commit c6cd23bb8bcfb50da6075765a905d1c07425ba78
parent 0aeeeed253053d31eed23962299d86200820eaa9
Author: Sebastiano Tronto <sebastiano@tronto.net>
Date: Fri, 31 May 2024 21:21:52 +0200
New blog post
Diffstat:
3 files changed, 92 insertions(+), 0 deletions(-)
diff --git a/src/blog/2024-04-07-expand-unexpand/expand-unexpand.md b/src/blog/2024-04-07-expand-unexpand/expand-unexpand.md
@@ -56,3 +56,5 @@ $ for f in *.rs; do unexpand $f > $f.2 && mv $f.2 $f; done
For some reason, the OpenBSD version of `unexpand` does not allow
using the `-t` option. So if I had brought my OpenBSD laptop at the
rust lecture I would have been stuck with spaces :(
+
+*Next in the series: [fold](../2024-05-31-fold)*
diff --git a/src/blog/2024-05-31-fold/fold.md b/src/blog/2024-05-31-fold/fold.md
@@ -0,0 +1,89 @@
+# UNIX text filters, part 2.6 of 3: fold
+
+*This post is part of a [series](../../series)*
+
+Today's text filter is `fold`, a program that can be used to format
+paragraphs of text so that lines do not exceed a given width.
+
+## fold
+
+First of all, let's take a moment to celebrate the fact that the
+[OpenBSD manual page for `fold`](http://man.openbsd.org/OpenBSD-7.4/fold)
+uses the same terminology as me:
+
+```
+DESCRIPTION
+ fold is a filter [...]
+```
+
+See? I didn't make this up!
+
+Anyway, back to the tool. What `fold` does is breaking up lines of text
+so that they take up a maximum of 80 characters - or any number of
+characters specified by the `-w` option. For example:
+
+```
+$ echo 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec pretium odio quis nisi vestibulum, at semper magna ornare. Nulla facilisi. Sed in magna lacus. Proin faucibus est non ligula vehicula, quis ultrices lectus ultricies. Aenean sit amet dignissim mauris. Sed luctus lobortis augue nec aliquet. Cras in felis tellus. Curabitur id purus feugiat enim posuere ultrices in viverra erat. Nulla facilisi. Donec et neque hendrerit, dignissim ipsum id, venenatis enim.' \
+| fold -w 72
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec pretium o
+dio quis nisi vestibulum, at semper magna ornare. Nulla facilisi. Sed in
+ magna lacus. Proin faucibus est non ligula vehicula, quis ultrices lect
+us ultricies. Aenean sit amet dignissim mauris. Sed luctus lobortis aug
+ue nec aliquet. Cras in felis tellus. Curabitur id purus feugiat enim po
+suere ultrices in viverra erat. Nulla facilisi. Donec et neque hendrerit
+, dignissim ipsum id, venenatis enim.
+```
+
+Although you probably want to use the `-s` option so that words are not
+broken halfway through:
+
+```
+$ (same echo command) \
+| fold -s -w 72
+Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec pretium
+odio quis nisi vestibulum, at semper magna ornare. Nulla facilisi. Sed
+in magna lacus. Proin faucibus est non ligula vehicula, quis ultrices
+lectus ultricies. Aenean sit amet dignissim mauris. Sed luctus
+lobortis augue nec aliquet. Cras in felis tellus. Curabitur id purus
+feugiat enim posuere ultrices in viverra erat. Nulla facilisi. Donec et
+neque hendrerit, dignissim ipsum id, venenatis enim.
+```
+
+As with
+[`rev`](../2024-03-27-rev) and [`cut`](../2024-03-28-cut), the environment
+variable `LC_CTYPE` is used to determine what a character is, but
+one can also specify to use *bytes* instead using the `-b` option.
+
+## Examples
+
+### Text formatting
+
+A classic use for `fold` is formatting emails (or markdown files
+for blog posts) to avoid long lines. For example if you use
+[vi](https://en.wikipedia.org/wiki/Vi_(text_editor)), the command
+
+```
+!} fold -s -w 72
+```
+
+will format the next paragraph. However,
+[`fmt`](http://man.openbsd.org/OpenBSD-7.4/fmt) can do the same and it is
+more convenient to use - check it out!
+
+### Generating passwords
+
+Here is a cool example that puts together `fold` with some of the
+other tools we have seen, [`tr`](../2024-01-13-tr) and
+[`head`](../2024-02-20-head-and-tail). To generate a random password,
+I use:
+
+```
+$ cat /dev/random | tr -cd 'a-z0-9' | fold -w 12 | head -1
+```
+
+This command reads the special file `/dev/random`, which contains a
+never-ending stream of random bytes, and passes it through various commands
+in a *pipeline*. First, every character that is *not* (`tr -c`) a
+lowercase letter or a number (`a-z0-9`) is deleted (`-d`); then
+the result is `fold`'d to 12 characters (`fold -w 12`); finally,
+the first line is taken (`head -1`).
diff --git a/src/series/series.md b/src/series/series.md
@@ -34,6 +34,7 @@ of complexity: `grep`, `sed` and `awk`. Work in progress.
* Part 2.3: [rev](../blog/2024-03-27-rev)
* Part 2.4: [cut](../blog/2024-03-28-cut)
* Part 2.5: [expand and unexpand](../blog/2024-04-07-expand-unexpand)
+* Part 2.6: [fold](../blog/2024-05-31-fold)
* Part 3: awk (coming "soon")
## The UNIX shell as an IDE
