Shell执行:时间与/ usr/bin /时间

Question

当bash/zsh执行以下操作时会发生什么:

~ » /usr/bin/time -l sleep 1                                                                                                              
    1.00 real         0.00 user         0.00 sys
516096  maximum resident set size
     0  average shared memory size
     0  average unshared data size
     0  average unshared stack size
   145  page reclaims
     0  page faults
     0  swaps
     0  block input operations
     0  block output operations
     0  messages sent
     0  messages received
     0  signals received
     0  voluntary context switches
     2  involuntary context switches
------------------------------------------------------------
~ » time -l sleep 1                                                                                                                       
zsh: command not found: -l
-l sleep 1  0.00s user 0.00s system 52% cpu 0.001 total
------------------------------------------------------------
~ » /usr/bin/time foo                                                                                                                     
foo: No such file or directory
        0.00 real         0.00 user         0.00 sys
------------------------------------------------------------
~ » time foo                                                                                                                              
zsh: command not found: foo
foo  0.00s user 0.00s system 52% cpu 0.001 total

为什么我的使用时间有所不同,为什么zsh试图执行-l？

奇怪的是,zsh说

~ » which time                                                                                                                            
time: shell reserved word

虽然bash没有:

~ » bash                                                                                                                                  
bash-3.2$ which time
/usr/bin/time
bash-3.2$ time foo
bash: foo: command not found

real    0m0.006s
user    0m0.000s
sys 0m0.003s
bash-3.2$ /usr/bin/time foo
foo: No such file or directory
        0.00 real         0.00 user         0.00 sys
bash-3.2$ time -l sleep 1
bash: -l: command not found

real    0m0.001s
user    0m0.000s
sys 0m0.001s
bash-3.2$ /usr/bin/time -l sleep 1
        1.00 real         0.00 user         0.00 sys
    516096  maximum resident set size
         0  average shared memory size
         0  average unshared data size
         0  average unshared stack size
       144  page reclaims
         0  page faults
         0  swaps
         0  block input operations
         1  block output operations
         0  messages sent
         0  messages received
         0  signals received
         2  voluntary context switches
         2  involuntary context switches

Answer 1

time是在zsh和bash中内置的.但是,which仅内置于zsh.在bash中,当你使用which它时,它/usr/bin/which不知道shell内置函数.

所以在bash中,你应该使用:

$ type time
time is a shell keyword

原因time -l ...不起作用的是time语法不包含-l标志.

在这两种情况下,说这time是一个内置函数并不正确.将其称为"保留字"或"shell关键字"更准确,因为它适用于整个管道; 它不能作为函数或外部命令实现.在这个意义上,它类似于像其他语法元素if和while.