For a fast system-wide check, run free -h. It shows RAM, swap and an available estimate—the figure that matters more than the raw free column when deciding whether Linux can start more applications without swapping. Use top or ps to identify large processes, vmstat 1 to watch memory pressure over time, /proc/meminfo for kernel counters, and sudo dmidecode -t 17 to inventory physical RAM modules.
The Linux Foundation published the original Jack Wallen article on March 6, 2022, demonstrating on Ubuntu Server 18.04. Its “five commands” title is technically imprecise: /proc/meminfo is a virtual procfs file, not a command. Availability of these utilities and their options varies among distributions, package versions, minimal images, containers and virtual machines.
What “memory usage” means in Linux
Several different measurements are commonly called memory usage:
- Total: usable physical RAM.
- Free: RAM currently idle.
- Available: an estimate of memory that can be allocated to new applications without swapping. Linux derives it from reclaimable cache and other information, so it is generally more useful than free alone.
- Used: in current
freeoutput, total minus available; it is not simply application memory. - Buffers and cache: kernel buffers, page cache and reclaimable kernel memory that Linux can often reuse.
- Swap used: occupied swap space. It does not by itself prove that the system is currently thrashing.
- Resident process memory: physical pages currently resident for a process.
- Virtual memory: address space mapped or reserved by a process, not necessarily RAM in use.
A low free value is therefore not automatically an emergency. Check available, active swap movement and application behavior.
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Fastest one-time check: free
free -hThe -h option uses human-readable binary units such as KiB, MiB and GiB. The utility reads /proc/meminfo and reports physical and swap memory.
free -h -s 1
free -h -w
free -h -s 1 -c 5
free -m
free -g
free -b-s 1 repeats every second, -c 5 limits the run to five reports, and -w keeps buffers and cache in separate columns. The older -m, -g and -t forms remain useful when a script or person requires a particular scale or a total row. See the current free manual for implementation details.
| Column | Meaning |
|---|---|
| total | Physical RAM or swap capacity visible to the system. |
| used | Total minus available in current procps-style output. |
| free | RAM not currently allocated. |
| buff/cache | Buffers, page cache and reclaimable memory. |
| available | Estimated memory that can be used for new applications without swapping. |
Find the largest memory-consuming process
Interactive view with top
top
top -o %MEMIn current procps-ng top, press M to sort by memory, c to toggle command names and full command lines, h or ? for help, and q to quit. E or e changes display scaling on versions that support those keys. The -o %MEM startup sort is supported by procps-ng-style implementations, not every top.
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| Field | Interpretation |
|---|---|
| %MEM | Process resident share of physical memory. |
| RES | Resident memory currently in RAM. |
| VIRT | Virtual address space and mappings; not equivalent to RAM use. |
| SHR | Memory that may be shared with other processes. |
| SWAP | Process pages currently located in swap, where reported. |
Consult the top manual. A large process at one instant is not proof of a leak; record observations over time.
Noninteractive process listing
ps aux --sort=-%mem | head
ps -eo pid,ppid,comm,%mem,rss,vsz --sort=-%mem | head -n 15RSS is usually shown in KiB and represents resident memory; VSZ is virtual size. Shared pages mean that adding every process’s RSS will not produce an exact system total.
Watch swapping and pressure over time with vmstat
vmstat 1
vmstat 1 5
vmstat -y 1
vmstat -svmstat 1 emits one report per second. The first report summarizes averages since boot; later reports describe each sampling interval. -y suppresses the since-boot first line, and -s prints cumulative counters rather than an interval stream. Refer to the vmstat manual.
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| Column | What to watch |
|---|---|
| swpd | Swap memory currently used. |
| si | Memory swapped in per second. |
| so | Memory swapped out per second. |
| free | Idle memory. |
| buff/cache | Buffer and cache memory. |
| r | Runnable processes. |
| b | Processes blocked, often waiting for I/O. |
High swpd with near-zero si and so can indicate past swap use without current pressure. Sustained nonzero swap-in or swap-out, falling available memory and worsening latency are stronger evidence of active pressure.
Read kernel counters from /proc/meminfo
cat /proc/meminfo
less /proc/meminfo
grep -E '^(Mem|Swap|SReclaimable|Shmem|Slab)' /proc/meminfo
grep -E '^(MemTotal|MemFree|MemAvailable|Buffers|Cached|SwapCached|SwapTotal|SwapFree):' /proc/meminfoThe kernel documentation describes this procfs interface as memory-distribution information and warns that counters can overlap, so they should not be treated as independent addends. Definitions are documented in the kernel proc filesystem documentation and the proc_meminfo manual.
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Inspect installed RAM modules with dmidecode
sudo dmidecode -t 17
sudo dmidecode --type memory
sudo dmidecode -t 17 | less
sudo dmidecode -t 17 > dmi_info.txtType 17 queries memory-device records. Depending on firmware, output can include module size, slot locator, form factor, DDR type, speed, manufacturer, part number, serial number and empty slots. This is hardware inventory—not live utilization—and normally requires elevated privileges.
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dmidecode relies on firmware-provided DMI/SMBIOS data. Virtual machines may expose synthetic or incomplete records, and some systems restrict access. For a simple visible total, compare it with free -h or grep MemTotal /proc/meminfo.
A practical diagnostic workflow
- Establish capacity and headroom: run
free -hand inspectavailable, not justfree. - Check activity: run
vmstat 1 5; sustainedsi/sois more informative than swap allocation alone. - Attribute usage: run
ps -eo pid,ppid,comm,%mem,rss,vsz --sort=-%mem | head -n 15or sorttopwithM. - Investigate kernel and shared memory: inspect the filtered
/proc/meminfofields. - Confirm ownership before action: identify the service, capture diagnostics and use its service manager where appropriate, for example
sudo systemctl status service-namefollowed bysudo systemctl stop service-name. Do not kill a process merely because it is large.
When a command is missing or the environment is unusual
command -v free top vmstat dmidecode
top -h
free --help
vmstat --help
dmidecode --helpfree, top and vmstat commonly come from procps or procps-ng; dmidecode is often a separate package. Minimal distributions and BusyBox may provide reduced implementations, so verify the local help output rather than assuming every option exists.
In containers, output may reflect the host, a namespace or a cgroup-limited view, depending on runtime, procps version and configuration. Distinguish host RAM from the container’s memory limit and in-container usage. In a VM, dmidecode describes exposed virtual hardware, not necessarily the physical host.
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Command chooser
| Question | Use first | Main limitation |
|---|---|---|
| How much RAM and swap are available now? | free -h |
No process attribution. |
| Which process is largest? | top or ps |
A snapshot does not establish a leak. |
| Is swapping active? | vmstat 1 |
Requires interpreting interval columns. |
| What kernel categories contribute to memory? | /proc/meminfo |
Counters can overlap. |
| What physical modules are installed? | sudo dmidecode -t 17 |
Needs privileges and reliable SMBIOS data. |
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