使用和不使用“bash -c”在 shell 中运行命令有什么区别?

使用和不使用“bash -c”在 shell 中运行命令有什么区别?

bash -c <command>直接在 bash shell 中运行命令与在 bash shell 中运行命令有什么区别?我的问题的目的是我想知道bash -c在 shell 中和不在 shell 中如何执行命令。这些信息可以帮助我思考之前的问题https://unix.stackexchange.com/questions/261595/do-a-bash-script-and-bash-c-command-run-in-a-subshel​​l-environment, 和为什么通过命令替换调用传递到子 shell 的非环境变量?

以下是我尝试通过的一种方式来理解上述问题strace。但您可以在不使用 的情况下解释差异strace

我考虑两种情况,runningbash -c date和 running bash -c 'date&'

  1. 当我bash -c date在 pid 6913 的 bash shell 中运行时,并在另一个 shell 中跟踪该 shell:

    $ sudo strace -f -e trace=process -p 6913
    Process 6913 attached
    clone(Process 13010 attached
    child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7f457c05ca10) = 13010
    [pid  6913] wait4(-1,  <unfinished ...>
    [pid 13010] execve("/bin/bash", ["bash", "-c", "date"], [/* 66 vars */]) = 0
    [pid 13010] arch_prctl(ARCH_SET_FS, 0x7f920c591740) = 0
    [pid 13010] execve("/bin/date", ["date"], [/* 66 vars */]) = 0
    [pid 13010] arch_prctl(ARCH_SET_FS, 0x7f67e34df740) = 0
    [pid 13010] exit_group(0)               = ?
    [pid 13010] +++ exited with 0 +++
    <... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 13010
    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13010, si_status=0, si_utime=0, si_stime=0} ---
    wait4(-1, 0x7ffea6781518, WNOHANG|WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes)
    

    与在 bash shell 中运行(见下文)相比date,区别仅在于我们在第一行 clone()和之间多了一行execve() date

    execve("/bin/bash", ["bash", "-c", "date"], [/* 66 vars */]) = 0
    

    区别是什么意思?

    跟踪运行的输出date而不是bash -c date

    $ sudo strace -f -e trace=process -p 6913
    [sudo] password for t: 
    Process 6913 attached
    clone(Process 12918 attached
    child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7f457c05ca10) = 12918
    [pid  6913] wait4(-1,  <unfinished ...>
    [pid 12918] execve("/bin/date", ["date"], [/* 66 vars */]) = 0
    [pid 12918] arch_prctl(ARCH_SET_FS, 0x7ff00c632740) = 0
    [pid 12918] exit_group(0)               = ?
    [pid 12918] +++ exited with 0 +++
    <... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 12918
    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12918, si_status=0, si_utime=0, si_stime=0} ---
    wait4(-1, 0x7ffea6781518, WNOHANG|WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes)
    
  2. 当我bash -c 'date&'在 pid 6913 的 bash shell 中运行时,并在另一个 shell 中跟踪该 shell:

    $ sudo strace -f -e trace=process -p 6913
    Process 6913 attached
    clone(Process 13023 attached
    child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7f457c05ca10) = 13023
    [pid  6913] wait4(-1,  <unfinished ...>
    [pid 13023] execve("/bin/bash", ["bash", "-c", "date&"], [/* 66 vars */]) = 0
    [pid 13023] arch_prctl(ARCH_SET_FS, 0x7f154970b740) = 0
    [pid 13023] clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7f154970ba10) = 13024
    [pid 13023] exit_group(0)               = ?
    [pid 13023] +++ exited with 0 +++
    <... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WSTOPPED|WCONTINUED, NULL) = 13023
    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=13023, si_status=0, si_utime=0, si_stime=0} ---
    wait4(-1, 0x7ffea6781518, WNOHANG|WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes)
    Process 13024 attached
    [pid 13024] execve("/bin/date", ["date"], [/* 66 vars */]) = 0
    [pid 13024] arch_prctl(ARCH_SET_FS, 0x7f8045f35740) = 0
    [pid 13024] exit_group(0)               = ?
    [pid 13024] +++ exited with 0 +++
    

    与在 bash shell 中运行(见下文)相比date&,区别似乎不仅仅是多了一个execve

    execve("/bin/bash", ["bash", "-c", "date&"], [/* 66 vars */]) = 0
    

    还安排子流程的创建,哪个子流程 execve() date和哪个子流程execve() /bin/bash

    区别是什么意思?

    运行时跟踪输出date&而不是bash -c 'date&'

    $ sudo strace -f -e trace=process -p 6913
    Process 6913 attached
    clone(Process 12931 attached
    child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,child_tidptr=0x7f457c05ca10) = 12931
    [pid 12931] execve("/bin/date", ["date"], [/* 66 vars */]) = 0
    [pid 12931] arch_prctl(ARCH_SET_FS, 0x7f530c5ee740) = 0
    [pid 12931] exit_group(0)               = ?
    [pid 12931] +++ exited with 0 +++
    --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=12931, si_status=0, si_utime=0, si_stime=0} ---
    wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], WNOHANG|WSTOPPED|WCONTINUED, NULL) = 12931
    wait4(-1, 0x7ffea6780718, WNOHANG|WSTOPPED|WCONTINUED, NULL) = -1 ECHILD (No child processes)
    

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