Possible to pipe into an if-statement?

Question

I have a script that outputs about 10 lines every time if it run. The content of these lines varies.

I would really like to be able to grep in the output and do different things depending on the output.

In pseudo this is what I would like to do

cat /etc/password | \\
if [ grep "root" $STDOUT ]; then
   echo "root is found"

elif [ grep "nobody" $STDOUT ]; then
   echo "nobody is found"

fi

Here have I used cat /etc/password as an example, but it should be replaced with my scripts mentioned above.

The problem is, how do I get hold of the output from cat /etc/password in the if/elif conditions?

Answer 1

You just do :

if grep -q "root" /etc/passwd ; then
   ...
fi

which will play the ... commands if grep exit code is 0.

remember that \[ is a external command, probably located in /usr/bin/[ (normally it's a hard link to test and when invoked as [ it requires a matching ] argument). Also see the pitfalls page here, many of them deal are related to that command.

Answer 2

I'd suggest using awk:

cat /etc/passwd | awk '/root/{ do something }/nobody/{ do something else }'

You can achieve the same in bash using an expression like:

cat /etc/passwd |
while read; do
  if echo "$REPLY" | fgrep root; then
    something
  fi
  if echo "$REPLY" | fgrep nobody; then
    something_else
  fi
done

However the pure bash solution is less efficient for large inputs because it runs separate instances of grep for every line.

Answer 3

As @Benoit recommends, just use grep directly.

As @larsmans notes, you can avoid a double-read of the file by reading it into a variable once.

Given the availability of bash I'd do it like this:

password=$(< /etc/passwd)

if grep -q root <<< "$password" ; then
    echo root found
elif grep -q nobody <<< "$password" ; then
    echo nobody found
fi

One read of the file, one or two invocations of grep, no other processes or subshells launched.