Question about Bash script for Linux
 

Question about Bash script for Linux

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1) When running a script and it is piped to /dev/null &, what is it mean ?
/usr/local/sbin/quotacheck1 > /dev/null &

2) When running a script and it is piped to /dev/null, what is it mean ?
killall checkin2.sh > /dev/null

3) When running a script and it is piped to /dev/null 2>&1 &, what is it mean ?
What is a 2 for ?
What is a &1 for ?
What is a & that goes after a 1 means ?
/etc/checkin2.sh > /dev/null 2>&1 &

Thank you for your help.

/dev/null is like a black hole - everything you send there disappears.
>/dev/null means to redirect the output to go to /dev/null -- in other words not have coammnd out appear on the screen

The ampersand at the end of a line means to execute the line in a background process.

> /dev/null 2>&1 &

means: send standard output to the bit bucket (/dev/null), plus whatever would come out on stderr (2 - standard error output) make that go wherever stdout ( 1 - standard output ) is going. & at the end of the line = run in background. the &> is called a dup (for duplicate): creating a duplicate file descriptor, in this case, whatever goes to 2 ends up in 1

0 - stdin
1 - stdout
2 - stderr

1) The output just gets sent into oblivion. The ampersand sends the command into the background.
2) See above minus the backgrounding
3) The last ampersand backgrounds the task. 1=stdout (standard output stream) and 2=stderr (standard error stream). "2>&1" redirects messages from stderr to stdout.

Redirecting (the > symbol) makes all standard output from the program go to the specified file. Redirecting to /dev/null means to send the output to oblivion. So basically the script doesn't want any of the output from quotacheck1, etc. to be shown.

The & at the end of a line tells the shell to run the program in the background. Otherwise you would have to wait for the program to finish before you got a prompt back again.

The 2>&1 redirection means to send standard output and error output messages to the file (/dev/null = oblivion).

Reply
 

Thank you jim mcnamara and david_ross for your help.

"2>&1" redirects messages from stderr to stdout.

Is it true that the standard in is by default the screen which is the terminal where the user type it in on a keyboard ?
Is it true that the standard out is by default the screen when 2>&1 is used?
Is it true that the standard error is by default the screen when 2>&1 is used?

Yes to all 3

unix was written in C.

By default C opens three file descriptors - 0, 1, 2
and they are called stdin, stdout, stderr.

The same concept carries over to the command shell.

A lot of C langauge conventions permeate unix.

Thank you david_ross and jim mcnamara for your help.

Re: Reply
 

Yes, but both are also true when 2>&1 is not used.

reply
 

Thank you HKo for the additional answer.