Posted By: Anonymous
I would like to have the
echo command executed when
cat /etc/passwd | grep "sysa" is not true.
What am I doing wrong?
if ! [ $(cat /etc/passwd | grep "sysa") ]; then echo "ERROR - The user sysa could not be looked up" exit 2 fi
if ! grep -q sysa /etc/passwd ; then
true if it finds the search target, and
false if it doesn’t.
if evaluation in shells are designed to be very flexible, and many times doesn’t require chains of commands (as you have written).
Also, looking at your code as is, your use of the
$( ... ) form of cmd-substitution is to be commended, but think about what is coming out of the process. Try
echo $(cat /etc/passwd | grep "sysa") to see what I mean. You can take that further by using the
-c (count) option to grep and then do
if ! [ $(grep -c "sysa" /etc/passwd) -eq 0 ] ; then which works but is rather old school.
BUT, you could use the newest shell features (arithmetic evaluation) like
if ! (( $(grep -c "sysa" /etc/passwd) == 0 )) ; then ...`
which also gives you the benefit of using the c-lang based comparison operators,
==,<,>,>=,<=,% and maybe a few others.
In this case, per a comment by Orwellophile, the arithmetic evaluation can be pared down even further, like
if ! (( $(grep -c "sysa" /etc/passwd) )) ; then ....
if (( ! $(grep -c "sysa" /etc/passwd) )) ; then ....
Finally, there is an award called the
Useless Use of Cat (UUOC). 🙂 Some people will jump up and down and cry gothca! I’ll just say that
grep can take a file name on its cmd-line, so why invoke extra processes and pipe constructions when you don’t have to? 😉
I hope this helps.