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+# bash(1)
+
+## Expansion
+
+### Generator
+
+```bash
+# generate sequence from n to m
+{n..m}
+# generate sequence from n to m step by s
+{n..m..s}
+
+# expand cartesian product
+{a,b}{c,d}
+```
+
+### Parameter
+```bash
+# default param
+# if $foo set, then bar=$foo else bar=some_val
+bar=${foo:-some_val}
+```
+
+## I/O redirection
+
+> Note: The trick with bash I/O redirection is to interpret from left-to-right.
+
+```bash
+# stdout & stderr to file
+command >file 2>&1
+# equivalent
+command &>file
+
+# stderr to stdout & stdout to file
+command 2>&1 >file
+```
+
+### Explanation
+
+```bash
+j>&i
+```
+Duplicate `fd i` to `fd j`, making `j` a copy of `i`. See [dup2(2)][dup2].
+
+Example:
+```bash
+command 2>&1 >file
+```
+1. duplicate `fd 1` to `fd 2`, effectively redirecting `stderr` to `stdout`
+2. redirect `stdout` to `file`
+
+[dup2]: http://man7.org/linux/man-pages/man2/dup.2.html