The Linux Command Tutorial series provides rigorous, upstream-verified references for essential system commands across Linux distributions and UNIX-like environments. Each article focuses on a single executable, combining exhaustive option documentation, verified real-world examples, security boundaries, and best practices directly derived from official source documentation and POSIX standards.

---

1. Introduction

Upstream: procps-ng 4.0.4 | POSIX: Linux-Specific (procps-ng extension) | Safety Tier: safe-read-only | Scope: memory-inspection

free displays the total amount of free and used physical memory (RAM) and swap memory in the system, as well as the memory used by kernel buffers and page caches. It parses /proc/meminfo to report accurate, kernel-derived memory statistics.

• Upstream Project & Provenance: Maintained within procps-ng (procps-ng).
• Portability & Standards Baseline: free is a Linux-specific utility reflecting Linux virtual memory architecture; it is not defined in POSIX.1-2024.
• Target Research Implementation: Audited against procps-ng 4.0.4 (free(1)).
• Applicability & Lifecycle: The standard command for checking available memory and diagnosing Out-Of-Memory (OOM) risk.

---

2. Syntax and Command Model

2.1 Canonical Synopsis

free [options]

2.2 Execution Model & Memory Metric Definitions

free queries /proc/meminfo and computes tabular memory columns:

• total: Total installed memory (MemTotal).
• used: Calculated as total - free - buff/cache.
• free: Unused memory (MemFree) that contains no data.
• shared: Memory utilized by tmpfs and POSIX shared memory (Shmem).
• buff/cache: Combined kernel block buffers (Buffers) and page cache (Cached + SReclaimable).
• available: The kernel's estimate of memory available for starting new applications without swapping (MemAvailable).

---

3. Options

3.1 Display and Unit Flags

Short Flag	Long Flag	Description	Default
-b	--bytes	Display memory in bytes.	KiB
-k	--kibi	Display memory in kibibytes (1024 bytes).	Default
-m	--mebi	Display memory in mebibytes (1,048,576 bytes).	Off
-g	--gibi	Display memory in gibibytes (1,073,741,824 bytes).	Off
-h	--human	Human-readable output; auto-scales to shortest unit.	Off
-w	--wide	Wide mode: separate buffers and cache into two columns.	Combined
-s N	--seconds N	Continuously display memory metrics every N seconds.	Single-shot
-c N	--count N	Display N updates when used with -s.	Infinite
-t	--total	Display a line showing total RAM + Swap.	Off

---

4. Basic Usage

4.1 Quick-Reference Cheatsheet Card

Operation	Command	Notes
Human-readable summary	free -h	Displays RAM/swap in GiB/MiB
Megabytes format	free -m	Deterministic numeric output for scripting
Wide breakdown	free -h -w	Separates buffers from cache
Include total row	free -h -t	Appends total line combining RAM + Swap
Continuous monitoring	free -h -s 2 -c 5	Updates every 2 seconds, 5 times
Check available memory	free -m | awk '/^Mem:/ {print $7}'	Extracts usable memory without swapping

4.2 Human-Readable Memory Summary

free -h

               total        used        free      shared  buff/cache   available
Mem:            15Gi       5.8Gi       4.2Gi       412Mi       5.5Gi       9.1Gi
Swap:          2.0Gi          0B       2.0Gi

4.3 Detailed Wide Output

free -h -w

               total        used        free      shared     buffers       cache   available
Mem:            15Gi       5.8Gi       4.2Gi       412Mi       320Mi       5.2Gi       9.1Gi
Swap:          2.0Gi          0B       2.0Gi

---

5. Practical Operations

5.1 Understanding Why "Free" Memory is Low

IMPORTANT

The Available vs Free Distinction: Linux aggressively utilizes unallocated memory for disk caching (buff/cache) to maximize system performance. A low free figure is normal and expected. Always evaluate the available column, which reflects memory that can be immediately provided to applications without forcing swap activity.

• Linux Kernel Architecture: The Linux kernel intentionally borrows unused RAM for disk caching (buff/cache) to accelerate disk reads.
• The Golden Rule: Look at the available column, not the free column.
• If applications require more RAM, the kernel instantaneously reclaims page cache without swapping. The available column accurately reflects memory ready to be allocated.

5.2 Polling Memory Usage During Load Testing

Monitoring memory allocation every 2 seconds for 5 iterations:

free -h -s 2 -c 5

5.3 Automated OOM Health Check in Shell Scripts

Extracting the available memory in megabytes for automated threshold alerting:

free -m | awk '/^Mem:/ {print $7}'

9318

• Column 7 represents available memory in MiB.

---

6. Advanced Usage

6.1 Inspecting Kernel Slable Cache Reclaimability

Wide mode (-w) exposes buffers vs cache. In modern Linux kernels, cache includes both the standard page cache and SReclaimable (reclaimable kernel slab memory used for dentries and inodes). free accurately accounts for reclaimable slab in the available metric.

---

7. Exit Status, Environment, and Configuration

7.1 Exit Status Codes

Exit Code	Meaning
0	Success: /proc/meminfo parsed and displayed cleanly.
>0	An error occurred (invalid command-line option, /proc/meminfo unreadable).

---

8. Safety, Security, and Portability

8.1 Linux-Specific Kernel Metrics

NOTE

free directly parses /proc/meminfo and is specific to the Linux kernel. It is not available on BSD or macOS systems (which use vm_stat or sysctl vm).

---

9. Best Practices

1. Evaluate available, Never Just free:
   • Guidance: Base all monitoring and automated alerts on the available metric.
   • Authoritative Justification: procps-ng documentation notes that free reflects strictly idle memory, whereas available accounts for reclaimable caches.
2. Use -h for Terminal Inspection and -m for Scripts:
   • Guidance: Use free -h for human eyes and free -m or free -b for shell scripts.
   • Authoritative Justification: -h produces changing unit suffixes (Mi, Gi) that complicate numeric comparisons in shell scripts.
3. Use -w When Diagnosing Filesystem Pressure:
   • Guidance: Pass -w to inspect separate buffer and cache allocations.
   • Authoritative Justification: Discloses whether disk block buffers or VFS inode/dentry caches are driving memory utilization.

---

References

1. procps-ng free(1) Manual: https://man7.org/linux/man-pages/man1/free.1.html
2. Linux /proc/meminfo Documentation: https://docs.kernel.org/filesystems/proc.html#meminfo