sebastiano.tronto.net

commit f0b6a7f1c683446651b72ef9a968b90f548a42b2
parent 480851093e36799c4ccfaddf68d34819175377f7
Author: Sebastiano Tronto <sebastiano@tronto.net>
Date:   Fri, 20 Sep 2024 14:26:43 +0200

new blog post

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src/blog/2024-09-20-c-scripting/c-scripting.md
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src/blog/2024-09-20-c-scripting/script.c
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diff --git a/src/blog/2024-09-20-c-scripting/c-scripting.md b/src/blog/2024-09-20-c-scripting/c-scripting.md
@@ -0,0 +1,95 @@
+# Shell scripting in C
+
+Recently, one of my [shell scripts](https://git.tronto.net/scripts)
+grew too large. It is probably time to rewrite it in a proper
+language. If possible I would like to still keep it as a script,
+so I don't have to add a compilation step to the installation process
+of my scripts.  Sure, I could use a scripting language like Python
+or Perl... but what if I wanted to rewrite this script in C?
+
+To be more precise, I want to write a single C file `script.c`,
+make it executable with `chmod +x`, move it to somewhere in my
+`$PATH` and then be able to run it simply by typing `script.c` in
+my shell. It turns out that this is actually possible. Let's see
+how *right now* so we don't have the time to wonder if it is a wise
+thing to do (it does not sound like it).
+
+## The trick
+
+The single thing that makes it all possible is that lines starting
+with `#` are preprocessor directives for C and comments for a UNIX
+shell. We can then use the `#if` directive for conditional compilation
+to "hide" a shell script in our souce file:
+
+```
+#if 0 /* Always false, skipped by the C compiler */
+
+[Your shell script here]
+
+exit 0 # Shell script terminates
+#endif
+
+[Your C source code here]
+```
+
+And now we just have to come up with a shell script that compiles
+itself - that it, that runs a C compiler on its own source file.
+For this we can run the `realpath` command on the shell parameter
+`$0` to obtain the full path to the source file. Then we can compile
+this file and save the executable to a temporary file, which we
+finally run. The "C Script Hello World" looks like this:
+
+```
+#if 0
+
+bin="$(mktemp)"
+cc -o "$bin" "$(realpath $0)"
+"$bin"
+
+exit 0
+#endif
+
+#include <stdio.h>
+
+int main() {
+	printf("Hello, world!\n");
+	return 0;
+}
+```
+
+If you want to try this for yourself, you can download
+[script.c](./script.c) instead of copy-pasting.
+
+## Caveats
+
+There are few caveats with this approach. First of all, you are
+running a C compiler every time you launch this "script". This is
+obviously less efficient than running a normal shell script.
+
+Secondly, the `realpath` command I used above is not standard. I
+thought it was in POSIX, but only the C library function with the
+same name is, not the command itself. However, it is present in
+most Linux distros since around 2012 and OpenBSD since version 7.1
+- surprisingly, after I started using each of these operating
+systems! The command is also included in FreeBSD (since 4.3) and
+MacOS (since 13), and there are probably workarounds to make the
+same concept work without it.
+
+## Credits
+
+I did not come up with this trick - I read about it in at least
+three separate occasions from different places. Had I saved any of
+the links I would add them here.
+
+## For completeness, a python version
+
+In case you did not know, making this work with an interpreted
+language is much simpler. You just need to use a
+[shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) to make
+the shell use the correct interpreter:
+
+```
+#!/usr/bin/env python3
+
+print("Hello, World!")
+```
diff --git a/src/blog/2024-09-20-c-scripting/script.c b/src/blog/2024-09-20-c-scripting/script.c
@@ -0,0 +1,15 @@
+#if 0
+
+bin="$(mktemp)"
+cc -o "$bin" "$(realpath $0)"
+"$bin"
+
+exit 0
+#endif
+
+#include <stdio.h>
+
+int main() {
+	printf("Hello, world!\n");
+	return 0;
+}