the ${param/pat[/string]} parameter pattern substitution operator
${param/pat[/string]} 参数模式替换的操作
expansions to perform substring removal (${p%[%]w}, ${p#[#]w})
扩展子串删除命令：(${p%[%]w}, ${p#[#]w})
expansion of positional parameters beyond $9 with ${num}
扩展了为止参数超过9的限制
variables: BASH, BASH_VERSION, BASH_VERSINFO, UID, EUID, REPLY,
TIMEFORMAT, PPID, PWD, OLDPWD, SHLVL, RANDOM, SECONDS,
LINENO, HISTCMD, HOSTTYPE, OSTYPE, MACHTYPE, HOSTNAME,
ENV, PS3, PS4, DIRSTACK, PIPESTATUS, HISTSIZE, HISTFILE,
HISTFILESIZE, HISTCONTROL, HISTIGNORE, GLOBIGNORE, GROUPS,
PROMPT_COMMAND, FCEDIT, FIGNORE, IGNOREEOF, INPUTRC,
SHELLOPTS, OPTERR, HOSTFILE, TMOUT, FUNCNAME, histchars,
auto_resume
上边一堆的环境变量
DEBUG trap
ERR trap
调试和错误陷阱
variable arrays with new compound assignment syntax
数组和复合赋值运算符的语法规则
redirections: <>, &>, >|, <<<, [n]<&word-, [n]>&word-
重定向规则： <>, &>, >|, <<<, [n]<&word-, [n]>&word-
prompt string special char translation and variable expansion
提示字符串特殊字符的转化和变量展开
auto-export of variables in initial environment
自动继承初始化的环境变量
command search finds functions before builtins
命令搜查函数限于内建命令
