bash return builtin will exit a file sourced with `.'
bash返回内建命令 将退出一个.文件源
builtins: cd -/-L/-P, exec -l/-c/-a, echo -e/-E, hash -d/-l/-p/-t.
export -n/-f/-p/name=value, pwd -L/-P,
read -e/-p/-a/-t/-n/-d/-s/-u,
readonly -a/-f/name=value, trap -l, set +o,
set -b/-m/-o option/-h/-p/-B/-C/-H/-P,
unset -f/-v, ulimit -m/-p/-u,
type -a/-p/-t/-f/-P, suspend -f, kill -n,
test -o optname/s1 == s2/s1 < s2/s1 > s2/-nt/-ot/-ef/-O/-G/-S
上边一堆内建命令
bash reads ~/.bashrc for interactive shells, $ENV for non-interactive
bash reads这样写交互脚本，环境变量非交互的。
bash restricted shell mode is more extensive
bash更广泛的受限模式
bash allows functions and variables with the same name
允许函数名变量名一样
brace expansion
括号扩展
tilde expansion
～波浪号扩展
arithmetic expansion with $((...)) and `let' builtin
算术扩展：$((...)) 和let命令
the `...' extended conditional command
`...` 命令扩展
process substitution
进程替换
aliases and alias/unalias builtins
别名的内建命令
local variables in functions and `local' builtin
函数内本地变量
readline and command-line editing with programmable completion
