如何查看系统内置的shell命令及帮助信息

[tansheng@localhost ~]$ help
GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
These shell commands are defined internally.  Type `help‘ to see this list.
Type `help name‘ to find out more about the function `name‘.
Use `info bash‘ to find out more about the shell in general.
Use `man -k‘ or `info‘ to find out more about commands not in this list.

A star (*) next to a name means that the command is disabled.

 job_spec [&]                            history [-c] [-d offset] [n] or hist>
 (( expression ))                        if COMMANDS; then COMMANDS; [ elif C>
 . filename [arguments]                  jobs [-lnprs] [jobspec ...] or jobs >
 :                                       kill [-s sigspec | -n signum | -sigs>
 [ arg... ]                              let arg [arg ...]
 [[ expression ]]                        local [option] name[=value] ...
 alias [-p] [name[=value] ... ]          logout [n]
 bg [job_spec ...]                       mapfile [-n count] [-O origin] [-s c>
 bind [-lpvsPVS] [-m keymap] [-f filen>  popd [-n] [+N | -N]
 break [n]                               printf [-v var] format [arguments]
 builtin [shell-builtin [arg ...]]       pushd [-n] [+N | -N | dir]
 caller [expr]                           pwd [-LP]
 case WORD in [PATTERN [| PATTERN]...)>  read [-ers] [-a array] [-d delim] [->
 cd [-L|-P] [dir]                        readarray [-n count] [-O origin] [-s>
 command [-pVv] command [arg ...]        readonly [-af] [name[=value] ...] or>

[tansheng@localhost ~]$ help fg
fg: fg [job_spec]
    Move job to the foreground.
    
    Place the job identified by JOB_SPEC in the foreground, making it the
    current job.  If JOB_SPEC is not present, the shell‘s notion of the
    current job is used.
    
    Exit Status:
    Status of command placed in foreground, or failure if an error occurs.
[tansheng@localhost ~]$ 

[tansheng@localhost ~]$ help alias
alias: alias [-p] [name[=value] ... ]
    Define or display aliases.
    
    Without arguments, `alias‘ prints the list of aliases in the reusable
    form `alias NAME=VALUE‘ on standard output.
    
    Otherwise, an alias is defined for each NAME whose VALUE is given.
    A trailing space in VALUE causes the next word to be checked for
    alias substitution when the alias is expanded.
    
    Options:
      -p    Print all defined aliases in a reusable format
    
    Exit Status:
    alias returns true unless a NAME is supplied for which no alias has been
    defined.
[tansheng@localhost ~]$