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Need help understanding a back-up script (lemmy.world)

submitted 1 year ago by Natal@lemmy.world to c/selfhosted@lemmy.world

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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"

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[–] Doomdoxrulz@lemmy.ml 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