The Advisory Boar

By Abhijit Menon-Sen <>

How are Unix pipes implemented?

; updated

This article is about how pipes are implemented the Unix kernel. I was a little disappointed that a recent article titled “How do Unix pipes work?” was not about the internals, and curious enough to go digging in some old sources to try to answer the question.

What are we talking about?

Pipes are “perhaps the single most striking invention in Unix” — a defining characteristic of the Unix philosophy of composing small programs together, and a familiar sight in the Unix shell:

$ echo hello | wc -c

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What is fsck up to now?

We had unexpectedly heavy snowfall the other day and, as always, the mains power supply came back only after a few days of repairing broken power lines in the forest. Meanwhile, the days were so overcast that the solar inverter couldn't charge the batteries enough to keep up with our minimal domestic load.

Which meant that when the sun came out again, I was left staring at something like this for a long time:

root@soot:~# ps -eo pid,cmd|grep '[f]sck'
756 /lib/systemd/systemd-fsck /dev/mapper/sdb1_crypt
757 /sbin/fsck -a -T -l -M -C4 /dev/mapper/sdb1_crypt
758 /lib/systemd/systemd-fsckd
759 fsck.ext4 -a -C4 /dev/mapper/sdb1_crypt

Long enough, in fact, that I began to wonder if I could tell what it was doing. (The volume in question is exported via iSCSI from a Synology NAS and fsck is still running long after the machine has otherwise finished booting up, so I have ordinary shell access.)

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Understanding sudoers(5) syntax

This straightforward guide to configuring sudo is for anyone who didn't expect to see “Don't despair” and a “Quick guide to EBNF” in the sudoers(5) manpage.

Sudo (su "do") allows a system administrator to delegate authority to give certain users (or groups of users) the ability to run some (or all) commands as root or another user while providing an audit trail of the commands and their arguments.

This guide is intended to supplement the manpage. The various environment, security, and logging options are not covered; the explanations in the manpage are easy to follow.

The first 90%

It's possible that the only sudo explanation you will ever need is:


This means “any user in the adm group on any host may run any command as any user without a password”. The first ALL refers to hosts, the second to target users, and the last to allowed commands. A password will be required if you leave out the "NOPASSWD:".

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