Cleaned up the documentation and code a little.

README.md

@@ -1,25 +1,40 @@
 # arg - A simple command line argument parser
 This is a simple tool with a simple job: figure out the value of a single
-command line argument. Tools like `getopt` and `getopts` are much too
-complicated. It's much nicer to be able to write shell scripts like this:
+command line argument. Tools like `getopt` and `getopts` are unnecessarily
+complex and difficult to use, while also not supporting common use cases well.
+Parsing an argument in a shell script shouldn't require a tutorial or dozen
+lines of shell code!
+
+To quote the `getopt` manpage:
+
+> Each shellscript has to carry complex code to parse arguments halfway
+> correcty (like the example presented here). A better getopt-like tool would
+> move much of the complexity into the tool and keep the client shell scripts
+> simpler.
+
+## Usage
+Simply run: `arg <flag> <extra args...>`.  `<flag>` can be anything, like `-f`,
+`-flag`, `--flag`, or `flag`.  If the flag occurs among the rest of the
+command line arguments, `arg` will print the flag's value (if any) and exit
+successfully, otherwise, it will fail (exit status 1). In a shell script, this
+would look like:
 
 ```
 #!/bin/sh
 dir=`arg --dir "$@" || arg -d "$@" || echo "$HOME/Downloads"`
+if ! server=`arg --server "$@" || arg -s "$@"`; then
+    echo "No server provided :(" && exit 1
+fi
 if arg --verbose "$@" || arg -v "$@"; then
     echo "Downloading to $dir"
 fi
-cd $path && curl -O example.com/file.zip
+curl "$server/file.zip" > "$dir/file.zip"
 ```
 
-## Usage
-The following forms are accepted: `arg -f ...`, `arg -flag ...`, or `arg --flag
-...`. If `arg` finds the first given flag among the rest of the command line
-arguments, it will print the flag's value (if any) and exit successfully,
-otherwise, it will fail (exit status 1). The value may be of the form
-`--flag=value`, `--flag value`, `-f value`, or `-f=value`. Single-letter flags
-will match when grouped with other single letter flags, but will not have a
-value, i.e. `arg -b -abc foo` will exit successfully without printing anything.
+The flag's value may be of the form `<flag>=<value>` or `<flag> <value>`.
+Single-letter flags will also match when grouped with other single letter
+flags, but will not have a value (e.g. `arg -b -abc foo` will succeed without
+printing anything).
+
 When using `arg` in a shell script, it is best to use quotes around `$@`, as in
-`arg --foo "$@"`, so arguments with spaces will parse properly, like
-`my_script.sh --flag "one two"`.
+`arg --foo "$@"`, so arguments with spaces will be forwarded correctly.

arg.c

@@ -12,39 +12,43 @@
 #include <stdio.h>
 #include <string.h>
 
+#define EXIT_SUCCESS 0
+#define EXIT_NO_MATCH 1
+#define EXIT_BAD_USAGE -1
+
 int main(int argc, char **argv)
 {
     char *flag = argv[1];
-    if (!flag || flag[0] != '-') {
-        fprintf(stderr, "Usage: arg [-<f> | --<flag>] ...\n");
-        return 1;
+    if (!flag) {
+        fprintf(stderr, "Usage: arg <flag> ...\n");
+        return EXIT_BAD_USAGE;
     }
-    size_t n = strlen(flag);
+    size_t flaglen = strlen(flag);
     // Look for: --flag=val, --flag val, --flag, -f=val, -f val, or -f
     for (int i = 2; i < argc; i++) {
         char *arg = argv[i];
-        if (strncmp(arg, flag, n) != 0) continue;
-        if (arg[n] == '\0') { // --flag
-            if (argv[i+1] && argv[i+1][0] != '-')
-                puts(argv[i+1]); // value of flag, if any
-            return 0;
-        } else if (arg[n] == '=') { // --flag=...
-            puts(&arg[n+1]);
-            return 0;
+        if (strncmp(arg, flag, flaglen) != 0) continue;
+        if (arg[flaglen] == '\0') { // --flag ...
+            if (argv[i+1] && argv[i+1][0] != '-') // --flag <value>
+                puts(argv[i+1]); // value of the flag
+            return EXIT_SUCCESS;
+        } else if (arg[flaglen] == '=') { // --flag=...
+            puts(&arg[flaglen+1]);
+            return EXIT_SUCCESS;
         }
     }
-    // If flag is single-character, e.g. -f, look for it among other single
-    // character flags like -xfy, in which case the next argument will not
-    // be considered its value.
-    if (n == 2) {
+    // If flag is single-character without a value, e.g. -f, look for it
+    // among other single character flags like -xfy.
+    if (flag[0] == '-' && flaglen == 2) {
         for (int i = 2; i < argc; i++) {
             char *arg = argv[i];
             if (arg[0] != '-' || arg[1] == '-')
                 continue; // skip foo and --foo
             for (char *c = &arg[1]; *c; c++) {
-                if (*c == flag[1]) return 0;
+                if (*c == flag[1])
+                    return EXIT_SUCCESS;
             }
         }
     }
-    return 1;
+    return EXIT_NO_MATCH;
 }