Mastering the 'swapon' and 'swap' Linux Commands

Specify the location of your system's swapfile with 'swapon'

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Swapon specifies the devices on which paging and file swapping will take place. Calls to swapon normally occur in the system multi-user initialization file /etc/rc that makes all swap devices available, so that the paging and swapping activity is interleaved across several devices and files.

Most people never execute this command in a shell prompt. It's useful primarily during system startup and for specific forms of troubleshooting.

Synopsis

The command takes any of the following forms:

/sbin/swapon [-h -V]
/sbin/swapon -a [-v] [-e]
/sbin/swapon [-v] [-p priority] specialfile ...
/sbin/swapon [-s]
/sbin/swapoff [-h -V]
/sbin/swapoff -a
/sbin/swapoff specialfile ...  

Switches

Swapon supports several switches to extend or refine the command's execution.

  • -h: Provide help.
  • -V: Display version.
  • -s: Display swap usage summary by device. Equivalent to cat /proc/swaps.
  • -a: All devices marked as swap will swap devices in /etc/fstab are made available. Devices that are already running as swap are silently skipped.
  • -e: When -a is used with swapon-e makes swapon silently skip devices that do not exist.
  • -p priority: Specify a priority for swapon. This option is only available if swapon was compiled under and is used under a 1.3.2 or later kernel. The priority is a value between 0 and 32767. Add pri=value to the option field of/etc/fstab for use with swapon -a.

Swapoff disables swapping on the specified devices and files. When the -a flag is given, swapping is disabled on all known swap devices and files (as found in /proc/swaps or /etc/fstab).

Notes

You should not use swapon on a file with holes. Swap over NFS may not work. 

The specific use of swapon may vary by distribution and kernel-release level. Use the man command to see how a command is used in your specific environment.

Check out the swapoff command for additional insight.