Erlang Shell下有很多内置的命令，在平时交互的时候很好用，文档里面都是一行带过，大家可能没什么感觉。
我来重点讲解和演示下：
$ erl
Erlang R14B04 (erts-5.8.5) 1 [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.8.5 (abort with ^G)
1> help().
** shell internal commands **
b() -- display all variable bindings
e(N) -- repeat the expression in query
f() -- forget all variable bindings
f(X) -- forget the binding of variable X
h() -- history
history(N) -- set how many previous commands to keep
results(N) -- set how many previous command results to keep
catch_exception(B) -- how exceptions are handled
v(N) -- use the value of query
rd(R,D) -- define a record
rf() -- remove all record information
rf(R) -- remove record information about R
rl() -- display all record information
rl(R) -- display record information about R
rp(Term) -- display Term using the shell\'s record information
rr(File) -- read record information from File (wildcards allowed)
rr(F,R) -- read selected record information from file(s)
rr(F,R,O) -- read selected record information with options我最经常用的有以下几个，熟悉了感觉就很爽：
抹掉变量：
f() - forget all variables
f(X) - forget X
1> Pid=spawn(fun()-> receive _->ok end end).
<0.33.0>
2>
2> Pid=spawn(fun()-> receive _->ok end end).
** exception error: no match of right hand side value <0.35.0>
3> f(Pid).
ok
4> Pid=spawn(fun()-> receive _->ok end end).
<0.39.0>
5> f().
ok
6> Pid=spawn(fun()-> receive _->ok end end).
<0.42.0>
7>找回命令执行结果：
v(42) - recall result from line 42
v(-1) - recall result from previous line
$ erl
1> spawn(fun()-> receive _->ok end end). %%忘记赋值，有spawn这样的命令有副作用，不能重复执行
<0.33.0>
2> Pid=v(-1). %% 补救
<0.33.0>
3> v(2).
<0.33.0>读入记录定义：
rr(foo) - read record definitions from module foo
$ touch test.dat
$ locate file.hrl
/usr/local/lib/erlang/lib/kernel-2.14/include/file.hrl
$ erl
1> file:read_file_info("test.dat"). %%信息很乱
{ok,{file_info,38914,regular,read_write,
{{2011,10,5},{14,56,49}},
{{2011,10,5},{14,56,49}},
{{2011,10,5},{14,56,49}},
33261,1,234881026,0,4577608,501,20}}
2>
2> rr("/usr/local/lib/erlang/lib/kernel-2.14/include/file.hrl").
[file_descriptor,file_info]
3>
3> file:read_file_info("test.dat"). %%自描述
{ok,#file_info{size = 38914,type = regular,
access = read_write,
atime = {{2011,10,5},{14,56,49}},
mtime = {{2011,10,5},{14,56,49}},
ctime = {{2011,10,5},{14,56,49}},
mode = 33261,links = 1,major_device = 234881026,
minor_device = 0,inode = 4577608,uid = 501,gid = 20}}
4>找回命令历史记录：
h()
$ erl
1> X=1.
1
2> Y=2.
2
3> Z=3.
3
4> h().
1: X = 1
-> 1
2: Y = 2
-> 2
3: Z = 3
-> 3
ok
5>祝大家玩得开心！