this post was submitted on 05 Oct 2023
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28
Need help understanding a back-up script (lemmy.world)
submitted 1 year ago by [email protected] to c/[email protected]
13 comments fedilink hide all child comments
 

Hello everyone!

I had a container with a DB crap itself yesterday so I'm trying to speed up my learning to back up stuff.

I came across a script that taught me how to back-up a containerized postgres db at given intervals and it works. I managed to create db dumps and restore them. I've documented everything and now my whole docker-compose/env etc are on git control.

There's one part of the script I don't decypher but I'd like to maybe change it. It is about the number of back-up copies.

Here's the line from the tutorial: ls -1 /backup/*.dump | head -n -2 | xargs rm -f

Can someone explain to me what this line does? I'd like to keep maybe 3 copies just in case the auto-backup backs up a rotten one.

Thanks!

Full code below:

backup:
    image: postgres:13
    depends_on:
      - db_recipes
    volumes:
      - ./backup:/backup
    command: >
      bash -c "while true; do
        PGPASSWORD=$$POSTGRES_PASSWORD pg_dump -h db-postgresql -U $$POSTGRES_USER -Fc $$POSTGRES_DB > /backup/$$(date +%Y-%m-%d-%H-%M-%S).dump
        echo ""Backup done at $$(date +%Y-%m-%d_%H:%M:%S)""
        ls -1 /backup/*.dump | head -n -2 | xargs rm -f
        sleep 86400
      done"
all 14 comments
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[–] [email protected] 14 points 1 year ago* (last edited 1 year ago) (1 children)

This line seems to list all dumps and then deletes all but the two most recent ones.

In detail:

  • ls -1 /backup/*.dump lists all files ending with .dump alphabetically inside the /backup directory
  • head -n -2 returns all filenames except the two most recent ones from the end of the list
  • xargs rm -f passes the filenames to rm -f to delete them

Take a look at explainshell.com.

[–] [email protected] 7 points 1 year ago (1 children)

I just looked up the man page, and actually head -n -2 means "everything up to but not including the last two lines", so this should always leave two files remaining.

[–] [email protected] 3 points 1 year ago (1 children)

You're right, I edited my comment. Thanks!

[–] [email protected] 3 points 1 year ago (1 children)

Your xargs comment is still wrong tho. It deletes ALL but the most recent two files.

[–] [email protected] 2 points 1 year ago

Fixed, thanks.

[–] [email protected] 5 points 1 year ago* (last edited 1 year ago) (2 children)

Ah! This is a shell pipe! It's composing several smaller commands together, cool stuff.

  • ls -1 is the grep-friendly version of ls, it prints one entry per line, like a shopping list.

  • head takes a set number of entries from the head of a list, in this case ~~2 items.~~ negative two, meaning "all but the last two."

  • xargs takes the incoming pipe and converts it into extra arguments, in this case applying those arguments to rm.

So, combined, this says "list all the .dump files, pick ~~the first two,~~ all but the last two, and delete them." Presumably the first are the oldest ones and the last are the newest, if the .dump files are named chronologically.

[–] [email protected] 2 points 1 year ago

Great response 👍🏾

[–] [email protected] 2 points 1 year ago

Backups are created to /backup directory and are ended with .dump file extention.

ls -1 is listing all those files chronologically, -1 is to keep one file per one line.

head -n -2 is getting lines from the top to the last two at bottom.

xargs rm -f is calling rm -f on every line of the input.

| is pipe symbol, that gets output from command before and gives it to command after

So TLDR it's removing all backups except the last 2 ones.

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago)

The first command (ls -1 /backup/*.dump) just creates a list of files in the backup folder that have the extension .dump. the output of the prior command is then sent to the next command (head -n -2) this cuts the list down to everything except the last 2 items in the list this is then sent to the final command which takes the list and runs the final (rm -f) command with the items in the list as the targets to delete.

heres a solution based on this post https://stackoverflow.com/questions/25785/delete-all-but-the-most-recent-x-files-in-bash

ls -tp /backup/*.dump | grep -v '/$' | tail -n +4 | tr '\n' '\0' | xargs -0 rm -f

There is an explanation on that post that explains it in better detail but in simple terms it deletes all files but the most recent 3 files in the directory that have the .dump extension

[–] [email protected] 0 points 1 year ago (2 children)

Others have explained the line.

Worth noting that not all implementations of head accept negative line counts (i.e. last n lines), and you might substitute tail.

i.e.: ls -1 /backup/*.dump | tail -2 | xargs rm -f

[–] [email protected] 6 points 1 year ago

Won't this delete the two newest files, as opposed to everything except the two newest files?

[–] [email protected] 4 points 1 year ago* (last edited 1 year ago)

~~Yeah, tail would be the more obvious choice instead of negating head.~~

Fuck, I need coffee. @[email protected] is right (again).

[–] [email protected] 0 points 1 year ago

If you want to get more in depth, I've been using this container:

https://github.com/jareware/docker-volume-backup

It can be setup in the same compose or in it's own, and it supports pre/post commands if you want to dump a db or stop a container before backup.

Additionally, Setting a post backup command like in their docs:

POST_BACKUP_COMMAND: "docker run --rm -e DRY_RUN=false -e DAILY=3 -e WEEKLY=1 -e MONTHLY=1 -v /backup:/archive ghcr.io/jan-brinkmann/docker-rotate-backups"

Lets you specify the number of backups retained per period, E.G. 3 daily, 1 weekly, 1 monthly.

You could also mix and match.