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free Command Cheat Sheet

free gives a snapshot of the system's memory usage. It parses /proc/meminfo and displays total, used, free, shared, buffer, and swapped memory.

Synopsis

free [options]

Basic Usage

Standard Output

free

Output (in kilobytes by default): 

              total        used        free      shared  buff/cache   available
Mem:        8106604     2125640     1568832      302456     4412132     5412520
Swap:       2097148           0     2097148

Understanding the Columns

Most confusion comes from "free" vs "available".

Column Meaning
total Total installed RAM (excluding kernel reserved bits)
used Memory currently in use by processes (total - free - buff/cache)
free Completely unused memory (wasted RAM!)
shared Memory used by tmpfs (shmem)
buff/cache Memory used by kernel buffers and page cache
available The most important number. Estimate of how much memory is available for starting new applications.

Why is free so low? Linux caches accessed files in RAM to speed up performance. This memory is marked as buff/cache but can be instantly reclaimed if apps need it. Therefore, check available, not free.

Display Options

Human Readable (-h)

Uses standard suffixes (Ki, Mi, Gi - base 1024).

free -h

Output: 

              total        used        free      shared  buff/cache   available
Mem:          7.7Gi       2.0Gi       1.5Gi       295Mi       4.2Gi       5.2Gi
Swap:         2.0Gi          0B       2.0Gi

SI Units (--si)

Uses power of 1000 (KB, MB, GB).

free -h --si

Specific Units

Force output in specific unit (numbers can be huge).

  • -b: bytes
  • -k: kilobytes (default)
  • -m: megabytes
  • -g: gigabytes
  • --tera: terabytes
free -m

Continuous Monitoring

Watch Mode (-s)

Run free continuously with a delay (seconds).

free -h -s 3
(Updates every 3 seconds. Press Ctrl+C to stop).

Repeated Count (-c)

Run N times and exit.

free -h -c 5 -s 1

Detailed View

Show Low/High Memory (-l)

Relevant for 32-bit systems with PAE or unusual architectures. Shows memory mapping.

free -l

Total Line (-t)

Adds a line at the bottom summing Physical RAM + Swap.

free -h -t
Output: 
Total:        9.7Gi       2.0Gi

Troubleshooting High Memory Usage

  1. Check available: If this is close to zero, your system will start Swapping.
  2. Check Swap used: If Swap is high and actively changing (thrashing), performance will tank.
  3. Check buff/cache: If this is high but available is also ok, this is healthy behavior. Do not try to "clear RAM" manually; let the kernel manage it.

Clearing Cache (For Testing Only)

If you really need to flush buffers (e.g., before a benchmark):

sudo sync; echo 3 | sudo tee /proc/sys/vm/drop_caches
Note: This spikes CPU I/O as the cache is rebuilt.

Exit Status

Code Meaning
0 Success
1 Error

Comparison

  • free: Quick overview.
  • vmstat: Memory + IO/CPU context.
  • top: Per-process memory.
  • cat /proc/meminfo: The raw, extremely detailed source.