使用 set -x 时提示字符串 PS4 复制自身

这里的区别不在于您放置 的位置-x，而在于您是否指定要使用的 shell（通过 shebang 或通过使用 显式调用脚本bash -x）。

根据man bash, PS4:

   PS4    The value of this parameter is expanded  as  with  PS1  and  the
          value  is  printed  before  each command bash displays during an
          execution trace.  The first character of PS4 is replicated  mul‐
          tiple  times, as necessary, to indicate multiple levels of indi‐
          rection.  The default is ``+ ''.

但是，在这种情况下，“间接级别”的含义还没有公布。

当您尝试在未指定要使用的解释器的情况下执行文本文件时，系统调用会execve失败并显示ENOEXEC (Exec format error). 接下来会发生什么取决于您尝试从中执行它的 shell，如下所述：

• 哪个 shell 解释器运行没有 shebang 的脚本？

特别是，如果调用 shell 是bash，则脚本似乎直接由调用 shell 执行：这似乎是手册中提到的“间接”：

$ strace -e trace=%process -f bash -c 'echo "Starting $0"; ./hw_no_shebang; echo "Finished."'
execve("/bin/bash", ["bash", "-c", "echo \"Starting $0\"; ./hw_no_sheb"...], 0x7ffd3dadbee8 /* 25 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7f95fe939740) = 0
Starting bash
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f95fe939a10) = 26515
strace: Process 26515 attached
[pid 26514] wait4(-1,  <unfinished ...>
[pid 26515] execve("./hw_no_shebang", ["./hw_no_shebang"], 0x555e5058a4f0 /* 25 vars */) = -1 ENOEXEC (Exec format error)
++ echo Hello
Hello
++ echo World
World
++ set +x
[pid 26515] exit_group(0)               = ?
[pid 26515] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 26515
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=26515, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
wait4(-1, 0x7fffba5272d0, WNOHANG, NULL) = -1 ECHILD (No child processes)
Finished.
exit_group(0)                           = ?
+++ exited with 0 +++

请注意，bash -c此处的 strace 调用中的 代替了您的交互式 shell，而不是bash -x您在上一个示例中使用的那个。

一旦我们添加一个shebang，例如：

$ cat hw_shebang
#!/bin/bash

set -x
echo "Hello"
echo "World"
set +x

然后（无论是否-x使用set或其他方式调用），然后execve成功并且脚本在其自己的外壳中执行，没有“间接”：

$ strace -e trace=%process -f bash -c 'echo "Starting $0"; ./hw_shebang; echo "Finished."'
execve("/bin/bash", ["bash", "-c", "echo \"Starting $0\"; ./hw_shebang"...], 0x7ffccc0af338 /* 25 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7f81b9ed2740) = 0
Starting bash
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f81b9ed2a10) = 26553
strace: Process 26553 attached
[pid 26552] wait4(-1,  <unfinished ...>
[pid 26553] execve("./hw_shebang", ["./hw_shebang"], 0x55879d9924f0 /* 25 vars */) = 0
[pid 26553] arch_prctl(ARCH_SET_FS, 0x7fa55de83740) = 0
+ echo Hello
Hello
+ echo World
World
+ set +x
[pid 26553] exit_group(0)               = ?
[pid 26553] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 26553
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=26553, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
wait4(-1, 0x7ffda2983b10, WNOHANG, NULL) = -1 ECHILD (No child processes)
Finished.
exit_group(0)                           = ?
+++ exited with 0 +++

例如，如果您调用了 no-shebang 脚本dash，则行为会有所不同：在初始 failed 之后，execve它将执行脚本：execve/bin/sh

$ strace -e trace=%process -f dash -c 'echo "Starting $0"; ./hw_no_shebang; echo "Finished."'
execve("/bin/dash", ["dash", "-c", "echo \"Starting $0\"; ./hw_no_sheb"...], 0x7ffc283d8638 /* 25 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7fd2cc787540) = 0
Starting dash
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fd2cc787810) = 26590
strace: Process 26590 attached
[pid 26589] wait4(-1,  <unfinished ...>
[pid 26590] execve("./hw_no_shebang", ["./hw_no_shebang"], 0x557143e3c8d8 /* 25 vars */) = -1 ENOEXEC (Exec format error)
[pid 26590] execve("/bin/sh", ["/bin/sh", "./hw_no_shebang"], 0x557143e3c8d8 /* 25 vars */) = 0
[pid 26590] arch_prctl(ARCH_SET_FS, 0x7f3b9aa14540) = 0
+ echo Hello
Hello
+ echo World
World
[pid 26590] exit_group(0)               = ?
[pid 26590] +++ exited with 0 +++
<... wait4 resumed> [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 26590
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=26590, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
Finished.
exit_group(0)                           = ?
+++ exited with 0 +++