df (disk free space) in Linux CLI - Time & Space Complexity

Time Complexity: df (disk free space)

O(n)

Understanding Time Complexity

We want to understand how the time taken by the df command changes as the number of mounted file systems grows.

How does the command's work increase when there are more disks or partitions?

Scenario Under Consideration

Analyze the time complexity of this df command usage.

df -h

This command lists disk space usage for all mounted file systems in a human-readable format.

Identify Repeating Operations

The command internally processes each mounted file system one by one.

• Primary operation: Reading and calculating disk usage for each mounted file system.
• How many times: Once per mounted file system.

How Execution Grows With Input

As the number of mounted file systems increases, the command does more work linearly.

Input Size (number of file systems)	Approx. Operations
10	About 10 disk usage checks
100	About 100 disk usage checks
1000	About 1000 disk usage checks

Pattern observation: The work grows directly in proportion to the number of file systems.

Final Time Complexity

Time Complexity: O(n)

This means the time taken grows in a straight line as the number of mounted file systems increases.

Common Mistake

[X] Wrong: "The df command always runs instantly no matter how many disks there are."

[OK] Correct: The command must check each file system, so more disks mean more work and longer time.

Interview Connect

Understanding how commands scale with input size helps you reason about system performance and scripting efficiency in real tasks.

Self-Check

"What if the command was run with a specific file system path instead of all? How would the time complexity change?"

Practice

1. What does the df command do in Linux?

easy

A. Shows disk free space on mounted filesystems

B. Displays running processes

C. Lists files in a directory

D. Shows network connections

Solution

1. Step 1: Understand the purpose of df

   The df command is used to report disk space usage on mounted filesystems.

2. Step 2: Compare with other commands

   Other options like listing processes or files are done by different commands like ps or ls.

3. Final Answer:

   Shows disk free space on mounted filesystems -> Option A

4. Quick Check:

   df = disk free space [OK]

Hint: Remember: df = disk free space quick check [OK]

Common Mistakes:

• Confusing df with ls or ps commands
• Thinking df shows network info
• Assuming df lists files

2. Which of these is the correct syntax to show disk space in human-readable format?

easy

A. df -r

B. df -h

C. df --list

D. df -x

Solution

1. Step 1: Identify the option for human-readable sizes

   The -h option in df shows sizes in easy units like MB, GB.

2. Step 2: Check other options

   Options like -r, --list, or -x do not provide human-readable output.

3. Final Answer:

   df -h -> Option B

4. Quick Check:

   -h means human-readable [OK]

Hint: Use -h for human-readable sizes with df [OK]

Common Mistakes:

• Using -r or -x instead of -h
• Forgetting the dash before h
• Trying --list which is invalid

3. What will be the output of the command df -h /home?

medium

A. Shows disk usage of the /home filesystem in human-readable format

B. Shows disk usage of the root filesystem

C. Shows all mounted filesystems in bytes

D. Shows network disk usage

Solution

1. Step 1: Understand the command arguments

   The command df -h /home checks disk space for the /home directory only, with human-readable sizes.

2. Step 2: Eliminate other options

   It does not show root filesystem unless /home is on root, nor does it show all filesystems or network usage.

3. Final Answer:

   Shows disk usage of the /home filesystem in human-readable format -> Option A

4. Quick Check:

   df -h /home = /home usage human-readable [OK]

Hint: Add path after df -h to check specific folder space [OK]

Common Mistakes:

• Assuming it shows all filesystems
• Confusing /home with root filesystem
• Expecting output in bytes without -h

4. You run df -h /nonexistent and get an error. What is the likely cause?

medium

A. You need root permissions to run df

B. The df command does not support -h option

C. The path /nonexistent does not exist or is not mounted

D. The disk is full

Solution

1. Step 1: Analyze the error cause

   Running df on a path that does not exist or is not mounted causes an error.

2. Step 2: Check other options

   df supports -h, does not require root, and disk full does not cause this error.

3. Final Answer:

   The path /nonexistent does not exist or is not mounted -> Option C

4. Quick Check:

   Invalid path = error [OK]

Hint: Check if path exists before using df on it [OK]

Common Mistakes:

• Thinking -h is unsupported
• Assuming root needed for df
• Confusing disk full with path error

5. You want to check disk space for all mounted filesystems but exclude the tmpfs type. Which command will do this?

hard

A. df -h --exclude tmpfs

B. df -h --exclude=tmpfs

C. df -h --ignore-type tmpfs

D. df -h -x tmpfs

Solution

1. Step 1: Identify the correct option to exclude filesystem type

   The -x option excludes a filesystem type in df. So df -h -x tmpfs excludes tmpfs.

2. Step 2: Verify other options

   Options like --exclude-type, --exclude, or --ignore-type are invalid for df.

3. Final Answer:

   df -h -x tmpfs -> Option D

4. Quick Check:

   -x excludes filesystem type [OK]

Hint: Use -x to exclude filesystem types in df [OK]

Common Mistakes:

• Using invalid options like --exclude-type
• Confusing exclude with ignore
• Forgetting dash before x