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System resource monitoring (free, uptime, vmstat) in Linux CLI - Cheat Sheet & Quick Revision

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Recall & Review
beginner
What does the free command show in Linux?
The free command displays the amount of used and free memory in the system, including RAM and swap space. It helps you understand how much memory is available and how much is being used.
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beginner
What information does the uptime command provide?
The uptime command shows how long the system has been running, the current time, how many users are logged in, and the system load averages for the last 1, 5, and 15 minutes.
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intermediate
What is the purpose of the vmstat command?
The vmstat command reports information about processes, memory, paging, block IO, traps, and CPU activity. It helps monitor system performance and resource usage over time.
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intermediate
How can you interpret the load average numbers shown by uptime?
Load averages represent the average number of processes waiting to run over 1, 5, and 15 minutes. A load average close to the number of CPU cores means the system is busy but not overloaded. Higher numbers indicate more demand than the CPU can handle.
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intermediate
Which vmstat fields indicate CPU usage?
In vmstat, the CPU usage fields are us (user time), sy (system time), id (idle time), and wa (time waiting for IO). These help understand how the CPU is spending its time.
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What does the free command NOT show?
AMemory usage details
BSwap space usage
CAvailable RAM
DCPU load averages
Which command shows how long the system has been running?
Avmstat
Buptime
Cfree
Dtop
In vmstat, what does the id field represent?
AIdle CPU time
BInput device usage
CInterrupts per second
DDisk IO
What does a high load average compared to CPU cores indicate?
ASystem is overloaded
BSystem is under low load
CSystem is idle
DSystem has no users logged in
Which command would you use to monitor memory and CPU usage over time?
Auptime
Bfree
Cvmstat
Dls
Explain how you would use free, uptime, and vmstat together to monitor a Linux system's health.
Think about what each command tells you and how they complement each other.
You got /4 concepts.
    Describe what the load average numbers mean and how they relate to CPU cores.
    Imagine how busy your kitchen is with cooks compared to the number of stoves.
    You got /4 concepts.

      Practice

      (1/5)
      1. What does the free command primarily display on a Linux system?
      easy
      A. System uptime and load average
      B. Disk usage statistics
      C. CPU temperature
      D. Memory usage and availability

      Solution

      1. Step 1: Understand the purpose of free

        The free command shows information about the system's memory usage, including total, used, free, and available memory.

      2. Step 2: Compare with other commands

        uptime shows system running time and load, vmstat shows detailed CPU and memory stats, but free focuses on memory.

      3. Final Answer:

        Memory usage and availability -> Option D

      4. Quick Check:

        free = Memory info [OK]
      Hint: Remember: free = memory info, uptime = running time [OK]
      Common Mistakes:
      • Confusing free with uptime output
      • Thinking free shows CPU or disk info
      • Mixing vmstat details with free
      2. Which of the following is the correct syntax to display memory usage in megabytes using the free command?
      easy
      A. free -k
      B. free -m
      C. free -g
      D. free --bytes

      Solution

      1. Step 1: Identify the option for megabytes

        The -m option in free shows memory in megabytes.

      2. Step 2: Check other options

        --bytes shows bytes, -g shows gigabytes, -k shows kilobytes. So -m is correct for MB.

      3. Final Answer:

        free -m -> Option B

      4. Quick Check:

        free -m = megabytes [OK]
      Hint: Use -m for MB, -k for KB, -g for GB in free [OK]
      Common Mistakes:
      • Using -k expecting MB output
      • Confusing --bytes with -m
      • Using wrong option like -g for MB
      3. What output will the command uptime produce on a system that has been running for 3 days, 4 hours, 12 minutes with load averages 0.15, 0.10, 0.05?
      medium
      A. 10:30:00 up 3 days, 4:12, 1 user, load average: 0.15, 0.10, 0.05
      B. 3 days, 4 hours, 12 minutes uptime with load 0.15, 0.10, 0.05
      C. uptime: 3d 4h 12m, load: 0.15 0.10 0.05
      D. System up for 3 days 4 hours 12 minutes, load averages: 0.15 0.10 0.05

      Solution

      1. Step 1: Understand uptime output format

        The uptime command shows current time, system uptime in days and hours:minutes, number of users, and load averages.

      2. Step 2: Match given data to output style

        The correct format includes current time, "up 3 days, 4:12", user count, and load averages separated by commas.

      3. Final Answer:

        10:30:00 up 3 days, 4:12, 1 user, load average: 0.15, 0.10, 0.05 -> Option A

      4. Quick Check:

        uptime shows time, up, users, load [OK]
      Hint: Look for 'up' and load averages format in uptime output [OK]
      Common Mistakes:
      • Expecting uptime to print in verbose sentences
      • Missing commas or user count in output
      • Confusing uptime with other commands
      4. You run vmstat 1 3 but get an error saying "command not found". What is the most likely cause?
      medium
      A. vmstat is not installed on the system
      B. The interval must be greater than 1 second
      C. The command syntax is incorrect; count must be last
      D. You need to run vmstat as root user

      Solution

      1. Step 1: Verify vmstat syntax

        vmstat 1 3 uses correct syntax: 1-second delay, 3 samples.

      2. Step 2: Analyze the error

        "command not found" means vmstat (from procps package) is not installed.

      3. Final Answer:

        vmstat is not installed on the system -> Option A

      4. Quick Check:

        command not found = not installed [OK]
      Hint: vmstat missing? Install procps package [OK]
      Common Mistakes:
      • Assuming syntax error for missing command
      • Thinking root needed for vmstat
      • Believing interval too small
      5. You want to monitor CPU and memory usage every 2 seconds for 5 samples using vmstat. Which command will correctly do this and what will be the total monitoring time?
      hard
      A. vmstat 2 5; total time 10 seconds
      B. vmstat 5 2; total time 10 seconds
      C. vmstat 2 5; total time 8 seconds
      D. vmstat 5 2; total time 8 seconds

      Solution

      1. Step 1: Understand vmstat interval and count

        vmstat 2 5 runs vmstat every 2 seconds, 5 times total.

      2. Step 2: Calculate total monitoring time

        vmstat prints the first line immediately, then 4 intervals of 2 seconds each, so total time = 2 * (5 - 1) = 8 seconds.

      3. Final Answer:

        vmstat 2 5; total time 8 seconds -> Option C

      4. Quick Check:

        Total time = interval * (count - 1) [OK]
      Hint: Total time = interval x (count - 1) for vmstat [OK]
      Common Mistakes:
      • Multiplying interval by count instead of count-1
      • Swapping interval and count values
      • Assuming total time equals interval x count
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