free Memory Usage

free cheatsheet — check RAM and swap usage on Linux. free -h for human-readable, free -m for MB, free -s 5 to watch every 5s. Understand available vs used memory.

4 min read

What it is

A command-line utility to display the amount of free and used memory in the system.

Installation

Linux

free is typically pre-installed on most Linux distributions. If not:

Debian/Ubuntu: sudo apt update && sudo apt install procps
Fedora/CentOS/RHEL: sudo dnf install procps-ng or sudo yum install procps-ng

macOS

free is not available by default on macOS. You can install it using Homebrew: brew install coreutils After installation, you might need to use gfree instead of free to avoid conflicts with the built-in free command.

Windows

free is not available on Windows. You can use tools like Task Manager or PowerShell cmdlets (Get-ComputerInfo) for memory information.

Core Concepts

total: Total amount of physical RAM in the system.
used: Amount of memory currently being used by processes.
free: Amount of memory that is completely unused.
shared: Memory used by tmpfs (RAM-based file system).
buff/cache: Memory used by the kernel for buffering and caching disk I/O. This memory can be reclaimed if applications need it.
available: An estimate of how much memory is available for starting new applications, without swapping. This includes free memory plus a portion of buff/cache that can be easily freed. This is often the most useful metric for understanding available memory.

Commands / Usage

Basic Memory Information

free Display memory usage in kibibytes (KiB).

free

Output example:

              total        used        free      shared  buff/cache   available
Mem:        8071140     3876580     1385444      211884     2809116     3673940
Swap:       2097148           0     2097148

free -b Display memory usage in bytes.

free -b

Output example:

              total        used        free      shared  buff/cache   available
Mem:        8462467072  4064718848  1452642304   222119936  2945105920  3852767232
Swap:       2199023104           0  2199023104

free -k Display memory usage in kibibytes (KiB). This is the default.
```
free -k
```

free -m Display memory usage in mebibytes (MiB).

free -m

Output example:

              total        used        free      shared  buff/cache   available
Mem:            7882        3786        1353         207        2743        3588
Swap:           2047           0        2047

free -g Display memory usage in gibibytes (GiB).

free -g

Output example:

              total        used        free      shared  buff/cache   available
Mem:               7           3           1           0           2           3
Swap:              2           0           2

free -h Display memory usage in human-readable format (e.g., KB, MB, GB).

free -h

Output example:

              total        used        free      shared  buff/cache   available
Mem:           7.9Gi       3.8Gi       1.3Gi       207Mi       2.7Gi       3.6Gi
Swap:          2.0Gi          0B       2.0Gi

free -l Display detailed memory information, including Huge Pages.
```
free -l
```
free -s 5 Update memory usage every 5 seconds. Press Ctrl+C to exit.
```
free -s 5
```
free -c 10 Display memory usage 10 times and then exit.
```
free -c 10
```
free --wide Display the output on a wider terminal, showing all columns without wrapping.
```
free --wide
```
free --si Display units in powers of 1000 (e.g., KB, MB, GB) instead of powers of 1024 (KiB, MiB, GiB).
```
free --si
```

Displaying Specific Information

free | grep Mem Show only the memory line.
```
free | grep Mem
```
free -m | awk '/^Mem:/ {print $4}' Print only the free memory in MiB.
```
free -m | awk '/^Mem:/ {print $4}'
```
free -h | awk '/^Mem:/ {print $7}' Print only the available memory in human-readable format.
```
free -h | awk '/^Mem:/ {print $7}'
```

Common Patterns

Checking available memory in GB, updating every 2 seconds:
```
free -g -s 2
```

Getting total RAM in MiB and piping to a script:

total_ram_mib=$(free -m | awk '/^Mem:/ {print $2}')
echo "Total RAM: ${total_ram_mib} MiB"

Monitoring memory usage continuously in human-readable format:
```
watch -n 1 free -h
```
(Requires watch to be installed: sudo apt install watch or brew install watch)
Checking swap usage:
```
free | grep Swap
```
Calculating percentage of used memory:
```
free -m | awk '/^Mem:/ {printf "%.2f%%\n", $3/$2*100}'
```
This calculates the percentage of used memory from the Mem: line.

Gotchas

free vs available: The free column is often misleading. available is a much better indicator of how much memory can actually be used for new applications, as it accounts for reclaimable cache and buffer memory.
Buff/Cache is not wasted: The buff/cache memory is actively used by the Linux kernel to speed up disk operations. This memory is not lost; it will be freed automatically if applications require more RAM.
Swap usage: High swap usage can indicate that your system is running out of physical RAM and performance may degrade significantly.
macOS gfree: If you install coreutils on macOS, remember to use gfree instead of free to access the GNU version of the command.
Output interpretation: The units can be confusing. Always pay attention to the unit displayed (bytes, KiB, MiB, GiB, or human-readable) and use the appropriate flags (-b, -k, -m, -g, -h) to get the desired output.