Linux script start,stop,restart [closed]

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Closed 4 years ago.

Can anyone please tell me what exactly this script does?

#! /bin/sh
test –f /usr/bin/sshd || exit 0
case “$1” in
    start)
            echo –n “Starting sshd: sshd”
            /usr/sbin/sshd
            echo “.”
            ;;
   stop) 
          echo –n “Stopping sshd: sshd”
          kill `cat /var/run/sshd.pid`
          echo “.”
           ;;
   restart)
          echo –n “Stopping sshd: sshd”
          kill `cat /var/run/sshd.pid`
          echo “.”
          echo –n “Starting sshd: sshd”
            /usr/sbin/sshd
            echo “.”
            ;;
        *)
          echo “Usage: /etc/init.d/sshd start|stop|restart”
          exit 1
          ;;
    esac

I want to know what exactly this part:

#! /bin/sh
test –f /usr/bin/sshd || exit 0
case “$1” in
    start)
            echo –n “Starting sshd: sshd”
            /usr/sbin/sshd
            echo “.”
            ;;

does because the other part is the same! Please ;)

回答1:

Which other part is the same? The way that script works is it checks the value of $1, which is the first parameter to the script supplied on the command-line. If it's 'start', then the part after start) is executed. If it's 'stop', then the part after stop) is executed. If it's 'restart', then the part after restart) is executed.

Line by line for that first part:

#! /bin/sh

Hey, it's a shell script! Specifically, execute this script using the sh shell.

test –f /usr/bin/sshd || exit 0

Is there a file called /usr/bin/sshd? If not, exit with a 0 return status.

case “$1” in

Check the value of $1, the first command-line option.

    start)

If $1 is 'start'...

            echo –n “Starting sshd: sshd”

Print "Starting sshd: sshd".

            /usr/sbin/sshd

Execute /usr/sbin/sshd.

            echo “.”

Print ".".

            ;;

Exit the case statement.

回答2:

sshd writes its process ID to the file in /var/run. The backticks cause the script inside them to be executed by the shell, and the result is used in its place.

The net result is kill [sshd pid]

回答3:

The part you mention starts the "sshd" program. It's the Secure Shell (SSH) daemon (server process).

Depending on the command-line argument, your script starts, stops or restarts the SSH server. $1 represents the command-line argument, and this value gets compared to the different possible values between case and esac.

回答4:

The test –f /usr/bin/sshd part returns true if the file exists. The || is an or, so the || exit 0 gets executed (ending the script) only if the first part returned false.

In the case "$1" part, the $1 is the first argument passed to the script.

回答5:

"I want to know what exactly this part... does because the other part is the same!"

start) assumes that sshd is not already started, and starts it.

This is different from restart), which first stops the sshd process (as Joe describes), and then starts it again.