Master Linux Memory Monitoring: 6 Essential Commands Explained

Why Monitoring Memory Matters

System memory is a critical resource; regularly checking its usage helps detect abnormal consumption, prevent service outages, and ensure stable operation of applications and servers.

1. free Command

The free utility displays total, used, free, shared, buffer/cache, and available memory. Common options include -b, -k, -m, -g for units, -o to hide the buffer column, -s<seconds> for continuous monitoring, -t to show totals, and -V for version.

free -t               # show totals
free -h -s 10        # update every 10 seconds
free -h -c 10        # output 10 times (requires -s)
free -V               # version

Key fields:

Mem : overall memory usage line.

Swap : swap usage line.

Total : physical memory available to the OS.

Used : memory allocated (interpretation differs between kernel versions).

Free : memory not yet allocated.

Shared : shared memory.

Buff/Cache : memory used for buffers and cache.

Available : memory that can be allocated without swapping (newer kernels).

2. vmstat Command

vmstat

reports virtual memory statistics, including processes, memory, paging, block I/O, traps, and CPU activity. It is part of the sysstat package.

vmstat -s               # display memory statistics
vmstat 2 1             # sample every 2 seconds, 1 time

Typical columns:

procs : r (running/ready), b (blocked).

memory : swpd, free, buff, cache.

swap : si (swap‑in), so (swap‑out).

io : bi (blocks in), bo (blocks out).

system : in (interrupts), cs (context switches).

cpu : us (user), sy (system), id (idle), wa (IO wait).

Common troubleshooting clues:

High r with low id → CPU overload.

Non‑zero pi/po → memory pressure.

High disk and queue >3 → IO bottleneck.

3. /proc/meminfo

Reading /proc/meminfo provides raw memory statistics directly from the kernel. Example: cat /proc/meminfo Important fields include MemTotal, MemFree, MemAvailable, Buffers, Cached, SwapTotal, SwapFree, Active, Inactive, and many others that give a detailed view of memory allocation, cache, swap usage, and kernel structures.

Values are in kB (kilobytes). Use less /proc/meminfo to scroll.

4. top Command

top

shows real‑time CPU and memory usage per process, along with system‑wide summaries. Basic syntax: top [-] [d delay] [q] [c] [S] [s] [i] [n] [b] Key options: d: change refresh delay. q: fastest refresh (requires root). c: toggle full command line. S: cumulative mode. i: hide idle processes. n: number of updates before exit. b: batch mode (useful for logging).

Typical output sections:

System time and load average.

Task summary (total, running, sleeping, stopped, zombie).

CPU usage percentages (us, sy, ni, id, wa, hi, si, st).

Memory summary (total, free, used, buff/cache).

Swap summary.

Per‑process table (PID, USER, PR, NI, VIRT, RES, SHR, S, %CPU, %MEM, TIME+, COMMAND).

5. htop Command

htop

is an interactive, color‑enhanced alternative to top that allows vertical and horizontal scrolling, process tree view, and easy management (kill, renice) via function keys.

# htop          # launch interactive view
# top -c        # show full command line in top

Useful shortcuts:

F1 / h / ? – help.

F2 – settings.

F3 – search.

F4 – filter.

F5 – tree view.

F6 – sort column.

F7 / [ – decrease nice.

F8 / ] – increase nice.

F9 / k – kill selected process.

F10 / q – quit.

6. Inspecting a Specific Process’s Memory

Read /proc/<pid>/status to see detailed memory usage for a given process. cat /proc/4398/status Key fields include Name, State, VmSize, VmRSS, Threads, and various capability and context‑switch counters.

Conclusion

Understanding and using these Linux tools—free, vmstat, /proc/meminfo, top, htop, and per‑process status—gives administrators the ability to quickly diagnose memory‑related issues, identify CPU or IO bottlenecks, and keep systems running smoothly.