How to get advisory lock in shell?

Recently we had interesting problem. There are some maintenance tools, that do stuff that touch database. One can be calling repack, another can be specific dump, and yet another can be applying migrations from application.

The problem is that they can step over each other toes, and cause issues.

So we needed to add some way to prevent them from running at the same time…

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How to run short ALTER TABLE without long locking concurrent queries

Recently I've seen case like:

  1. application had to add column to table.
  2. application ran ALTER TABLE ADD COLUMN (without default!)
  3. everything stopped for many MINUTES

Why? How to avoid the problem?

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Wait for program(s) in tmux panes to end…

I am doing quite a lot of work inside tmux. I especially love that I can start multiple windows/panes and use them to run the same thing across multiple servers, while still having normal shell access between steps.

This thing was greatly improved when I wrote tmux_send_to_many, but one thing was missing – waiting for the thing, running in tmux window to end.

Generally, I had to look at all the windows, and decide when to launch next step on my own.

Not anymore.

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visual sleep in shell, and shell_utils repo information

I wrote previously about tmux_send_to_many and group_by shell tools that I authored.

Since then I got some more ideas for changes, and for new tool, so figured I'll make an honest git repo for it. Repo is on GitLab, and all the tools inside can be freely used.

While I was doing it, I fixed some things in tmux_send_to_many.

And then – I remembered that for long time I was missing “visual-sleep" type of tool. One that will show some progress information while it's working.

Enter vsleep.

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Automation for doing stuff in multiple windows in tmux

Tmux is terminal multiplexer. Kinda like old screen, but with much more functionality.

When I work on my servers, it's pretty common that I have to do the same things to multiple servers. To make my life easier I start tmux, and in there start many “windows", each related to work on single server.

I name the windows in a way that let's me quickly find them, without false positives.

For example, if I'd have to upgrade servers db1..db5 then I'd create windows “up-db1" .. “up-db5", and each window would work on single server.

This is already scriptable – let's assume I'd want to show uptime in all of the windows, I can:

tmux lsw -F '#W' | grep -E '^up-db[0-9]+$' | xargs -r -d$'\n' -I% tmux send-keys -t % uptime Enter

But this gets tedious fast.

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Why does “sudo ls -l /proc/1/fd/*” fail?

I usually write about PostgreSQL, but lately someone asked for help, and one of the problems was similar to sudo command from title.

This was not the first time I saw it, so figured, I'll write a blogpost about it, just so I can refer people to it in the future.

Continue reading Why does “sudo ls -l /proc/1/fd/*" fail?