Hello everyone,

I'm having trouble understanding how the following command works:

'wd' is a company proprietary software piece written in C. It's a process that monitors other proprietary programs written in C that reside in a Linux environment.

The command above restarts wd after an upgrade has been implemented.

I'm trying to understand how it works. One thing that baffles me is the use of minus signs /dev/null instead of greater than signs. Can anyone explain to me why they are used this way, or perhaps point me in the direction of a tutorial that explains it? Not having much luck searching on google.

Thanks.

'-s' command line switch, most progs understand -h,--help or -? to give a summary of options that can be passed to it.

'<','>' io redirection '<' get input from ( in your example /dev/null ) '>' send output to ( again /dev/null ), /dev/null can be thought of as a black hole nothing gets out once its gone in.

2>/dev/null - slightly more complex , send stream 2 to /dev/null, all programs start with 3 basic streams open for them 0,1 and 2 which are stdin stdout and stderr. so this command sends error messages ( from stream 2 ) to /dev/null

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/usr1/vision/bin/wd </dev/null 2</dev/null >/dev/null 2>/dev/null
/usr1/vision/bin/wd is the program you executed
</dev/null the standard input is redirected, the data is coming from /dev/null (means no data at all).
2</dev/null I have no idea, but I think it is completely meaningless
>/dev/null the standard output is redirected to /dev/null that means all the messages will be dropped
2>/dev/null the standard error is redirected to /dev/null, all the error messages are dropped

& means the program will be executed in the background (shell will not wait for it)

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most important here is the semicolon ";". This tells the interpeter (shell) That the commands end here and is to be executed. So you can read the command as this:
So start wd with the option s
after its done
start wd reading from stdin and stderr both inputs coming from /dev/null and we also redirect the output of wd into /dev/null on stdout and stderr whith everything put into background through the ampersand "&".

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Thanks everyone for your help in my understanding of this command.

A wild guess is that the "-s" switch is telling the process to shut down gracefully much like passing "stop" to scripts in init.d.

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I agree: "command above restarts wd"

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