Run another command if there is any input on the standard input

Question

How can I run another command if there is any input on the standard input

commonly used in situations like this:

some command with no normal output | ifinput mail -s 'some output' me

Answer 1

The simplest solution would be using a file (if you dare!). The -s switch means "file is not zero size". The &> redirections includes errors as well (means that stdout and stderr will be sent).

OUTFILE=/tmp/command.out
some command &> $OUTFILE
[ -s "$OUTFILE" ] && mail -s "subject here" email@example.com < "$OUTFILE"

Edit: the comment below by glenn jackman contains even a better solution.

Answer 2

A little trickery with read:

$ echo 1 | { read -t 0 -N 0 && cat ; } 
1
$ { read -t 0 -N 0 && cat ; } 
$ 

Answer 3

You could also create a shell script. That script could first call some command that redirects the output to a file and then call the mail command from within the script to send it out which is essentially what SamKrieg is doing without the need to create a shell script. A reason for the script may depend on if you want to run this command as a cron job or somehting.