No code will run after `exec` in bash script [duplicate]

问题

Sample bash script where I'm testing using variable expansion in command names:

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!

Output:

$ ./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.

Questions:

Now, if I uncomment this line: # exec "${CMD_PREFIX}ho" "hey" # works, but then prevents the code below from running!, the code below it no longer runs, and I get this output instead! Notice I no longer get the 3 printouts of This is a test script found in ~/bin. Why?
```
$ ./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
```
Also, the eval command just below it doesn't work. If I uncomment out that line I get the exact same erroneous output as posted just above, which still doesn't execute my call to gs_test_script three times like it should. Why?

回答1:

Because the exec command will replace the current bash process with the new command to be executed. It does not return to the calling process. So you should not use exec in your script.

  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.

来源：https://stackoverflow.com/questions/60795129/no-code-will-run-after-exec-in-bash-script

标签

bash

variables

exec

eval