add shebang

1 files changed, 32 insertions, 0 deletions

diff --git a/shebang b/shebang
new file mode 100755
index 0000000..09ecd76
--- /dev/null
+++ b/shebang
@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+"""
+Reads a script with a shebang from standard input and executes it.
+"""
+import os
+import subprocess
+import sys
+
+# We are not using sys.stdin because it does internal buffering:
+# sys.stdin.read(2) can read up to 8192 bytes, we however don't
+# want that because we want everything after the shebang to be
+# read by the subprocess.
+
+if os.read(0, 2) != b'#!':
+    sys.exit('expected #! shebang')
+
+line = b''
+
+while True:
+    byte = os.read(0, 1)
+    if byte == b'':
+        sys.exit('unexpected EOF')
+    if byte == b'\n':
+        break
+    line += byte
+
+args = line.strip().split(maxsplit=1)
+args.append('/dev/stdin')
+subprocess.run(args)
+
+# the subprocess inherits the standard input file
+# descriptor and interprets the remaining input