"its" -> "it's" for grammar

[wiki.git] / doc / safe-shell.mdwn

diff --git a/doc/safe-shell.mdwn b/doc/safe-shell.mdwn
index e57eba1c642b29d0cf41a2d98019fb546b86eb0d..7b24040f27a906a60df551fbf105c2c4fb4f289f 100644 (file)
--- a/doc/safe-shell.mdwn
+++ b/doc/safe-shell.mdwn
@@ -18,11 +18,8 @@ variety of command-line utilities available. Much of that functionality will be
 available through libraries in Python or other languages. For the handful of
 things that aren't, you can still call external programs. In Python, the
 [subprocess](http://docs.python.org/2/library/subprocess.html) module is very
-useful for this. It also has two big advantages over shell — it's a lot
-easier to avoid
-[word-splitting](http://www.gnu.org/software/bash/manual/html_node/Word-Splitting.html)
-or similar issues, and since calls to subprocess will tend to be relatively
-uncommon, it's easy to scrutinize them especially hard.
+useful for this. You should try to avoid passing `shell=True` to `subprocess` (or using `os.system` or similar functions at all), since that will run a shell, exposing you to many of the same issues as plain shell has. It also has two big advantages over shell — it's a lot easier to avoid
+[word-splitting](http://www.gnu.org/software/bash/manual/html_node/Word-Splitting.html) or similar issues, and since calls to subprocess will tend to be relatively uncommon, it's easy to scrutinize them especially hard. When using `subprocess` or similar tools, you should still be aware of the suggestions in "Passing filenames or other positional arguments to commands" below.
 
 ## Shell settings
 
@@ -107,7 +104,7 @@ If you get filenames from the user or from shell globbing, or any other kind of
 
 Fixing this depends on what command you're running.
 
-For many commands, however, `--` is accepted to indicate that any options are done, and future arguments should be parsed as positional parameters – even if they look like options. In the `sudo` example above, `sudo -u nobody -- "$@"` would avoid this attack (though obviously specifying in the `sudo` configuration that commands can only be run as `nobody` is also a good idea).
+For many commands, however, `--` is accepted to indicate that any options are done, and future arguments should be parsed as positional parameters — even if they look like options. In the `sudo` example above, `sudo -u nobody -- "$@"` would avoid this attack (though obviously specifying in the `sudo` configuration that commands can only be run as `nobody` is also a good idea).
 
 Another approach is to prefix each filename with `./`, if the filenames are expected to be in the current directory.
 
@@ -115,9 +112,13 @@ Another approach is to prefix each filename with `./`, if the filenames are expe
 
 TODO: mumble `mktemp`?
 
+## Other resources
+
+Google has a [Shell Style Guide](http://google-styleguide.googlecode.com/svn/trunk/shell.xml). As the name suggests, it primarily focuses on good style, but some items are safety/security-relevant.
+
 ## Conclusion
 
 When possible, instead of writing a "safe" shell script, *use a higher-level
 language like Python*. If you can't do that, the shell has several *options* that
 you can enable that will reduce your chances of having bugs, and you should be
-sure to *quote liberally*.
+sure to *quote liberally*.