # -p creates parent directories if they don't exist
 
# Create a file
touch notes.txt              # creates empty file, or updates timestamp if exists
echo "Hello" > notes.txt     # creates file with content (overwrites if exists)
echo "More" >> notes.txt     # appends to file
 
# Copy files
cp notes.txt backup.txt               # copy file to new name in same directory
cp notes.txt /tmp/                     # copy to different directory
cp -r projects/ projects_backup/      # -r copies directory recursively
cp -p notes.txt backup.txt            # -p preserves permissions and timestamps
 
# Move and rename (same command)
mv notes.txt renamed.txt              # rename file
mv notes.txt /tmp/                    # move to different directory
mv projects/ /opt/myprojects          # move and rename directory
 
# Delete
rm notes.txt                          # delete file
rm -i notes.txt                       # -i asks for confirmation before deleting
rm -r projects/                       # delete directory and all contents recursively
rm -rf projects/                      # force delete, no confirmation — use with care
 
# View file contents
cat notes.txt                         # print entire file
less notes.txt                        # page through file (q to quit, / to search)
head -20 notes.txt                    # first 20 lines
tail -20 notes.txt                    # last 20 lines
tail -f /var/log/syslog               # follow a file in real time as lines are added

The danger of rm -rf: There is no recycle bin in the Linux terminal. rm -rf /important/data permanently destroys data immediately. The safest habit: always specify exactly what you are deleting, use -i when in doubt, and never run rm -rf / (deletes the entire filesystem).

2.4 File Paths: Absolute and Relative

Every file has two kinds of path:

Absolute path: starts from / and specifies the complete location.

/home/alice/projects/notes.txt

Relative path: starts from your current location.

# If you are in /home/alice:
projects/notes.txt          # means /home/alice/projects/notes.txt
 
# If you are in /home/alice/projects:
notes.txt                   # means /home/alice/projects/notes.txt
../notes.txt                # means /home/alice/notes.txt (one level up)

The special names:

. (single dot) — the current directory
.. (two dots) — the parent directory
~ (tilde) — your home directory, expands to /home/yourusername

2.5 Wildcards and Globbing

The shell expands wildcard patterns before passing them to commands. The expansion happens in the shell, not in the command itself.

# * matches any sequence of characters (except /)
ls *.txt              # all files ending in .txt
ls report_*.csv       # all CSV files starting with report_
rm /tmp/*.log         # delete all .log files in /tmp
 
# ? matches exactly one character
ls file?.txt          # matches file1.txt, fileA.txt but not file10.txt
 
# [...] matches one character from the set
ls file[123].txt      # matches file1.txt, file2.txt, file3.txt
ls file[a-z].txt      # matches filea.txt through filez.txt
 
# Practical example: backup all .conf files
cp /etc/*.conf /tmp/conf_backup/

2.6 Input, Output, and Redirection

Every program in Linux has three standard data streams:

stdin (0): input, by default reads from keyboard
stdout (1): normal output, by default prints to terminal
stderr (2): error messages, by default prints to terminal

You can redirect these streams to and from files.

# Redirect stdout to a file (overwrites)
ls -l > directory_listing.txt
 
# Redirect stdout to a file (appends)
echo "new line" >> notes.txt
 
# Redirect stderr to a file
ls /nonexistent 2> errors.txt
 
# Redirect both stdout and stderr to the same file
ls /var /nonexistent > output.txt 2>&1
# 2>&1 means "send stderr (2) to wherever stdout (1) is going"
 
# Discard output entirely (send to /dev/null, the black hole)
noisy_command > /dev/null 2>&1
 
# Read stdin from a file
sort < unsorted.txt
 
# Here document: provide multi-line input inline
cat << EOF
Line one
Line two
Line three
EOF

2.7 Pipes

A pipe | connects the stdout of one command to the stdin of the next. This is one of the most powerful ideas in Unix.

# Count how many files are in /etc
ls /etc | wc -l
# ls prints filenames, wc -l counts lines
 
# Find the ten largest files in the current directory
ls -lS | head -10
 
# Find all running processes and search for python
ps aux | grep python
 
# See which processes are using the most memory
ps aux --sort=-%mem | head -10
 
# Count how many times a word appears in a file
cat /var/log/syslog | grep "error" | wc -l
 
# Chain many commands
cat /etc/passwd | cut -d: -f1 | sort | head -10
# /etc/passwd has colon-separated fields, field 1 is username
# cut extracts field 1, sort alphabetises, head shows top 10

The philosophy: Instead of one complex program that does everything, Linux has many small programs that do one thing well, and you compose them with pipes.

2.8 Searching for Files

# find: search by name, type, size, date, permissions
find /home -name "notes.txt"
find /home -name "*.txt"
find /var/log -name "*.log" -mtime -7    # modified in last 7 days
find /tmp -type f -size +10M             # files larger than 10MB
find /home -user alice                   # files owned by alice
find /etc -name "*.conf" -exec ls -l {} \;   # run ls -l on each result
 
# locate: searches a pre-built index (faster but may be out of date)
locate sshd_config
# Update the index
sudo updatedb
 
# which: find where a command's binary lives
which python3
# /usr/bin/python3
 
# whereis: find binary, source, and man page
whereis ssh
# ssh: /usr/bin/ssh /etc/ssh /usr/share/man/man1/ssh.1.gz

2.9 Reading Files Efficiently

# cat: print an entire file
cat /etc/hostname
 
# less: interactive pager (better than cat for large files)
less /var/log/syslog
# Keys inside less:
# Space or f    — page down
# b             — page up
# /pattern      — search forward
# n             — next match
# N             — previous match
# g             — go to beginning
# G             — go to end
# q             — quit
 
# head and tail
head /etc/passwd           # first 10 lines (default)
head -5 /etc/passwd        # first 5 lines
tail /var/log/syslog       # last 10 lines
tail -100 /var/log/syslog  # last 100 lines
tail -f /var/log/syslog    # follow: print new lines as they are added
 
# wc: word count (also counts lines and characters)
wc -l /etc/passwd          # number of lines (= number of user accounts)
wc -w notes.txt            # number of words
wc -c notes.txt            # number of bytes

2.10 Command History and Shortcuts

These shortcuts save enormous amounts of typing.

# History
history                    # show command history list
history | grep apt         # search history for apt commands
!!                         # repeat the last command
!ls                        # repeat the last command starting with 'ls'
!500                       # repeat command number 500 from history
Ctrl+R                     # interactive reverse search through history

Keyboard shortcuts in the terminal:

Shortcut	Action
`Ctrl+C`	Kill the running command
`Ctrl+Z`	Suspend the running command (resume with `fg`)
`Ctrl+D`	End of input / log out of shell
`Ctrl+L`	Clear the screen
`Ctrl+A`	Move cursor to beginning of line
`Ctrl+E`	Move cursor to end of line
`Ctrl+W`	Delete the word before the cursor
`Tab`	Auto-complete command or filename
`Tab Tab`	Show all possible completions
Up/Down arrows	Scroll through history

2.11 Links: Hard and Symbolic

A hard link is a second name pointing to the same file data. A symbolic link (symlink) is a file that points to another file by path, like a shortcut.

# Create a hard link
ln original.txt hardlink.txt
# Both names now refer to the same data. Deleting one leaves the other intact.
 
# Create a symbolic link
ln -s /etc/nginx/nginx.conf ~/nginx.conf
# ~/nginx.conf is a pointer to /etc/nginx/nginx.conf
 
# See where a symlink points
ls -l ~/nginx.conf
# lrwxrwxrwx 1 alice alice 22 Jan 15 nginx.conf -> /etc/nginx/nginx.conf
 
# Find broken symlinks (target no longer exists)
find /etc -type l ! -exec test -e {} \; -print

Practical use: Many Linux systems use symlinks extensively. The python command is often a symlink to python3. Nginx configuration uses symlinks to enable/disable sites (a site config lives in sites-available/ and is enabled by symlinking it into sites-enabled/).

Phase 2 Exercises

Exercise 1: Navigate to /var/log. List the files sorted by size (largest first). What are the five largest log files?

Exercise 2: Create a directory structure: ~/practice/scripts/ and ~/practice/data/. Create three text files inside ~/practice/data/. Copy all .txt files from ~/practice/data/ to /tmp/ using a wildcard.

Exercise 3: Use head and tail to read lines 20 through 30 of /etc/passwd. Hint: head -30 /etc/passwd | tail -11

Exercise 4: Count the number of lines in /var/log/syslog. Then count how many of those lines contain the word "error" (case insensitive, use grep -i).

Exercise 5: Use find to locate every file in /etc that was modified in the last 3 days. Save the output to a file called /tmp/recent_changes.txt.

Common Mistakes in Phase 2

Mistake: Using cat to read large files cat /var/log/syslog on a 500 MB log file will flood your terminal. Use less, head, or tail instead.

Mistake: Forgetting that > overwrites echo "test" > important_file.txt will destroy whatever was in that file. Use >> to append.

Mistake: Spaces around = in variable assignment name = alice fails. The shell requires name=alice (no spaces). Spaces make the shell think name is a command.

Mistake: Not using tab completion Typing long paths by hand wastes time and introduces typos. Press Tab after typing the first few letters of any path or command.

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