Hmm, this was sitting on my laptop

1 files changed, 50 insertions, 0 deletions

diff --git a/public/bash-redirection.md b/public/bash-redirection.md
new file mode 100644
index 0000000..8da02b3
--- /dev/null
+++ b/public/bash-redirection.md
@@ -0,0 +1,50 @@
+Bash redirection
+================
+---
+date: "2014-02-13"
+---
+
+Apparently, too many people don't understand Bash redirection.  They
+might get the basic syntax, but they think of the process as
+declarative; in Bourne-ish shells, it is procedural.
+
+In Bash, streams are handled in terms of "file descriptors" of "FDs".
+FD 0 is stdin, FD 1 is stdout, and FD 2 is stderr.  The equivalence
+(or lack thereof) between using a numeric file descriptor, and using
+the associated file in `/dev/*` and `/proc/*` is interesting, but
+beyond the scope of this article.
+
+Step 1: Pipes
+-------------
+
+To quote the Bash manual:
+
+	A 'pipeline' is a sequence of simple commands separated by one of the
+	control operators '|' or '|&'.
+
+	   The format for a pipeline is
+	     [time [-p]] [!] COMMAND1 [ [| or |&] COMMAND2 ...]
+
+Now, `|&` is just shorthand for `2>&1 |`, the pipe part happens here,
+but the `2>&1` part doesn't happen until step 2.
+
+First, if the command is part of a pipeline, the pipes are set up.
+For every instance of the `|` metacharacter, Bash creates a pipe
+(`pipe(3)`), and duplicates (`dup2(3)`) the write end of the pipe to
+FD 1 of the process on the left side of the `|`, and duplicate the
+read end of the pipe to FD 0 of the process on the right side.
+
+Step 2: Redirections
+--------------------
+
+*After* the initial FD 0 and FD 1 fiddling by pipes is done, Bash
+looks at the redirections.  **This means that redirections can
+override pipes.**
+
+Redirections are read left-to-right, and are executed as they are
+read, using `dup2(right-side, left-side)`.  This is where most of the
+confusion comes from, people think of them as declarative, which leads
+to them doing the first of these, when they mean to do the second:
+
+	cmd 2>&1 >file # stdout goes to file, stderr goes to stdout
+	cmd >file 2>&1 # both stdout and stderr go to file