Cfrgtkky

I don't understand the bash command exec. I have seen it used inside scripts to redirect all output to a file (as seen in this). But I don't understand how it works or what it does in general. I have read the man pages but I don't understand them.

man bash says:

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.

The last two lines are what is important: If you run exec by itself, without a command, it will simply make the redirections apply to the current shell. You probably know that when you run command > file, the output of command is written to file instead of to your terminal (this is called a redirection). If you run exec > file instead, then the redirection applies to the entire shell: Any output produced by the shell is written to file instead of to your terminal. For example here

bash-3.2$ bash

bash-3.2$ exec > file

bash-3.2$ date

bash-3.2$ exit

bash-3.2$ cat file

Thu 18 Sep 2014 23:56:25 CEST

I first start a new bash shell. Then, in this new shell I run exec > file, so that all output is redirected to file. Indeed, after that I run date but I get no output, because the output is redirected to file. Then I exit my shell (so that the redirection no longer applies) and I see that file indeed contains the output of the date command I ran earlier.

exec is a command with two very distinct behaviors, depending on whether at least one argument is used with it, or no argument is used at all.

• If at least one argument is passed, the first one is taken as a command name and exec try to execute it as a command passing the remaining arguments, if any, to that command and managing the redirections, if any.




• If the command passed as first argument doesn't exist, the current shell, not only the exec command, exits in error.




• If the command exists and is executable, it replaces the current shell. That means that if exec appears in a script, the instructions following the exec call will never be executed (unless exec is itself in a subshell). exec never returns. Shell traps like "EXIT" won't get triggered either.




• If no argument is passed, exec is only used to redefine the current shell file descriptors. The shell continue after the exec, unlike with the previous case, but the standard input, output, error or whatever file descriptor has been redirected take effect.




• If some of the redirections uses /dev/null, any input from it will return EOF and any output to it will be discarded.




• You can close file descriptors by using - as source or destination, e.g. exec <&-. Subsequent read or writes will then fail.

Here are two examples:

echo foo > /tmp/bar

exec < /tmp/bar # exec has no arguments, will only affect current shell descriptors, here stdin

cat # simple command that read stdin and write it to stdout

This script will output "foo" as the cat command, instead of waiting for user input as it would have done in the usual case will take its input from the /tmp/bar file which contains foo.

echo foo > /tmp/bar

exec wc -c < /tmp/bar # exec has two arguments, the control flow will switch to the wc command

cat

This script will display 4 (the number of bytes in /tmp/bar) and immediately ends. The cat command won't be executed.

To understand exec you need to first understand fork. I am trying to keep it short.

• When you come to a fork in the road you generally have two options. Linux programs reach this fork in the road when they hit a
  fork() system call.




• Normal programs are system commands that exist in a compiled form on
  your system. When such a program is executed, a new process is
  created. This child process has the same environment as its parent,
  only the process ID number is different. This procedure is called
  forking.



• Forking provides a way for an existing process to start a new one. However,
  there may be situations where a child process is not the part of the same
  program as parent process. In this case exec is used. exec
  will replace the contents of the currently running process with the
  information from a program binary.


• After the forking process, the address space of the child process is
  overwritten with the new process data. This is done through an exec
  call to the system.