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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
+bar=${foo:-some_val}  # if $foo set, then bar=$foo else bar=some_val
+
+# check param set
+bar=${foo:?msg}  # if $foo set, then bar=$foo else exit and print msg
+
+# indirect
+FOO=foo
+BAR=FOO
+bar=${!BAR}  # deref value of BAR -> bar=$FOO
+
+# prefix
+${foo#prefix}  # remove prefix when expanding $foo
+# suffix
+${foo%suffix}  # remove suffix when expanding $foo
+
+# substitute
+${foo/pattern/string}  # replace pattern with string when expanding foo
+# pattern starts with
+# '/'   replace all occurences of pattern
+# '#'   pattern match at beginning
+# '%'   pattern match at end
+```
+
+> Note: `prefix`/`suffix`/`pattern` are expanded as [pathnames](#pathname).
+
+### Pathname
+
+```bash
+*           match any string
+?           match any single char
+\\          match backslash
+[abc]       match any char of 'a' 'b' 'c'
+[a-z]       match any char between 'a' - 'z'
+[^ab]       negate, match all not 'a' 'b'
+[:class:]   match any char in class, available:
+              alnum,alpha,ascii,blank,cntrl,digit,graph,lower,
+              print,punct,space,upper,word,xdigit
+```
+
+Wit `extglob` shell option enabled it is possible to have more powerful
+patterns. In the following `pattern-list` is one ore more patterns separated
+by `|` char.
+
+```bash
+?(pattern-list)   matches zero or one occurrence of the given patterns
+*(pattern-list)   matches zero or more occurrences of the given patterns
++(pattern-list)   matches one or more occurrences of the given patterns
+@(pattern-list)   matches one of the given patterns
+!(pattern-list)   matches anything except one of the given patterns
+```
+> Note: `shopt -s extglob`/`shopt -u extglob` to enable/disable `extglob`
+> option.
+
+## 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`
+
+## Completion
+
+The `complete` builtin is used to interact with the completion system.
+```bash
+complete                    # print currently installed completion handler
+complete -F <func> <cmd>    # install <func> as completion handler for <cmd>
+complete -r <cmd>           # uninstall completion handler for <cmd>
+```
+
+Variables available in completion functions:
+```bash
+# in
+$1              # <cmd>
+$2              # current word
+$3              # privous word
+
+COMP_WORDS      # array with current command line words
+COMP_CWORD      # index into COMP_WORDS with current cursor position
+
+# out
+COMPREPLY       # array with possible completions
+```
+
+The `compgen` builtin is used to generate possible matches by comparing `word`
+against words generated by `option`.
+```bash
+compgen [option] [word]
+
+# usefule options:
+# -W <list>    specify list of possible completions
+# -d           generate list with dirs
+# -f           generate list with files
+# -u           generate list with users
+# -e           generate list with exported variables
+
+# compare "f" against words "foo" "foobar" "bar" and generate matches
+compgen -W "foo foobar bar" "f"
+
+# compare "hom" against file/dir names and generate matches
+compgen -d -f "hom"
+```
+
+### Example
+Skeleton to copy/paste for writing simple completions.
+
+Assume a program `foo` with the following interface:
+```bash
+foo -c green|red|blue -s low|high -f <file> -h
+```
+
+The completion handler could be implemented as follows:
+```bash
+function _foo() {
+    local curr=$2
+    local prev=$3
+
+    local opts="-c -s -f -h"
+    case $prev in
+        -c) COMPREPLY=( $(compgen -W "green red blue" -- $curr) );;
+        -s) COMPREPLY=( $(compgen -W "low high" -- $curr) );;
+        -f) COMPREPLY=( $(compgen -f -- $curr) );;
+        *)  COMPREPLY=( $(compgen -W "$opts" -- $curr) );;
+    esac
+}
+
+complete -F _foo foo
+```
+
+[dup2]: http://man7.org/linux/man-pages/man2/dup.2.html