commit 4fdc98c3094efc38d57bda69dbd6dafa98e27c9c
parent 315672cbd8eecc563b066fddea25d1440fe8fe89
Author: Sebastiano Tronto <sebastiano@tronto.net>
Date: Tue, 20 Feb 2024 08:59:34 +0100
Added blog post
Diffstat:
3 files changed, 100 insertions(+), 0 deletions(-)
diff --git a/src/blog/2024-01-13-tr/tr.md b/src/blog/2024-01-13-tr/tr.md
@@ -107,3 +107,5 @@ 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.
+
+*Next in the series: [head and tail](../2024-02-20-head-and-tail)*
diff --git a/src/blog/2024-02-20-head-and-tail/head-and-tail.md b/src/blog/2024-02-20-head-and-tail/head-and-tail.md
@@ -0,0 +1,97 @@
+# Unix text filters part 2.2 of 3: head and tail
+
+*This post is part of a [series](../../series)*
+
+Continuing on our series of small text-filtering utilities, we have
+`head` and `tail`. They are very simple, but also very useful.
+
+## head
+
+The `head` command is used to display the first few lines of a text file.
+For example, the command:
+
+```
+$ head -n 4 [file]
+```
+
+Prints the first 4 lines of `[file]`. If `[file]` is not specified,
+`head` reads from standard input. If the option `-n 4` is not specified,
+the first 10 lines are shown.
+
+One can also use the following alternative notation:
+
+```
+$ head -4 [file]
+```
+
+Of course, there is also the equivalent [`sed`](../2023-12-03-sed) command:
+
+```
+$ sed 5q [file]
+```
+
+## tail
+
+The `tail` command shows the *last* few lines of a specified file,
+or of standard input. It supports the same `-n [number]` (or
+`-[number]`) option where `n` defaults to 10.
+
+However, for some reason, even in its standard POSIX variant, `tail`
+has some extra features. First of all, one can add a `+` before
+`[number]` to show all lines *from the `[number]`-th onwards*,
+instead of the last `[number]`. For example:
+
+```
+$ printf '1\n2\n3\n4\n5\n' | tail -n +2
+2
+3
+4
+5
+```
+
+As you can see, the line numbering is 1-based.
+
+It is also possible to start at a specific *byte* in the text stream
+using the `-c` option:
+
+```
+$ echo 'Hello' | tail -c 3
+lo
+```
+
+Notice that the ending `\n` is also included in the count. The `-c` option
+also supports the `+[number]` notation:
+
+```
+$ echo 'Hello' | tail -c +2
+ello
+```
+
+There is also an `-r` option that reverses the order of the output:
+
+```
+$ printf '1\n2\n3\n4\n5\n' | tail -r -n 2
+5
+4
+```
+
+Perhaps the most interesting feature of `tail` is the `-f` option,
+which makes it stay open when the end of the file is reached,
+displaying in real time any lines that are sunbsequently added. It
+can be used like this:
+
+```
+$ tail -f my-log-file.log
+```
+
+This can be useful when the command writing to `my-log-file.log`
+is already running and it is not possible to redirect its output.
+
+The `-f` option does not work when `tail` is reading from standard
+input, so technically speaking we are not in "text filter" territory
+anymore, but it was mentioning.
+
+## Conclusions
+
+These two utilities don't do much, but can accomplish a lot when combined
+with I/O redirection and other text filters.
diff --git a/src/series/series.md b/src/series/series.md
@@ -30,6 +30,7 @@ of complexity: `grep`, `sed` and `awk`. Work in progress.
* 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 2.2: [head and tail](../blog/2024-02-20-head-and-tail)
* Part 3: awk (coming "soon")
## The UNIX shell as an IDE
