How do I add swap after system installation?

You need to edit /etc/fstab and add the new swap partition.

sudo nano /etc/fstab

You need to add a line that looks like

UUID=735b3be3-779c-4d21-a944-b033225f3ab4 none   swap    sw      0       0

and you get the UUID using the command

sudo blkid /dev/sda3

(substitute /dev/sda3 with the appropriate device name; in order to see the appropriate device name, we can use lsblk - as said here).

Related:

• How to edit files in a terminal with nano?

To create a swap partition after installation, create an empty partition. It should have no holes. You can then format this partition with:

sudo mkswap /dev/sdX

replacing /dev/sdX with your partition. Mount this partition as swap with

sudo swapon -U UUID

where UUID is that of your /dev/sdX as read from this:

blkid /dev/sdX

Bind your new swap in /etc/fstab by adding this line:

UUID=xxx    none    swap    sw      0   0

If you want to use your swap for hibernating then you need to update the UUID in /etc/initramfs-tools/conf.d/resume with this content RESUME=UUID=xxx. Don't forget to $ sudo update-initramfs -u.

To complete things: it is also possible to create a swap file if you do not have a spare partition. This answer gives you an idea of how to create a swap file and enable it on boot.

In case you don't want or you're not sure how to create a swap partition, you can create a swap file which will work in the same way as partition. Here are the steps (using terminal):

1. Create an empty file (1K * 4M = 4 GiB).

   sudo mkdir -v /var/cache/swap
   cd /var/cache/swap
   sudo dd if=/dev/zero of=swapfile bs=1K count=4M
   sudo chmod 600 swapfile
   

2. Convert newly created file into a swap space file.

   sudo mkswap swapfile
   

3. Enable file for paging and swapping.

   sudo swapon swapfile
   

   Verify by: swapon -s or top:

   top -bn1 | grep -i swap
   

   Should display line like: KiB Swap: 4194300 total, 4194300 free

   To disable, use sudo swapoff swapfile command.

4. Add it into fstab file to make it persistent on the next system boot.

   echo "/var/cache/swap/swapfile none swap sw 0 0" | sudo tee -a /etc/fstab
   

5. Re-test swap file on startup by:

   sudo swapoff swapfile
   sudo swapon -va
   

   ^{Note: Above commands re-checks the syntax of fstab file, otherwise your Linux could not boot up properly.}