Shell Programming and Scripting

What the folowing statements means and whats the use of it ?

expr $db_name + 2 >/dev/null 2>&1

Thanks.

This is my interpretation and very much open to correction,

this is finding the sum of a variable db_name and 2. And after this redirecting the output to dev/null.
2>&1 means if some error occurs then redirect the output to the same location as to which the normal output is redirected.

Gaurav

to /dev/null means where it redirects the output ...
whether its stored in the file or where ?

Error u mean to say in
expr command ...isnt it ?

hey Gaurav,
pipinng output /dev/null is like wrting to a file but the contents is not stored in the file.I think its character device file.
cat /dev/null will display nothing inidicating nothing is stored in it.

/dev/null is the bit bucket - a black hole for data. Everything goes in but never comes back out. Think of it as the perfect garbage bin for data.

Hi,
this may help,

a unix program opens three files stdin, stdout and stderr when it strarts executing.
stdin has file descriptor as 0
stdout has 1, and
stderr has 2

unix commands read from stdin
the output goes to stdout
stderr is where unix commands write commands if they encounter errors

by default stdout and stderr goes to terminal.

so the command:
command > opfile 2>&1
means that messages for file descriptor 2 (stderr) goes to the same place as stdout (&1), i.e. is opfile in this case.

And in the command given by OP the stderr is redirected to /dev/null

Gaurav

Thanks Gaurav .......Thanks a lot