在 bash 脚本中的 `exec` 之后没有代码会运行

Question

示例 bash 脚本，我在其中使用命令名称中的变量扩展进行测试：

test_command_w_variable_expansion_in_name.sh:

#!/bin/bash

# Gabriel Staples
# 21 Mar. 2020

echo "PATH = \"$PATH\""
# PATH="$HOME/bin:$PATH"
# echo "PATH = \"$PATH\""

# 1st, create a command in ~/bin to test here
mkdir -p ~/bin
echo -e "#!/bin/bash\necho \"This is a test script found in ~/bin.\"" > ~/bin/gs_test_script
chmod +x ~/bin/gs_test_script

# 1)
# command: `echo`
CMD_PREFIX="ec"

${CMD_PREFIX}ho "hey" # works
# exec "${CMD_PREFIX}ho" "hey" # works, but then prevents the code below from running!
# eval "${CMD_PREFIX}ho" "hey" # does NOT work, but also throws no error

# 2)
# command: `gs_test_script` from ~/bin
CMD_PREFIX="gs_test"

~/bin/gs_test_script # works!
gs_test_script # works!
${CMD_PREFIX}_script # works!

输出：

$ ./test_command_w_variable_expansion_in_name.sh
PATH = "/home/gabriel/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin"
hey
This is a test script found in ~/bin.
This is a test script found in ~/bin.
This is a test script found in ~/bin.

问题：

现在，如果我取消注释这一行：# exec "${CMD_PREFIX}ho" "hey" # works, but then prevents the code below from running!，它下面的代码将不再运行，而是我得到这个输出！请注意，我不再获得 This is a test script found in ~/bin. Why?
的 3 个打印输出
```
$ ./test_command_w_variable_expansion_in_name.sh
PATH = "/home/gabriel/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin"
hey
hey
```
此外，它下面的eval命令不起作用。如果我取消注释该行，我会得到与上面发布的完全相同的错误输出，它仍然没有像应该的那样执行我对 gs_test_script 的调用三次。为什么？

Answer 1

因为exec命令会将当前的bash进程替换为要执行的新命令。它不会 return 调用进程。所以你不应该在你的脚本中使用 exec。

  exec [-cl] [-a name] [command [arguments]]

  If command is specified, it replaces the shell.  No new  process
  is  created.  The arguments become the arguments to command.  If
  the -l option is supplied,  the  shell  places  a  dash  at  the
  beginning  of  the  zeroth  argument passed to command.  This is
  what login(1) does.  The -c option causes command to be executed
  with  an empty environment.  If -a is supplied, the shell passes
  name as the zeroth argument to the executed command.  If command
  cannot  be  executed  for  some  reason, a non-interactive shell
  exits, unless the execfail shell option  is  enabled.   In  that
  case,  it returns failure.  An interactive shell returns failure
  if the file cannot be executed.  If command  is  not  specified,
  any  redirections  take  effect  in  the  current shell, and the
  return status is 0.  If there is a redirection error, the return
  status is 1.

在 bash 脚本中的 `exec` 之后没有代码会运行

No code will run after `exec` in bash script

variables

bash

eval

exec

在 bash 脚本中的 `exec` 之后没有代码会 运行

No code will run after `exec` in bash script

variables

bash

eval

exec

在 bash 脚本中的 `exec` 之后没有代码会运行