[SOLVED] Bash conditional

the_gripmaster · 03-14-2013, 12:12 AM

I have a file

Code:

cat <<EOF > /tmp/test
Line one
Line two 
Line three
EOF

Now I am trying to get the exit stat ($?) of 1 if any text is found and 0 if any text is not found using grep, i.e. just reversing the exit status of grep

Code:

# (snippet 1) this one is not working!!! retval $? should be 1
grep -q 'one' /tmp/test && false || true
echo $?


# (snippet 2) but this one is working - retval is 1
grep -q 'one' /tmp/test
if [ $? -eq 0 ]; then 
    false
else 
    true
fi
echo $?

What am I missing in snippet 1?

pan64 · 03-14-2013, 01:42 AM

yes, grep sets return value itself, you do not need to check and set it.
grep <something> && <command> means command will be executed if grep was successful, so the return code was 0.
false simple returns 1.
grep <something> && false || <command> means command will be executed if false executed (and failed, but it cannot do anything else).
So grep 'one' will return 0, false and true will not be executed.
grep 'four' will return 1, therefore false will be executed and furthermore true will be executed also. True will return 0.

ntubski · 03-14-2013, 09:34 AM

Quote:

Originally Posted by the_gripmaster

just reversing the exit status

Consider using !:

Code:

$ ! true ; echo $?
1
$ ! false ; echo $?
0

Quote:

3.2.2 Pipelines

A pipeline is a sequence of simple commands separated by one of the control operators ‘|’ or ‘|&’.

The format for a pipeline is

Code:

[time [-p]] [!] command1 [ [| or |&] command2 …]

...

If the reserved word ‘!’ precedes the pipeline, the exit status is the logical negation of the exit status as described above.

mina86 · 03-14-2013, 09:49 AM

Quote:

Originally Posted by the_gripmaster

Code:

grep -q 'one' /tmp/test && false || true

Let's think about this code for a little. First of all, we have:

Code:

grep … && false

I wonder what that would evaluate to… What could it be… Could it be that “whatever and false” always yields “false”?

And later, after the above evaluates to false (ie. non-zero exit code), you end up with an equivalent of:

Code:

false || true

Again, what could this result in? Could it be that “whatever or true” always yields “true”?

David the H. · 03-15-2013, 09:02 AM

".. && .. || .." is not exactly equivalent to "if..then..else".

See Bash Pitfall #22

In short, while it's ok to use most of the time, if the second command can ever return false then it's probably not going to do what you want it to.

the_gripmaster · 03-16-2013, 03:16 AM

Quote:

Originally Posted by David the H.

".. && .. || .." is not exactly equivalent to "if..then..else".

Right. What I was looking for is

Code:

! grep -q 'one' /tmp/test