3 files changed, 113 insertions, 1 deletions

diff --git a/src/blog/2023-12-03-sed/sed.md b/src/blog/2023-12-03-sed/sed.md
index c422879..4d50ed9 100644
--- a/src/blog/2023-12-03-sed/sed.md
+++ b/src/blog/2023-12-03-sed/sed.md
@@ -399,4 +399,6 @@ to take a small detour and talk about some other simple, special-purpose
 text filtering commands, such as `tr`, `head`, `fmt` and so on. Expect
 some short posts in this series before part 3 - after all, there are
 [uncountably many](https://en.wikipedia.org/wiki/Uncountable_set)
-numbers between two and 3!
+numbers between 2 and 3!
+
+*Next in the series: [tr](../2024-01-13-tr)*
diff --git a/src/blog/2024-01-13-tr/tr.md b/src/blog/2024-01-13-tr/tr.md
new file mode 100644
index 0000000..207fde4
--- /dev/null
+++ b/src/blog/2024-01-13-tr/tr.md
@@ -0,0 +1,109 @@
+# UNIX text filters, part 2.1 of 3: tr
+
+*This post is part of a [series](../../series)*
+
+In the [post about sed](../2023-12-03-sed) I have not discussed the
+`y` command at all. This is because I realized it is just an
+underpowered version of the `tr` command, that we are going to
+explore in this post.
+
+The `tr` command is a simple utility that can perform
+character-by-character substitutions and a couple of other things.
+Like most UNIX utilities, it operates on standard input and standard
+output by default.
+
+## Replacing
+
+The most basic form of a `tr` command is
+
+```
+$ tr string1 string2
+```
+
+which replaces every occurrence of a character present in `string1`
+with its corresponding character in `string2` - the first with the
+first, the second with the second and so on. If `string2` is shorter
+than `string1`, the last character is repeated as needed.
+
+For example
+
+```
+$ echo 'Hello!' | tr le 13
+H311o!
+```
+
+An equivalent `sed` command would be `sed 'y/le/13/'`.
+
+Like sed and [grep](../2023-08-20-grep), also `tr` supports the
+standard character sets like `[:upper:]`, `[:alpha:]` and so on.
+For example, the following command capitalizes every letter in the
+input string:
+
+```
+$ echo 'Hello!' | tr [:lower:] [:upper:]
+HELLO!
+```
+
+## Deleting
+
+With the `-d` option one can delete characters:
+
+```
+$ echo 'R42emo3vin0g all n66umber3s!' | tr -d 0-9
+Removing all numbers!
+```
+
+Here I have used the *character range* `0-9` instead of `[:digit:]`.
+Other examples of valid character ranges are `A-Z`, `0-8` and `a-f`.
+
+The `-d` option can be combined with the `-c` option, which takes
+the *complement* of a given set of characters:
+
+```
+$ echo 'R42emo3vin0g all non-n66umber3s!' | tr -cd '0-9\n'
+4230663
+```
+
+Notice that I have also added `\n` to our list of
+characters, so that the newline at the end of the text is kept.
+
+A more complex example involving `tr -cd` is the following, which
+I use to generate random passowrds:
+
+```
+$ cat /dev/random | tr -cd 'a-z0-9' | fold -w 12 | head -n 1
+ft82mtfsy5ps
+```
+
+Here `/dev/random` spits out random data, while the commands
+`fold -w 12` and `head -n 1` are used to break the input text into
+lines of 12 characters and take the first line of the input,
+respectively.  We'll talk about them in future posts.
+
+## Squeezing
+
+One more thing `tr` can do is *squeezing* consecutive identical
+characters.  For example:
+
+```
+$ echo Helllllo | tr -s l
+Helo
+```
+
+The `-s` option can be combined with the `-c` or `-d` option, and
+in this case the squeezing is performed last, squeezing all the
+characters contained in the last given string:
+
+```
+$ echo 'Hellllo! 112233' | tr -s 'l_e' '123'
+H31o! 123
+$ echo 'Hello!' | tr -ds '!' 'l'
+Helo
+```
+
+## Conclusions
+
+This is pretty much all there is tu say about `tr`. All of this can
+probably be done with a sufficiently complicated `sed` or `awk`
+script, but it is definitely nice to have a simpler utility to perform
+easy changes.
diff --git a/src/series/series.md b/src/series/series.md
index 34f34d7..216c105 100644
--- a/src/series/series.md
+++ b/src/series/series.md
@@ -29,6 +29,7 @@ of complexity: `grep`, `sed` and `awk`. Work in progress.
 * Part 0: [Regular expressions](../blog/2023-06-16-regex)
 * Part 1: [grep](../blog/2023-08-20-grep)
 * Part 2: [sed](../blog/2023-12-03-sed)
+* Part 2.1: [tr](../blog/2024-01-13-tr)
 * Part 3: awk (coming "soon")
 
 ## The UNIX shell as an IDE