Fixed issue #1.

Author: Dmitry Grigoriev

README.md

@@ -2,55 +2,62 @@
 
 ## Concept
 
+### In short (for experienced linux users)
+
+- Create C++ source file, prepend it with shebang line `#!/path/to/build-n-run`, make it executable `chmod +x script.cpp` and run: `./script.cpp`.
+
+- `build-n-run` compiles `script.cpp` to `~/bin/build-n-run.compiled/` **only if source file is newer than compiled binary**, and then runs compiled binary.
+
+### In detail
+
 They say compiled languages are inconvenient for scripting because you have to recompile your script everytime you edit it. A little program `build-n-run.cpp` presented in this repo solves the problem.
 
 In bash scripts, first line always starts with "[shebang](https://en.wikipedia.org/wiki/Shebang_(Unix))" followed by name of a bash interpreter. When you run bash script, effectively bash interpreter is run and this script is passed to it as first argument.
 
         #!/bin/bash
         echo "Hello world!"
 
-Same approach is used for Python scripts, Scala scripts, etc. So I did the same: file `hello.cpp` in this repo is normal C++ program prepended with shebang line:
+Same approach is used for Python scripts, Scala scripts, etc. So I did the same: example script `hello.cpp` in this repo is normal C++ program prepended with shebang line:
 
         #!/usr/local/bin/build-n-run
         #include <stdio.h>
         int main() {
             puts("Hello world!");
         }
 
-`build-n-run` automatically recompiles script *if it was edited* (by comparing source and binary modification times) and then runs it:
+`build-n-run` automatically recompiles script **if it is new or edited** (i.e. if compiled binary does not exist or if source file's modification time is newer than binary's) and then runs it:
 
         $ hello.cpp
-        Recompiling...    <--- first time you run your script, it's recompiled
+        Recompiling...    <--- First time you run your script, it's recompiled.
         Hello world!
 
         $ hello.cpp
-        Hello world!
+        Hello world!      <--- No recompilation on next run if script is unchanged.
 
 ## Installation and use
 
 1. Download `build-n-run.cpp` from this repo.
 
-2. [Optional] Check its source code: update GCC options used to compile your scripts; remove "Recompiling..." message if it messes with our scripts' output.
+2. (Optional) Check its source code: you might want to update GCC options used to compile your scripts, or remove "Recompiling..." message if it messes with our scripts' output.
 
-3. Compile and install it. I install to `/usr/local/bin`, so I run this command as root. You can install it anywhere you want, just don't forget to edit path in `hello.cpp`'s shebang line. I use gcc 7.3.0, so I explicitly link `libstdc++fs.a` which implements `std::experimental::filesystem`.
+3. Compile and install it. I install to `/usr/local/bin`, so I run this command as root; you can install it anywhere you want, just don't forget to update path in scripts' shebang line. I use gcc 7.3.0, so I explicitly link `libstdc++fs.a` which implements `std::experimental::filesystem`.
 
         # g++ build-n-run.cpp -lstdc++fs -o /usr/local/bin/build-n-run
 
-4. Create directory for compiled scripts. Its full path must be the same as installed program's plus `.compiled` suffix. If that directory is shared among multiple users, grant global access to it.
-
-        # mkdir /usr/local/bin/build-n-run.compiled
-        # chmod a+rwx /usr/local/bin/build-n-run.compiled
-
-That's all. Now write C++ program with shebang at first line, *chmod it to be executable*, and run:
+That's all. Now write C++ program with shebang line, **chmod it to be executable**, and run:
 
         $ chmod +x hello.cpp
         $ ./hello.cpp
         Recompiling...
         Hello world!
 
-Now let's take a look at `build-n-run.compiled` directory:
+Let's take a look at `~/bin/build-n-run.compiled` directory:
 
-        $ ls /usr/local/bin/build-n-run.compiled
+        $ ls ~/bin/build-n-run.compiled
         home--me--tmp--hello
 
 I downloaded `hello.cpp` to `/home/me/tmp`, and compiled binary's name reflects source's full path, slashes replaced with `"--"`.
+
+## Security concerns
+
+Compiled cache is inside user's home, so scripts compiled by one user cannot be run by other users (see issue #1).

build-n-run.cpp

@@ -1,6 +1,5 @@
 // How to build with gcc-7.3.0: 
 // # g++ build-n-run.cpp -lstdc++fs -o /usr/local/bin/build-n-run
-// # mkdir /usr/local/bin/build-n-run.compiled; chmod a+rwx /usr/local/bin/build-n-run.compiled
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -25,7 +24,7 @@ string replaceAll(string s, const string& search, const string& replace) {
 }
 
 
-// I call gcc via `man 3 system` which starts shell. Slow. OK for compilation, but not for compiled script.
+// I call gcc via `system(3)` which starts shell. Slow. OK for compilation, but not for running compiled script.
 int fastExec(const char* cmd, char** argv) {
 	auto pid = fork();
 	if (pid == -1) {
@@ -49,33 +48,37 @@ int fastExec(const char* cmd, char** argv) {
 
 
 int main(int argc, char** argv) {
-	path me(argv[0]);
+	path me(*argv++);
 	if (argc < 2) {
 		fprintf(stderr, "ERROR: Usage: %s SCRIPT.cpp [ARG...]\n", me.filename().c_str());
 		exit(1);
 	}
 
-	path source(argv[1]);
+	path source(*argv);
 	if (source.extension() != ".cpp") {
 		fputs("ERROR: Given script must have .cpp extension", stderr);
 		exit(1);
 	}
 
-	path target(me.string() + ".compiled/" + replaceAll(canonical(source), "/", "--").substr(2));
+	char* home = getenv("HOME");
+	if (home == nullptr || *home == '\0') {
+		fputs("ERROR: HOME environment variable is not set", stderr);
+		exit(1);
+	}
+	path target(string(home) + "/bin/" + me.filename().string() + ".compiled/" + replaceAll(canonical(source), "/", "--").substr(2));
 	target.replace_extension();
 
 	if (!exists(target) || last_write_time(source) > last_write_time(target)) {
 		puts("Recompiling...");
-
-		// Commented out: permission denied if executed by restricted user. Create manually with mode=777.
-//		create_directories(target.parent_path());
+		
+		create_directories(target.parent_path());
 
 		// https://stackoverflow.com/a/1003654/4247442
-		string cmd = "tail -n +2 \"" + source.string() + "\" | g++ -o \"" + target.string() + "\" -xc++ -lstdc++fs -";
+		string cmd = "tail -n +2 \"" + source.string() + "\" | g++ -o \"" + target.string() + "\" -xc++ -Wall -lstdc++fs -";
 		if (system(cmd.c_str()) != 0) {
 			exit(1);
 		}
 	}
 
-	return fastExec(target.c_str(), argv + 1);
+	return fastExec(target.c_str(), argv);
 }