free command
Display amount of free and used memory in the system.
Overview
The free command displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the kernel. It provides a snapshot of memory usage, helping users monitor system resources and diagnose memory-related issues.
Options
-b
Display the amount of memory in bytes.
$ free -b
total used free shared buff/cache available
Mem: 8273514496 3868327936 1535881216 602931200 2869305344 3459538944
Swap: 2147479552 0 2147479552
-k
Display the amount of memory in kilobytes (default).
$ free -k
total used free shared buff/cache available
Mem: 8079604 3777664 1500860 588800 2801080 3378456
Swap: 2097148 0 2097148
-m
Display the amount of memory in megabytes.
$ free -m
total used free shared buff/cache available
Mem: 7889 3689 1465 574 2735 3299
Swap: 2047 0 2047
-g
Display the amount of memory in gigabytes.
$ free -g
total used free shared buff/cache available
Mem: 7 3 1 0 2 3
Swap: 1 0 1
-h, --human
Show all output fields automatically scaled to shortest three-digit unit and display units.
$ free -h
total used free shared buff/cache available
Mem: 7.7Gi 3.6Gi 1.4Gi 574Mi 2.7Gi 3.2Gi
Swap: 2.0Gi 0B 2.0Gi
-s, --seconds N
Continuously display the result with N second delay between updates.
$ free -s 2
total used free shared buff/cache available
Mem: 8079604 3777664 1500860 588800 2801080 3378456
Swap: 2097148 0 2097148
total used free shared buff/cache available
Mem: 8079604 3778112 1500412 588800 2801080 3378008
Swap: 2097148 0 2097148
-t, --total
Display a line showing the column totals.
$ free -t
total used free shared buff/cache available
Mem: 8079604 3777664 1500860 588800 2801080 3378456
Swap: 2097148 0 2097148
Total: 10176752 3777664 3598008
-w, --wide
Switch to the wide mode. The wide mode produces lines longer than 80 characters. In this mode buffers and cache are reported in two separate columns.
$ free -w
total used free shared buffers cache available
Mem: 8079604 3777664 1500860 588800 245760 2555320 3378456
Swap: 2097148 0 2097148
Usage Examples
Basic memory information
$ free
total used free shared buff/cache available
Mem: 8079604 3777664 1500860 588800 2801080 3378456
Swap: 2097148 0 2097148
Human-readable output with totals
$ free -ht
total used free shared buff/cache available
Mem: 7.7Gi 3.6Gi 1.4Gi 574Mi 2.7Gi 3.2Gi
Swap: 2.0Gi 0B 2.0Gi
Total: 9.7Gi 3.6Gi 3.4Gi
Continuous monitoring with 5-second intervals
$ free -h -s 5
total used free shared buff/cache available
Mem: 7.7Gi 3.6Gi 1.4Gi 574Mi 2.7Gi 3.2Gi
Swap: 2.0Gi 0B 2.0Gi
Tips
Understanding Memory Output
- total: Total installed memory
- used: Memory currently in use
- free: Unused memory
- shared: Memory shared by multiple processes
- buff/cache: Memory used by kernel buffers and page cache
- available: Estimate of memory available for starting new applications without swapping
Interpreting "Available" vs "Free"
The "available" column is more important than "free" when assessing if your system has enough memory. It includes memory that can be freed and used by applications.
Monitoring Memory Over Time
Use free -s N to monitor memory usage over time, which helps identify memory leaks or usage patterns.
Clearing Cache Memory
System administrators can free pagecache, dentries, and inodes with:
$ sudo sh -c "sync; echo 3 > /proc/sys/vm/drop_caches"
(Note: This should be done carefully and is rarely necessary in normal operation)
Frequently Asked Questions
Q1. What does it mean if my "free" memory is very low?
A. Low free memory isn't necessarily a problem. Linux uses available memory for disk caching to improve performance. Look at the "available" column for a better indication of memory that can be allocated to applications.
Q2. Why is my swap memory not being used?
A. Swap is only used when physical memory is nearly exhausted or for inactive memory pages. If your system has plenty of RAM, swap might remain unused.
Q3. How can I monitor memory usage continuously?
A. Use free -s N where N is the number of seconds between updates. For example, free -s 5 will update every 5 seconds.
Q4. What's the difference between buffers and cache?
A. Buffers are used for block device I/O, while cache is used for file system pages. In the standard output, they're combined as "buff/cache", but can be viewed separately with the -w option.
References
https://www.man7.org/linux/man-pages/man1/free.1.html
Revisions
- 2025/05/05 First revision