bash脚本中的exec之后没有代码将运行

Question

我在命令名称中使用变量扩展进行测试的示例bash脚本：

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：

1. 现在，如果我取消注释此行：# 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，而是得到此输出！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
   

2. 另外，它下面的eval命令也不起作用。如果我取消注释该行，则会得到与上面刚刚发布的错误输出完全相同的错误输出，该错误输出仍不会像应有的那样执行我对gs_test_script的调用。为什么？

Answer 1

因为exec命令将用要执行的新命令替换当前的bash进程。它不会返回到调用过程。因此，您不应在脚本中使用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.