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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Ah! This is a shell pipe! It’s composing several smaller commands together, cool stuff.
-
ls -1is the grep-friendly version of ls, it prints one entry per line, like a shopping list. -
headtakes a set number of entries from the head of a list, in this case2 items.negative two, meaning “all but the last two.” -
xargstakes the incoming pipe and converts it into extra arguments, in this case applying those arguments torm.
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.Great response 👍🏾
deleted by creator
-
This line seems to list all dumps and then deletes all but the two most recent ones.
In detail:
ls -1 /backup/*.dumplists all files ending with .dump alphabetically inside the /backup directoryhead -n -2returns all filenames except the two most recent ones from the end of the listxargs rm -fpasses the filenames torm -fto delete them
Take a look at explainshell.com.
Backups are created to /backup directory and are ended with .dump file extention.
ls -1is listing all those files chronologically, -1 is to keep one file per one line.head -n -2is getting lines from the top to the last two at bottom.xargs rm -fis callingrm -fon every line of the input.|is pipe symbol, that gets output from command before and gives it to command afterSo TLDR it’s removing all backups except the last 2 ones.
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.
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
Won’t this delete the two newest files, as opposed to everything except the two newest files?
