bak

Copy a file aside before you edit it.

Synopsis

bak [options] file ... [-- cp options]
bak -l | -r | -d | --prune N file ...

Description

bak makes a dated copy of a file right next to it, so you can edit the file and still get the old version back. The copy of nginx.conf made at 14:30:12 on 29 September 2026 is called nginx.conf.2026-09-29-143012.bak. The copy keeps the owner, the mode and the times of the original, because bak copies with cp -a.

bak never overwrites a copy. When you make two copies in the same second, the second one gets a number, as in nginx.conf.2026-09-29-143012-1.bak, and it counts as the newer one.

It works on folders too. bak site copies the whole site folder to site.2026-09-29-143012.bak with cp -a, which keeps symlinks inside the folder as symlinks.

When the file you name is a symlink, such as a dotfile linked from a dotfiles repo, bak copies the file the link points to. The copy is a real file with the content, not a second link.

The other modes work on the copies that already exist:

• -l lists them, newest first, with their age and size.
• -r puts the newest copy back. Before it does, it saves the current file as a new copy, so a restore can be undone with another bak -r.
• -d shows what changed between the newest copy and the file as it is now.
• --prune N keeps the newest N copies and moves the older ones to the trash.

bak finds the copies of a file by name. A copy is any file beside it called name.YYYY-MM-DD-HHMMSS.bak or name.YYYY-MM-DD-HHMMSS-N.bak. Other files that end in .bak, such as nginx.conf.old.bak, are left alone.

Options

Use only one of -l, -r, -d and --prune at a time. With none of them, bak makes a copy.

Restoring

bak -r nginx.conf does three things, in this order:

1. It finds the newest copy, for example nginx.conf.2026-09-29-143012.bak, and asks Restore nginx.conf from the copy made 3h ago? The current one is saved first. [y/N].
2. It saves the current nginx.conf as a new copy. That copy is now the newest.
3. It copies the old version back over nginx.conf with cp -a, so the mode and times come back too.

Because step 2 makes the current file the newest copy, running bak -r a second time puts back the version you had before the first restore. Restores never lose anything.

For a file, the copy goes back through cp, which writes into the existing file. A symlinked dotfile stays a symlink, and the file it points to gets the old content.

For a folder, the current folder moves aside whole with mv and becomes the new copy, and then the old copy is copied into place. Files you added after the copy was made do not linger in the restored folder. They are still in the copy that step 2 made.

When the file no longer exists, bak -r copies the newest copy back without saving anything first.

Where the copies go

By default each copy sits beside its file, in the same folder. That keeps it easy to find, and it means you need write access to that folder. For files under /etc you need sudo, the same as for editing them.

--dir DIR puts the copies in another folder, such as ~/.bak. The copy keeps only the file name, so /etc/hosts and ~/projects/hosts would share names in the same --dir. Give each group of files its own folder if that matters to you.

Safety checks

• bak never overwrites a copy. A copy made in the same second gets a -1, -2 and so on before .bak.
• -r and --prune ask before they change anything. Without a terminal they refuse with exit 4 unless you pass -y. Answering no exits 5.
• --prune moves old copies to the trash in ~/.local/share/Trash, where your file manager can restore them. It never deletes them for good.
• -r refuses when the file is a folder and the copy is not, or the other way round.
• bak / is refused with exit 4.

Pass-through

Options after -- go to cp when bak makes a copy, and to diff with -d.

$ bak disk.img -- --reflink=auto
$ bak -d nginx.conf -- -w

--reflink=auto makes the copy instant on Btrfs and XFS, because the copy shares blocks with the original until one of them changes. -w makes diff ignore changes in white space.

Needs

cp from coreutils, or the cp in BusyBox. -d also needs diff, from the diffutils package on every distro. Debian and Ubuntu install it by default. On Alpine, apk add diffutils gives the full diff, and the BusyBox one also works.

Examples

Copy a config file before you edit it

$ bak nginx.conf
nginx.conf -> nginx.conf.2026-09-29-143012.bak

See what you changed

$ bak -d nginx.conf
--- nginx.conf.2026-09-29-143012.bak	2026-09-29 14:30:12.394283532 +0530
+++ nginx.conf	2026-09-29 14:41:56.426271431 +0530
@@ -1,4 +1,4 @@
 server {
-    listen 80;
+    listen 443 ssl;
     server_name example.com;
 }

List the copies

$ bak -l nginx.conf
2h ago       60B  nginx.conf.2026-09-29-143012.bak
2d ago       55B  nginx.conf.2026-09-27-091200.bak

The age comes from the date in the name, which is when the copy was made. The file's own time is the time of the original, because cp -a keeps it.

Put the old version back

$ bak -r nginx.conf
Restore nginx.conf from the copy made 2h ago? The current one is saved first. [y/N] y
saved the current nginx.conf as nginx.conf.2026-09-29-164356.bak
nginx.conf.2026-09-29-143012.bak -> nginx.conf

Keep copies of dotfiles in one folder

$ bak -v --dir ~/.bak ~/.bashrc
bak: making /home/khadir/.bak
+ cp -a -H -- /home/khadir/.bashrc /home/khadir/.bak/.bashrc.2026-09-29-164356.bak
/home/khadir/.bashrc -> /home/khadir/.bak/.bashrc.2026-09-29-164356.bak

Keep only the newest two copies

$ bak --prune 2 nginx.conf
trash  nginx.conf.2026-09-27-091200.bak
Move 1 old copy of nginx.conf (55B) to the trash? [y/N] y
1 old copy of nginx.conf in the trash, 2 kept.

Copy a whole folder

$ bak site
site -> site.2026-09-29-164356.bak

Troubleshooting

bak: /etc: permission denied, run it with sudo

The copy goes beside the file, and you cannot write to that folder. Run sudo bak /etc/fstab, or keep the copy somewhere you can write with --dir ~/.bak.

bak: no copies of nginx.conf in .

There are no copies with the name.YYYY-MM-DD-HHMMSS.bak pattern in that folder. If you made them with --dir, pass the same --dir again.

bak: not asking without a terminal, pass --yes to go ahead

-r and --prune ask before they change anything. In a script or a cron job, add -y.

bak: nginx.conf and nginx.conf.2026-09-29-143012.bak are not the same kind, one is a folder. Restore it by hand

The name now belongs to a folder and the copy is a file, or the other way round. Move one of them and restore by hand with cp -a.

Exit status

See also

mirror, lock, cp(1), diff(1)