What a Filesystem Journal Does During a Crash
A journal means you no longer wait an hour for fsck after a power cut. It does not mean the file you were writing survived — and understanding that gap…
Signals: How Processes Are Asked to Stop
Ctrl+C, kill, systemctl stop and a container shutting down are all the same mechanism. Knowing which signal is being sent, and who is allowed to ignore it, explains why some…
Memory, Swap and the OOM Killer
free says almost no memory is available and the machine is fine. Then one day a process disappears with no error in its own logs. Both of those make sense…
What Actually Happens When You Run a Program
You type a name and press enter. Between that and the first line of output sit fork, exec, an ELF header and a dynamic linker — and knowing the sequence…
xargs: Turning a List Into Commands (and When to Reach for parallel)
Argument list too long, and filenames with spaces in them. xargs solves the first and creates the second unless you know the one flag that matters.
openssl: The Six Commands You Actually Need
openssl has hundreds of subcommands and a manual page that assumes you already know cryptography. In practice there are six things people need, and each is one line.
timedatectl: Time, Time Zones and Why Your Logs Are an Hour Out
Setting the clock is easy. The parts that bite are the two clocks in every machine, the difference between the system time zone and a user's, and what happens to…
lsof and fuser: Finding Out What Is Holding a File, a Port or a Mount
Address already in use. Device or resource busy. Disk full but nothing is using the space. All three are the same question, and lsof answers it in one line.
Sending Email From Your Server (And Why You Probably Should Not Run a Mail Server)
Your application needs to send password resets. That is a solved problem and takes an afternoon. Receiving mail on your own box is a different job entirely, and this is…
Moving an Existing Service Into a Container
Taking something installed with apt and running it in a container instead, without discovering three weeks later that it was writing somewhere you forgot. The inventory, the parallel run, and…