Unit 2: System Utilities and Basic Commands - Subjective Questions

A Linux command follows a well-defined syntax that the shell parses before execution.

General Syntax:

command [options] [arguments]

• Command: The name of the program or utility to be executed (e.g., ls, cat, date).
• Options (Flags/Switches): Modify the behaviour of the command. They usually begin with a hyphen - (short form) or double hyphen -- (long form). Example: -l, --all.
• Arguments: The objects or data on which the command operates, such as filenames, directories, or text.

Example:

ls -l /home/user

• ls → command
• -l → option (long listing format)
• /home/user → argument (directory to list)

Key points:

• Commands are case-sensitive in Linux.
• Multiple options can be combined: ls -la.
• White space separates the command, options, and arguments.
• The shell interprets and passes these tokens to the program.

The cal command displays a calendar in the terminal.

Syntax:

cal [options] [[month] year]

Common usages:

• cal → Displays the calendar of the current month.
• cal 2026 → Displays the calendar for the entire year 2026.
• cal 9 2026 → Displays September 2026.
• cal -3 → Displays previous, current, and next month.
• cal -y → Displays the calendar for the whole current year.
• cal -j → Displays the Julian calendar (day of the year, 1–366).

Example:

$ cal 9 2026
September 2026
Su Mo Tu We Th Fr Sa
1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30

Key points:

• The current date is often highlighted.
• Useful for quick reference without a GUI.
• Month value ranges from 1 to 12.

The date command displays or sets the system date and time.

Syntax:

date [options] [+format]

Basic usage:

• date → Shows current date and time, e.g. Fri Sep 25 22:26:57 UTC 2026.

Format Specifiers (prefixed with +):

• %d → Day of month (01–31)
• %m → Month number (01–12)
• %Y → Four-digit year
• %y → Two-digit year
• %H → Hour (00–23)
• %M → Minute (00–59)
• %S → Second (00–59)
• %A → Full weekday name
• %B → Full month name
• %j → Day of the year
• %T → Time in HH:MM:SS

Examples:

$ date +"%d-%m-%Y"
25-09-2026

$ date +"%A, %B %d"
Friday, September 25

Setting the date (requires superuser):

sudo date -s "2026-09-25 10:00:00"

Key points:

• Only root can change the system date.
• Format strings allow flexible output for scripts and logs.

The echo command displays a line of text or the value of a variable on the standard output.

Syntax:

echo [options] [string]

Common Options:

• -n → Does not output the trailing newline.
• -e → Enables interpretation of backslash escape sequences.
• -E → Disables escape sequence interpretation (default).

Escape Sequences (used with -e):

• \n → New line
• \t → Horizontal tab
• \\ → Backslash
• \a → Alert (bell)
• \b → Backspace
• \c → Suppress trailing newline

Examples:

$ echo "Hello World"
Hello World

$ echo -e "Name:\tKiro\nRole:\tAI"
Name: Kiro
Role: AI

$ echo -n "No newline"

Displaying variables:

$ name="Linux"
$ echo "Value is $name"
Value is Linux

Key points:

• Widely used in shell scripts for messages and debugging.
• Behaviour of -e may differ between shells.

Both echo and printf send output to the terminal, but they differ in flexibility and formatting control.

printf Syntax:

printf "format" [arguments]

Format Specifiers:

• %s → String
• %d → Integer
• %f → Floating point number
• %c → Character

Examples:

$ printf "Name: %s, Age: %d\n" "Kiro" 5
Name: Kiro, Age: 5

$ printf "%.2f\n" 3.14159
3.14

Why printf is preferred:

• Precise control over width, precision, and alignment.
• Consistent, predictable behaviour across systems.
• Ideal for generating tabular or formatted reports in scripts.

The bc (Basic Calculator) command is an arbitrary-precision calculator language that supports interactive and scripted calculations, including floating-point arithmetic.

Interactive Mode:

$ bc
5 + 3
8
quit

Setting precision with scale:

• The scale variable controls the number of decimal places.

$ bc
scale=2
10 / 3
3.33

Using with a pipe (non-interactive):

$ echo "scale=4; 22/7" | bc
3.1428

Mathematical library (-l option):

Enables standard math functions with default scale 20.

$ echo "sqrt(2)" | bc -l
1.41421356237309504880

Operators supported:

• Arithmetic: +, -, *, /, %, ^
• Relational and logical operators

Example in a script:

result=$(echo "scale=2; 15.5 * 2" | bc)
echo "Result = $result"

Key points:

• Unlike expr, bc handles floating-point numbers.
• Supports variables, loops, and functions.

The expr command evaluates expressions and prints the result. It is mainly used for integer arithmetic and string operations in shell scripts.

Syntax:

expr expression

Arithmetic Examples:

$ expr 5 + 3
8

$ expr 10 - 4
6

$ expr 6 * 2
12

$ expr 20 / 3
6

Note: The multiplication operator * must be escaped as \* to prevent shell globbing.

String Operations:

$ expr length "Linux"
5

$ expr substr "Creative" 1 4
Crea

Limitations compared to bc:

• Only integer arithmetic — no floating-point support.
• Requires careful escaping of operators like *.
• Every operand and operator must be space-separated.
• Less powerful; bc supports precision, functions, and loops.

Key points:

• Useful for simple counters and integer math in scripts.
• Returns exit status based on the result value.

The script command makes a typescript (a recording) of everything displayed on the terminal, including commands typed and their output. It is useful for documentation, tutorials, and audit trails.

Syntax:

script [options] [filename]

• If no filename is given, output is saved in a file named typescript by default.

Workflow:

$ script session.log
Script started, file is session.log
$ date
$ whoami
$ exit
Script done, file is session.log

• Type exit or press Ctrl+D to stop recording.

Useful Options:

• -a → Append to an existing file instead of overwriting.
• -t → Output timing data.
• -c command → Run a single command and record it.

Example of appending:

script -a session.log

Key points:

• Captures the entire interactive session.
• The recorded file can be viewed with cat or a text editor.
• Helpful for reproducing steps or teaching.

The passwd command is used to change a user's password and manage password-related properties.

Syntax:

passwd [options] [username]

Basic usage:

• passwd → Changes the password of the current user. It prompts for the current password, then the new password twice.
• sudo passwd username → Root changes another user's password (no current password needed).

Important Options:

• -l → Locks the user account.
• -u → Unlocks the account.
• -d → Deletes the password (makes it empty).
• -e → Expires the password, forcing change at next login.
• -S → Displays password status information.

Example:

$ passwd
Changing password for user.
Current password:
New password:
Retype new password:
passwd: password updated successfully

Key points:

• Passwords are stored in encrypted form in /etc/shadow.
• A normal user can change only their own password.
• Only root can modify other users' passwords or lock accounts.

These commands provide information about users logged into the system.

who command output fields:

$ who
user1 tty1 2026-09-25 09:15
user2 pts/0 2026-09-25 10:02 (192.168.1.5)

• Username → Name of the logged-in user.
• Terminal (tty/pts) → The terminal line used.
• Login date & time → When the session started.
• Host/IP (in parentheses) → Remote host for network logins.

whoami example:

$ whoami
user1

• Equivalent to id -un.

Key points:

• who reports all users; whoami reports only you.
• Useful options for who: -H (headers), -b (last boot time), -r (run level).

The uname (Unix name) command prints system information about the operating system and hardware.

Syntax:

uname [options]

Options:

• -s → Kernel name (default output), e.g. Linux.
• -n → Network node hostname.
• -r → Kernel release version.
• -v → Kernel version (build info).
• -m → Machine hardware name (architecture), e.g. x86_64.
• -p → Processor type.
• -i → Hardware platform.
• -o → Operating system name.
• -a → All available information combined.

uname -a example:

$ uname -a
Linux myhost 5.15.0-73-generic #80-Ubuntu SMP x86_64 GNU/Linux

The output includes, in order:

• Kernel name (Linux)
• Hostname (myhost)
• Kernel release (5.15.0-73-generic)
• Kernel version (build date/details)
• Machine architecture (x86_64)
• Operating system (GNU/Linux)

Key points:

• Quick way to identify kernel and architecture.
• Frequently used in scripts to detect the platform.

The uptime command shows how long the system has been running along with current load information.

Syntax:

uptime [options]

Sample Output:

$ uptime
22:26:57 up 5 days, 3:14, 2 users, load average: 0.15, 0.20, 0.10

Fields explained:

• Current time → 22:26:57
• Up time → How long the system has been running (5 days, 3:14).
• Number of users → Users currently logged in (2 users).
• Load average → Three numbers: 0.15, 0.20, 0.10.

Load Average interpretation:

• The three values represent the average system load over the last 1, 5, and 15 minutes.
• Load is the average number of processes waiting for or using the CPU.
• On a single-core system:
  • 1.00 means the CPU is fully utilised.
  • < 1.00 means idle capacity.
  • > 1.00 means processes are waiting (overloaded).
• For an -core system, a load of indicates full utilisation.

Options:

• -p → Pretty format of uptime only.
• -s → Shows the time since the system booted.

Key points:

• Useful for monitoring system health and performance trends.

A terminal is an interface that allows users to interact with the operating system by entering commands and viewing text output.

tty command:

The tty command prints the file name of the terminal connected to standard input.

Syntax:

tty [options]

Example:

$ tty
/dev/pts/0

Options:

• -s → Silent mode; prints nothing, only sets exit status.

Difference between tty and pts:

Key points:

• tty helps identify the current terminal device.
• If input is not a terminal, it prints not a tty.
• Useful in scripts to check whether they are running interactively.

The stty (set terminal) command is used to display and change terminal line settings, such as input/output behaviour and control characters.

Syntax:

stty [options] [setting]

Viewing settings:

• stty → Shows basic terminal settings.
• stty -a → Shows all settings in detail.

$ stty -a
speed 38400 baud; rows 24; columns 80;
intr = ^C; erase = ^?; kill = ^U; ...

Changing settings — examples:

• Disable echo (useful for password input):

  stty -echo

• Enable echo again:

  stty echo

• Change the interrupt key:

  stty intr ^X

• Reset the terminal to sane defaults:

  stty sane

Common control settings:

• erase → Character to delete previous character.
• kill → Character to erase entire line.
• intr → Interrupt character (usually Ctrl+C).

Key points:

• Controls how the terminal handles input and special keys.
• stty -echo is commonly used in scripts to hide sensitive input.

The man (manual) command displays the reference manual pages for commands, system calls, files, and more. It is the primary source of built-in documentation in Linux.

Syntax:

man [section] command

Example:

$ man ls

Manual Page Navigation (uses the less pager):

• Spacebar / f → Move forward one screen.
• b → Move back one screen.
• Arrow keys → Scroll line by line.
• /pattern → Search forward for a pattern.
• ?pattern → Search backward.
• n / N → Next / previous search match.
• g / G → Go to beginning / end.
• q → Quit the man page.

Manual Sections:

1. Section 1 → User commands (executable programs).
2. Section 2 → System calls.
3. Section 3 → Library functions.
4. Section 4 → Special files (devices in /dev).
5. Section 5 → File formats and conventions.
6. Section 6 → Games.
7. Section 7 → Miscellaneous (macros, conventions).
8. Section 8 → System administration commands.

Example of specifying a section:

man 5 passwd # shows the /etc/passwd file format

Key points:

• Use man -k keyword (apropos) to search by keyword.
• Structured sections: NAME, SYNOPSIS, DESCRIPTION, OPTIONS, EXAMPLES, SEE ALSO.

Linux commands can be broadly classified as internal (built-in) and external commands.

Determining the type of a command:

The type command reveals whether a command is built-in or external.

$ type cd
cd is a shell builtin

$ type ls
ls is /usr/bin/ls

Other helpful commands:

• which command → Shows the path of an external command.
• command -V command → Similar to type.

Key points:

• Some commands (like echo) exist both as built-ins and external programs.
• The shell searches built-ins first, then the directories in the PATH variable.

These commands quickly report useful details about the system and user environment.

1. date

• Displays or sets the current date and time.
• Supports custom formats.

$ date +"%d-%m-%Y"
25-09-2026

2. uname

• Prints operating system and kernel information.
• uname -a shows all details.

$ uname -r
5.15.0-73-generic

3. uptime

• Shows how long the system has run, number of users, and load averages.

$ uptime
22:26:57 up 5 days, 2 users, load average: 0.15, 0.20, 0.10

4. whoami

• Displays the effective username of the current user.

$ whoami
student

Summary Table:

Key points:

• These commands are widely used in shell scripts and system monitoring.

In shell scripts, expr handles simple integer math while bc handles floating-point math. Combining them lets scripts perform a variety of calculations.

Using expr for integers:

$ count=$(expr 5 + 10)
$ echo $count
15

Using bc for floating-point:

$ result=$(echo "scale=2; 22/7" | bc)
$ echo $result
3.14

Example 1 — Area of a circle :

r=5
area=$(echo "scale=2; 3.14159 $r $r" | bc)
echo "Area = $area"

For :

Example 2 — Simple Interest :

P=1000
R=5
T=2
SI=P $R $T)/100" | bc)
echo "Simple Interest = $SI"

For , , :

Key points:

• Use expr for counters and integer indices.
• Use bc whenever decimals or precision are required.
• The scale variable in bc controls decimal places.

Commands in Linux gain flexibility through options and arguments that follow the command name.

1. Arguments:

Arguments are the data or objects on which a command acts, such as filenames or text strings.

$ cat file1.txt file2.txt

Here file1.txt and file2.txt are arguments.

2. Options (Flags):

Options modify the behaviour of a command.

• Short options: Single letter preceded by a single hyphen -.

  $ ls -l

• Long options: Full word preceded by a double hyphen --.

  $ ls --all

Difference between short and long options:

3. Combining options:

Multiple short options can be merged after a single hyphen.

$ ls -l -a -h # separate
$ ls -lah # combined (equivalent)

Options with values:

$ sort -k 2 file.txt # short with argument
$ head --lines=5 file.txt # long with =value

4. Order and precedence:

• Options generally come before arguments.
• Some commands allow options anywhere; others require a strict order.

Key points:

• Commands are case-sensitive.
• The -- marker signals the end of options, so filenames beginning with - can follow.
• Understanding options and arguments is essential for reading man pages and writing scripts.

Linux provides several complementary tools for learning about commands. Together they help a user discover, understand, and quickly reference commands.

1. man — full manual pages:

Provides complete, detailed documentation for a command.

$ man ls

• Structured into NAME, SYNOPSIS, DESCRIPTION, OPTIONS, etc.
• Best when you need in-depth information.

2. whatis — one-line description:

Displays a brief, one-line summary of a command.

$ whatis ls
ls (1) - list directory contents

• Best for a quick reminder of what a command does.

3. apropos — keyword search:

Searches man page descriptions for a keyword (same as man -k).

$ apropos copy
cp (1) - copy files and directories
scp (1) - secure copy

• Best when you don't know the command name but know the task.

4. --help option — quick syntax help:

Most commands support a built-in --help flag.

$ ls --help

• Shows a concise usage summary without opening the pager.

Comparison Table:

Key points:

• whatis and apropos rely on a database updated by mandb.
• Use apropos to discover, whatis to recall, --help for quick syntax, and man for depth.