| Tested on | Ubuntu 25.04 (Plucky Puffin) |
|---|---|
| Package | util-linux 2.40.2 |
| Applies to | Ubuntu, Debian, Kali Linux, Linux Mint, Pop!_OS, Raspberry Pi OS, RHEL, Rocky Linux, AlmaLinux, Oracle Linux, CentOS Stream, Fedora, Arch Linux |
| Privilege | sudo or root |
| Man page | swapon(8) |
| Scope | Check swap usage, enable swap from fstab or a swapfile, set priority, and disable swap safely on Linux. |
| Related guides | Linux commands cheat sheet |
swapon turns on swap backing store (partition or file); swapoff turns it off. You use them after creating swap with mkswap, at boot via fstab, or when troubleshooting memory pressure. This cheat sheet walks through check → create → enable → inspect → disable on a real Ubuntu host, with trimmed terminal output you can diff locally.
swapoff -a on a busy system can take minutes while pages are copied back to RAM and may trigger OOM if memory is tight. Use on lab hosts or maintenance windows; see Do you need swap on Linux? for when swap matters.
swapon and swapoff — quick cheat sheet
| Command | Purpose |
|---|---|
free -h |
Human-readable RAM and swap usage |
swapon --show |
List active swap (preferred over -s) |
cat /proc/swaps |
Kernel view of the same data (kB columns) |
sudo swapon -a |
Enable all swap lines from /etc/fstab |
sudo swapon /path/to/swapfile |
Enable one swapfile or partition |
sudo swapon -p 10 /path |
Enable with priority (higher used first) |
sudo swapon -v /path |
Verbose enable (signature, page size) |
sudo swapon -ae |
Enable from fstab; skip missing devices (-e needs -a) |
sudo swapoff /path |
Disable one swap device or file |
sudo swapoff -a |
Disable all active swap |
swapon --show=NAME,SIZE,USED,PRIO |
Custom columns |
swapon --bytes |
Sizes in bytes |
swapon --noheadings / --raw |
Script-friendly table output |
Command syntax
swapon [options] [<spec>]
swapoff [options] [<spec>]<spec> can be a path (/swap.img, /dev/sda2), or LABEL=…, UUID=… as shown in swapon --help. Most changes need sudo.
Check whether swap is active
Start with free -h and swapon --show:
free -h
swapon --show total used free shared buff/cache available
Mem: 5.2Gi 1.9Gi 2.2Gi 2.7Mi 1.3Gi 3.2Gi
Swap: 2.4Gi 719Mi 1.7Gi
NAME TYPE SIZE USED PRIO
/swap.img file 2.4G 719.1M -1USED in swapon --show matches the swap line in free. The kernel exposes the same entries in /proc/swaps (sizes in kilobytes):
cat /proc/swapsFilename Type Size Used Priority
/swap.img file 2536444 736388 -1For background on what swap is doing in memory pressure, see Linux memory management overview.
Create a swapfile (before swapon)
swapon only works on space already prepared with mkswap. On this host the installer created /swap.img; to add your own file on ext4 (not on tmpfs—/tmp will fail with Invalid argument):
sudo fallocate -l 512M /var/tmp/lab-swap-512m
sudo chmod 600 /var/tmp/lab-swap-512m
sudo mkswap /var/tmp/lab-swap-512mSetting up swapspace version 1, size = 512 MiB (536866816 bytes)
no label, UUID=712c564c-1abc-4308-8668-584bc1bce2d0Make it persistent by adding a line to /etc/fstab (example):
/var/tmp/lab-swap-512m none swap sw 0 0Ubuntu’s default fstab entry for the installer swapfile looks like:
grep swap /etc/fstab/swap.img none swap sw 0 0Enable swap with swapon
Enable all fstab swap entries
sudo swapon -a
swapon --showAfter swapoff -a, re-enabling restores the swapfile with zero use until pressure returns:
NAME TYPE SIZE USED PRIO
/swap.img file 2.4G 0B -1Enable one swapfile
sudo swapon /var/tmp/lab-swap-512m
swapon --showNAME TYPE SIZE USED PRIO
/swap.img file 2.4G 719.1M -1
/var/tmp/lab-swap-512m file 512M 0B -1Set swap priority
Higher PRIO values are used before lower ones. Disable first, then enable with -p:
sudo swapoff /var/tmp/lab-swap-512m
sudo swapon -p 10 /var/tmp/lab-swap-512m
swapon --showNAME TYPE SIZE USED PRIO
/swap.img file 2.4G 719.1M -1
/var/tmp/lab-swap-512m file 512M 0B 10Verbose enable
-v prints how util-linux detected the swap signature:
sudo swapon -v /var/tmp/lab-swap-512mswapon: /var/tmp/lab-swap-512m: found signature [pagesize=4096, signature=swap]
swapon: /var/tmp/lab-swap-512m: pagesize=4096, swapsize=536870912, devsize=536870912
swapon /var/tmp/lab-swap-512mSkip missing fstab entries (-e with -a)
-e / --ifexists only applies with -a. Without it, a missing swap path in fstab errors out; with -e, that entry is skipped:
# fstab contains /missing-swap (does not exist) and /swap.img
sudo swapon -aT /tmp/fstab-swap-testswapon: cannot open /missing-swap: No such file or directorysudo swapon -aeT /tmp/fstab-swap-test
swapon --showNAME TYPE SIZE USED PRIO
/swap.img file 2.4G 0B -1(-T points at an alternate fstab file; omit it to use /etc/fstab.)
Inspect swap usage (swapon display options)
swapon -s and swapon --summary still work but are deprecated—prefer --show:
swapon --showCustom columns:
swapon --show=NAME,SIZE,USED,PRIONAME SIZE USED PRIO
/swap.img 2.4G 719.1M -1swapon --show=NAME,UUIDNAME UUID
/swap.img d3a0797b-f50c-4a3a-af43-559bff2a1a65Script-friendly output:
swapon --noheadings
swapon --raw
swapon --bytes/swap.img file 2.4G 719.1M -1
NAME TYPE SIZE USED PRIO
/swap.img file 2.4G 719.1M -1
NAME TYPE SIZE USED PRIO
/swap.img file 2597318656 754061312 -1Legacy summary (kB, same data as /proc/swaps):
swapon -sFilename Type Size Used Priority
/swap.img file 2536444 736388 -1Disable swap with swapoff
Disable one swapfile or partition
sudo swapoff /var/tmp/lab-swap-512m
swapon --showOnly /swap.img remains active.
Disable all swap
sudo swapoff -a
free -h | grep Swap:Swap: 0B 0B 0BRe-enable when finished:
sudo swapon -a
free -h | grep Swap:Swap: 2.4Gi 0B 2.4GiOn a busy system, swapoff -a can take a minute or longer while swapped pages are migrated back to RAM.
Other swapon options
| Option | Role |
|---|---|
-d, --discard |
Enable discard on swap devices that support TRIM (swapfiles on ext4 may not) |
-f, --fixpgsz |
Reinitialize swap if page size changed |
-o, --options |
Comma-separated mount-style options (see man swapon) |
-T, --fstab |
Alternate fstab path (used with -a) |
Full flag list:
swapon --helpTroubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
swapon: cannot open … No such file |
Wrong path or missing file | Fix /etc/fstab or create the file; use swapon -ae to skip bad entries at boot |
swapon: … Invalid argument on a file |
Swapfile on tmpfs, btrfs subvolume without proper flags, or similar |
Place the swapfile on ext4/XFS per distro docs |
swapon: … needs -a for -e |
-e only valid with -a |
Run sudo swapon -ae, not swapon -e /path alone |
swapoff hangs |
Large amount of data in swap | Wait; reduce load first; avoid on production without a plan |
Browse the full index in our Linux commands reference.
References
- swapon(8) / swapoff(8) — util-linux manual
Summary
Use swapon --show and free -h to inspect swap, sudo swapon -a or sudo swapon /path to enable, and sudo swapoff /path or sudo swapoff -a to disable. Prepare new files with mkswap on a supported filesystem, persist with /etc/fstab, and use -p for priority. For tuning when swap is used—not just how to toggle it—see the swap and swappiness guide.
Frequently Asked Questions
What is the difference between swapon and swapoff?
swapon activates a swap partition or swapfile for paging; swapoff deactivates it. Both require root or sudo. Use swapon --show to list active swap and free -h to see used swap space.
How do I enable all swap entries from fstab?
Run sudo swapon -a. That reads /etc/fstab and enables every line with the swap filesystem type. On Ubuntu you will usually see /swap.img enabled this way after boot or after swapoff -a.
How do I disable swap temporarily on Linux?
sudo swapoff -a turns off all active swap devices and files. Re-enable with sudo swapon -a. Avoid swapoff on production unless you accept OOM risk while swap is off.
Is swapon -s still valid?
It works but is deprecated. Prefer swapon --show for a readable table, or swapon --bytes when you need exact byte counts.
Why does swapon fail on my swapfile?
Common causes: file not initialized with mkswap, permissions not 600, path on tmpfs or an unsupported filesystem, or a stale fstab entry. swapon -v path prints verbose errors.
