pipe & grep

Notes from the Linux command line. A small manual of commands, written by one person, tested on a real machine.

cmd1 | cmd2 | cmd3 [--read-the-output]

Short notes about the Linux command line: how to put pipes together, what an awk or sed one-liner is doing, systemd timers against cron, ssh config, safe shell scripts and dotfiles. Each note explains the commands it uses and says where they differ between systems.

The example below is the whole idea in one picture. A command is cut into its stages, and each stage shows what it hands to the next.

$ awk '{print $1}' | sort | uniq -c | sort -rn | head -n 2 access.log

1. in< access.logthe file, 5 lines

   203.0.113.7 - - [07/Nov/2026:10:12:01] "GET / HTTP/1.1" 200
   198.51.100.4 - - [07/Nov/2026:10:12:03] "GET /a HTTP/1.1" 200
   203.0.113.7 - - [07/Nov/2026:10:12:09] "GET /b HTTP/1.1" 404
   192.0.2.88 - - [07/Nov/2026:10:12:15] "GET / HTTP/1.1" 200
   203.0.113.7 - - [07/Nov/2026:10:12:22] "GET /c HTTP/1.1" 200

2. 1awk '{print $1}'keep field 1, the client address

   203.0.113.7
   198.51.100.4
   203.0.113.7
   192.0.2.88
   203.0.113.7

3. 2sortput equal lines side by side

   192.0.2.88
   198.51.100.4
   203.0.113.7
   203.0.113.7
   203.0.113.7

4. 3uniq -ccollapse neighbours, prefix a count

   1 192.0.2.88
   1 198.51.100.4
   3 203.0.113.7

5. 4sort -rnbiggest count first

   3 203.0.113.7
   1 198.51.100.4
   1 192.0.2.88

6. 5head -n 2stop after two lines

   3 203.0.113.7
   1 198.51.100.4

Safer shell scripts, strict mode and quoting

2026-11-04 What set -euo pipefail actually does, where it stays silent, how to quote properly, and a trap that cleans up after itself.

systemd timers or cron

2026-10-21 A nightly backup written both ways, the units and the commands to manage them, and where each one is the better tool.

Five awk and sed one-liners, taken apart

2026-10-02 Five GNU awk and sed one-liners worth memorising, each split into its tokens, plus the sed -i difference on macOS.

Build a pipeline one stage at a time

2026-09-14 How to grow a shell pipeline stage by stage, with a web server log as the example, and why uniq needs sort first.

All notes

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