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"
  • klay@lemmy.worldEnglish
    1·
    2 years ago

    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.

  • Doomdoxrulz@lemmy.mlEnglish
    1·
    2 years 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

  • tburkhol@lemmy.worldEnglish
    11·
    2 years ago

    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

    • klay@lemmy.worldEnglish
      4·
      2 years ago

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