Add shell commands guide

CONTRIBUTING.md

@@ -66,6 +66,7 @@ If you can, please submit a merge request with the fix or improvements including
 1. If the MR changes the UI it should include before and after screenshots
 1. Link relevant [issues](https://gitlab.com/gitlab-org/gitlab-ce/issues) and/or [feedback items](http://feedback.gitlab.com/) from the merge request description and leave a comment on them with a link back to the MR
 1. Be prepared to answer questions and incorporate feedback even if requests for this arrive weeks or months after your MR submittion
+1. If your MR touches code that executes shell commands, make sure it adheres to the [shell command guidelines](    doc/development/shell_commands.md).
 
 The **official merge window** is in the beginning of the month from the 1st to the 7th day of the month. The best time to submit a MR and get feedback fast. Before this time the GitLab.com team is still dealing with work that is created by the monthly release such as assisting subscribers with upgrade issues, the release of Enterprise Edition and the upgrade of GitLab Cloud. After the 7th it is already getting closer to the release date of the next version. This means there is less time to fix the issues created by merging large new features.
 

doc/development/shell_commands.md

+# Guidelines for shell commands in the GitLab codebase
+
+## Use File and FileUtils instead of shell commands
+
+Sometimes we invoke basic Unix commands via the shell when there is also a Ruby API for doing it.
+Use the Ruby API if it exists.
+http://www.ruby-doc.org/stdlib-2.0.0/libdoc/fileutils/rdoc/FileUtils.html#module-FileUtils-label-Module+Functions
+
+```ruby
+# Wrong
+system "mkdir -p tmp/special/directory"
+# Better (separate tokens)
+system *%W(mkdir -p tmp/special/directory)
+# Best (do not use a shell command)
+FileUtils.mkdir_p "tmp/special/directory"
+
+# Wrong
+contents = `cat #{filename}`
+# Correct
+contents = File.read(filename)
+```
+
+This coding style could have prevented CVE-2013-4490.
+
+## Bypass the shell by splitting commands into separate tokens
+
+When we pass shell commands as a single string to Ruby, Ruby will let `/bin/sh` evaluate the entire string.
+Essentially, we are asking the shell to evaluate a one-line script.
+This creates a risk for shell injection attacks.
+It is better to split the shell command into tokens ourselves.
+Sometimes we use the scripting capabilities of the shell to change the working directory or set environment variables.
+All of this can also be achieved securely straight from Ruby
+
+```ruby
+# Wrong
+system "cd /home/git/gitlab && bundle exec rake db:#{something} RAILS_ENV=production"
+# Correct
+system({'RAILS_ENV' => 'production'}, *%W(bundle exec rake db:#{something}), chdir: '/home/git/gitlab')
+
+# Wrong
+system "touch #{myfile}"
+# Better
+system "touch", myfile
+# Best (do not run a shell command at all)
+FileUtils.touch myfile
+```
+
+This coding style could have prevented CVE-2013-4546.
+
+## Separate options from arguments with --
+
+Make the difference between options and arguments clear to the argument parsers of system commands with `--`.
+This is supported by many but not all Unix commands.
+
+To understand what `--` does, consider the problem below.
+
+```
+# Example
+$ echo hello > -l
+$ cat -l
+cat: illegal option -- l
+usage: cat [-benstuv] [file ...]
+```
+
+In the example above, the argument parser of `cat` assumes that `-l` is an option.
+The solution in the example above is to make it clear to `cat` that `-l` is really an argument, not an option.
+Many Unix command line tools follow the convention of separating options from arguments with `--`.
+
+```
+# Example (continued)
+$ cat -- -l
+hello
+```
+
+In the GitLab codebase, we avoid the option/argument ambiguity by _always_ using `--`.
+
+```ruby
+# Wrong
+system(*%W(git branch -d #{branch_name}))
+# Correct
+system(*%W(git branch -d -- #{branch_name}))
+```
+
+This coding style could have prevented CVE-2013-4582.
+
+## Do not use the backticks
+
+Capturing the output of shell commands with backticks reads nicely, but you are forced to pass the command as one string to the shell.
+We explained above that this is unsafe.
+In the main GitLab codebase, the solution is to use `Gitlab::Popen.popen` instead.
+
+```ruby
+# Wrong
+logs = `cd #{repo_dir} && git log`
+# Correct
+logs, exit_status = Gitlab::Popen.popen(%W(git log), repo_dir)
+
+# Wrong
+user = `whoami`
+# Correct
+user, exit_status = Gitlab::Popen.popen(%W(whoami))
+```
+
+In other repositories, such as gitlab-shell you can also use `IO.popen`.
+
+```ruby
+# Safe IO.popen example
+logs = IO.popen(%W(git log), chdir: repo_dir).read
+```
+
+Note that unlike `Gitlab::Popen.popen`, `IO.popen` does not capture standard error.