/dev/zero /dev/null > /dev/null 2>&1

 

实践：
1）
u@u5:~/tmp$ ll -as
total 16
4 drwxrwxr-x 2 u u 4096 12月 3 11:21 ./
4 drwxr-xr-x 27 u u 4096 12月 2 13:54 ../
4 -rw-rw-r-- 1 u u 4 12月 3 11:22 a
4 -rw-rw-r-- 1 u u 4 12月 3 11:22 b
u@u5:~/tmp$ cat a
aaa
u@u5:~/tmp$ cat b
bbb
u@u5:~/tmp$ cat a b
aaa
bbb
u@u5:~/tmp$ cat a b c
aaa
bbb
cat: c: No such file or directory
u@u5:~/tmp$ cat a b c > f
cat: c: No such file or directory
u@u5:~/tmp$ cat f
aaa
bbb
u@u5:~/tmp$ cat a b c > f1 2>&1
u@u5:~/tmp$ cat f1
aaa
bbb
cat: c: No such file or directory
u@u5:~/tmp$ cat a b c 1> f11 2>&1
u@u5:~/tmp$ cat f11
aaa
bbb
cat: c: No such file or directory
u@u5:~/tmp$

2）
u@u5:~/tmp$ cat a b c > g 2>g
u@u5:~/tmp$ cat g
cat: c: No such file or directory
u@u5:~/tmp$ cat a b c > g1 2>&1
u@u5:~/tmp$ cat g1
aaa
bbb
cat: c: No such file or directory
u@u5:~/tmp$

 

(0)
a.
类型 文件描述符 默认情况 对应文件句柄位置
标准输入（standard input） 0 从键盘获得输入 /proc/self/fd/0
标准输出（standard output） 1 输出到屏幕（即控制台） /proc/self/fd/1
错误输出（error output） 2 输出到屏幕（即控制台） /proc/self/fd/2
b.
dev/null看作"黑洞". 它非常等价于一个只写文件. 所有写入它的内容都会永远丢失. 而尝试从它那儿读取内容则什么也读不到。
c.
& 表示等同于的意思，2>&1，表示2的输出重定向等同于1

(1)
分解这个组合：“>/dev/null 2>&1” 为五部分。

a > 代表重定向到哪里，例如：echo "123" > /home/123.txt
b /dev/null 代表空设备文件
c 2> 表示stderr标准错误
d 表示等同于的意思，2>&1，表示2的输出重定向等同于1
e 1表示stdout标准输出，系统默认值是1，所以">/dev/null"等同于 "1>/dev/null",“>/dev/null 2>&1” 也可以写成“1> /dev/null 2> &1”

(2)
最常用的方式有：

command > file 2>file 与command > file 2>&1

它们有什么不同的地方吗？
首先command > file 2>file 的意思是将命令所产生的标准输出信息,和错误的输出信息送到file 中.command > file 2>file 这样的写法,stdout和stderr都直接送到file中, file会被打开两次,这样stdout和stderr会互相覆盖,这样写相当使用了FD1和FD2两个同时去抢占file 的管道。
而command >file 2>&1 这条命令就将stdout直接送向file, stderr 继承了FD1管道后,再被送往file,此时,file 只被打开了一次,也只使用了一个管道FD1,它包括了stdout和stderr的内容。
从IO效率上,前一条命令的效率要比后面一条的命令效率要低,所以在编写shell脚本的时候,较多的时候我们会command > file 2>&1 这样的写法。

 

Shell脚本———— /dev/null 2>&1详解 - Tinywan - 博客园 https://www.cnblogs.com/tinywan/p/6025468.html
linux下详解shell中>/dev/null 2>&1 - 哪是什么大佬 - 博客园 https://www.cnblogs.com/ultranms/p/9353157.html

 

/etc/init.d$ cat mysql
#!/bin/bash
#
### BEGIN INIT INFO
# Provides:          mysql
# Required-Start:    $remote_fs $syslog
# Required-Stop:     $remote_fs $syslog
# Should-Start:      $network $time
# Should-Stop:       $network $time
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start and stop the mysql database server daemon
# Description:       Controls the main MySQL database server daemon "mysqld"
#                    and its wrapper script "mysqld_safe".
### END INIT INFO
#
set -e
set -u
${DEBIAN_SCRIPT_DEBUG:+ set -v -x}

test -x /usr/bin/mysqld_safe || exit 0

. /lib/lsb/init-functions

SELF=$(cd $(dirname $0); pwd -P)/$(basename $0)
CONF=/etc/mysql/my.cnf
MYADMIN="/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf"

# priority can be overriden and "-s" adds output to stderr
ERR_LOGGER="logger -p daemon.err -t /etc/init.d/mysql -i"

# Safeguard (relative paths, core dumps..)
cd /
umask 077

# mysqladmin likes to read /root/.my.cnf. This is usually not what I want
# as many admins e.g. only store a password without a username there and
# so break my scripts.
export HOME=/etc/mysql/

## Fetch a particular option from mysql's invocation.
#
# Usage: void mysqld_get_param option
mysqld_get_param() {
        /usr/sbin/mysqld --print-defaults \
                | tr " " "\n" \
                | grep -- "--$1" \
                | tail -n 1 \
                | cut -d= -f2
}

## Do some sanity checks before even trying to start mysqld.
sanity_checks() {
  # check for config file
  if [ ! -r /etc/mysql/my.cnf ]; then
    log_warning_msg "$0: WARNING: /etc/mysql/my.cnf cannot be read. See README.Debian.gz"
    echo                "WARNING: /etc/mysql/my.cnf cannot be read. See README.Debian.gz" | $ERR_LOGGER
  fi

  # check for diskspace shortage
  datadir=`mysqld_get_param datadir`
  if LC_ALL=C BLOCKSIZE= df --portability $datadir/. | tail -n 1 | awk '{ exit ($4>4096) }'; then
    log_failure_msg "$0: ERROR: The partition with $datadir is too full!"
    echo                "ERROR: The partition with $datadir is too full!" | $ERR_LOGGER
    exit 1
  fi
}

## Checks if there is a server running and if so if it is accessible.
#
# check_alive insists on a pingable server
# check_dead also fails if there is a lost mysqld in the process list
#
# Usage: boolean mysqld_status [check_alive|check_dead] [warn|nowarn]
mysqld_status () {
    ping_output=`$MYADMIN ping 2>&1`; ping_alive=$(( ! $? ))

    ps_alive=0
    pidfile=`mysqld_get_param pid-file`
    if [ -f "$pidfile" ] && ps `cat $pidfile` >/dev/null 2>&1; then ps_alive=1; fi

    if [ "$1" = "check_alive"  -a  $ping_alive = 1 ] ||
       [ "$1" = "check_dead"   -a  $ping_alive = 0  -a  $ps_alive = 0 ]; then
        return 0 # EXIT_SUCCESS
    else
        if [ "$2" = "warn" ]; then
            echo -e "$ps_alive processes alive and '$MYADMIN ping' resulted in\n$ping_output\n" | $ERR_LOGGER -p daemon.debug
        fi
        return 1 # EXIT_FAILURE
    fi
}

#
# main()
#

case "${1:-''}" in
  'start')
        sanity_checks;
        # Start daemon
        log_daemon_msg "Starting MySQL database server" "mysqld"
        if mysqld_status check_alive nowarn; then
           log_progress_msg "already running"
           log_end_msg 0
        else
            # Could be removed during boot
            test -e /var/run/mysqld || install -m 755 -o mysql -g root -d /var/run/mysqld

            # Start MySQL!
            su - mysql -s /bin/sh -c "/usr/bin/mysqld_safe > /dev/null 2>&1 &"

            # 6s was reported in #352070 to be too few when using ndbcluster
            # 14s was reported in #736452 to be too few with large installs
            for i in $(seq 1 30); do
                sleep 1
                if mysqld_status check_alive nowarn ; then break; fi
                log_progress_msg "."
            done
            if mysqld_status check_alive warn; then
                log_end_msg 0
                # Now start mysqlcheck or whatever the admin wants.
                output=$(/etc/mysql/debian-start)
                [ -n "$output" ] && log_action_msg "$output"
            else
                log_end_msg 1
                log_failure_msg "Please take a look at the syslog"
            fi
        fi
        ;;

  'stop')
        # * As a passwordless mysqladmin (e.g. via ~/.my.cnf) must be possible
        # at least for cron, we can rely on it here, too. (although we have
        # to specify it explicit as e.g. sudo environments points to the normal
        # users home and not /root)
        log_daemon_msg "Stopping MySQL database server" "mysqld"
        if ! mysqld_status check_dead nowarn; then
          set +e
          shutdown_out=`$MYADMIN shutdown 2>&1`; r=$?
          set -e
          if [ "$r" -ne 0 ]; then
            log_end_msg 1
            [ "$VERBOSE" != "no" ] && log_failure_msg "Error: $shutdown_out"
            log_daemon_msg "Killing MySQL database server by signal" "mysqld"
            killall -15 mysqld
            server_down=
            for i in 1 2 3 4 5 6 7 8 9 10; do
              sleep 1
              if mysqld_status check_dead nowarn; then server_down=1; break; fi
            done
          if test -z "$server_down"; then killall -9 mysqld; fi
          fi
        fi

        if ! mysqld_status check_dead warn; then
          log_end_msg 1
          log_failure_msg "Please stop MySQL manually and read /usr/share/doc/mysql-server-5.7/README.Debian.gz!"
          exit -1
        else
          log_end_msg 0
        fi
        ;;

  'restart')
        set +e; $SELF stop; set -e
        $SELF start
        ;;

  'reload'|'force-reload')
        log_daemon_msg "Reloading MySQL database server" "mysqld"
        $MYADMIN reload
        log_end_msg 0
        ;;

  'status')
        if mysqld_status check_alive nowarn; then
          log_action_msg "$($MYADMIN version)"
        else
          log_action_msg "MySQL is stopped."
          exit 3
        fi
        ;;

  *)
        echo "Usage: $SELF start|stop|restart|reload|force-reload|status"
        exit 1
        ;;
esac

# Some success paths end up returning non-zero so exit 0 explicitly. See
# bug #739846.
exit 0

　　

# Start MySQL!
su - mysql -s /bin/sh -c "/usr/bin/mysqld_safe > /dev/null 2>&1 &"