> Hi,
>
> I keep seeing a reference to what looks like two pre-populated variables
> in shell scripts. Actually this one is in a crontab.
>
> Something like this : /home/user-name/directory-name/scriptname.php
> >/dev/null 2>&1
>
> What is the number 1 and 2? Is there others?
Unix and copycats, like Linux use small non-negative integers to identify
files that may be read from or written to. They are known as file
descriptors. By convention, the first 3 file descriptors have a special
meaning. 0 is standard input, 1 is standard output, and 2 is standard
error.
Normally, each process inherits these 3 file descriptors from its
parent process but the shell allows you to "redirect" them to
other files.
Examples:
echo hello >newfile # Creates the file "newfile" and writes
# "hello\n" to it.
echo hello >/dev/null # Writes "hello\n" to /dev/null, where it
# disappears
echo hello 2>newfile2 # Creates the file "newfile2" and puts
# error messages from "echo hello" into it.
# Since "echo hello" won't output any error
# messages, "newfile2" will be an empty file.
echo hello 2>&1 # Redirects the error output of the command
# into the same file where the standard
# output goes.
So, "scriptname.php >/dev/null 2>&1" says to redirect the standard output
of scriptname.php into the bit bucket and then to redirect the standard
error output to the same place.
-Dale
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