# nohup

```bash
nohup is a POSIX command to ignore the HUP (hangup) signal. 
& would run the process in background using a subshell.
```

```bash
$ nohup --help
Usage: nohup COMMAND [ARG]...
  or:  nohup OPTION
Run COMMAND, ignoring hangup signals.

      --help     display this help and exit
      --version  output version information and exit

If standard input is a terminal, redirect it from an unreadable file.
If standard output is a terminal, append output to 'nohup.out' if possible,
'$HOME/nohup.out' otherwise.
If standard error is a terminal, redirect it to standard output.
To save output to FILE, use 'nohup COMMAND > FILE'.

NOTE: your shell may have its own version of nohup, which usually supersedes
the version described here.  Please refer to your shell's documentation
for details about the options it supports.

GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Full documentation at: <https://www.gnu.org/software/coreutils/nohup>
or available locally via: info '(coreutils) nohup invocation'
```

```bash
sh myprocess.sh

sh myprocess.sh &
# kill -HUP 可以杀死

nohup sh myprocess.sh
# kill -HUP 杀不死，kill -TERM可以杀死，kill -KILL可以杀死

nohup sh myprocess.sh &
# kill -HUP 杀不死，kill -TERM可以杀死，kill -KILL可以杀死

```


```bash
       Name     Num   Action    Description
       0          0   n/a       exit code indicates if a signal may be sent
       ALRM      14   exit
       HUP        1   exit
       INT        2   exit
       KILL       9   exit      cannot be blocked
       PIPE      13   exit
       POLL           exit
       PROF           exit
       TERM      15   exit
       USR1           exit
       USR2           exit
       VTALRM         exit
       STKFLT         exit      might not be implemented
       PWR            ignore    might exit on some systems
       WINCH          ignore
       CHLD           ignore
       URG            ignore
       TSTP           stop      might interact with the shell
       TTIN           stop      might interact with the shell
       TTOU           stop      might interact with the shell
       STOP           stop      cannot be blocked
       CONT           restart   continue if stopped, otherwise ignore
       ABRT       6   core
       FPE        8   core
       ILL        4   core
       QUIT       3   core
       SEGV      11   core
       TRAP       5   core
       SYS            core      might not be implemented
       EMT            core      might not be implemented
       BUS            core      core dump might fail
       XCPU           core      core dump might fail
       XFSZ           core      core dump might fail
```       



# io-redirection
```bash
0: 标准输入(stdin)
1: 标准输出(stdout)
2: 标准错误(stderr)
```

```bash
ls a b 1>f.out 2>f.err
# 标准输出到f.out, 标准错误到 f.err
通常`1>`省略成`>`
```

```bash
ls a b 1>f.out 2>&1
# stdout、stderr到f.out
ls a b 2>f.out 1>&2
# stdout、stderr到f.out
```
```bash
&是一个描述符，如果1或2前不加&，会被当成一个普通文件

>	        1> 的简写
1>&2	    标准输出重定向到标准错误
2>&1	    标准错误重定向到标准输出
&> file	    把标准输出和标准错误输出都重定向到文件file中
>& file	    把标准输出和标准错误输出都重定向到文件file中


# # stdout、stderr到/dev/null，/dev/null代表黑洞

>& /dev/null
&> /dev/null
2> /dev/null 1>&2
1> /dev/null 2>&1
> /dev/null 2>&1
```

# ref

* [linux - What&#39;s the difference between nohup and ampersand - Stack Overflow](https://stackoverflow.com/questions/15595374/whats-the-difference-between-nohup-and-ampersand)
* [nohup - Wikipedia](https://en.wikipedia.org/wiki/Nohup)
* [Ampersand - Wikipedia](https://en.wikipedia.org/wiki/Ampersand#Unix_shells)
* [SIGHUP - Wikipedia](https://en.wikipedia.org/wiki/SIGHUP)
* [Termination Signals (The GNU C Library)](https://www.gnu.org/software/libc/manual/html_node/Termination-Signals.html)
* [kill man page](http://linuxcommand.sourceforge.net/lc3_man_pages/kill1.html)
* [bash - What&#39;s the difference between eval and exec? - Unix &amp; Linux Stack Exchange](https://unix.stackexchange.com/questions/296838/whats-the-difference-between-eval-and-exec)
* [输入输出重定向 | aimuke](https://aimuke.github.io/linux/2019/05/29/redirect/)