Filtering content with bash instead of grep

I get it, grep is an advanced content filtering tool. I’m not bashing its use. I do it on a daily basis, too. However, most of grep calls I’ve seen are for simple words or sometimes simple patterns. And that might come easier while you’re typing at the bash prompt, but might be too expensive to do it inside a script.

The problem with using grep in these simple situations is that you’re dragging a whole lot of executable code. This code is most of the times executed in a subshell. That’s because you’re either using a pipe or using a command substitution, each of these spawning a shell of their own. There is the case when you’re using grep as a conditional - if grep pattern; then, but this is less common. The point is that this might be expensive for a very simple piece of code which could be written in pure bash.

The solution to content filtering in bash is using the [[ ... ]] compound command in a while loop. Here’s an example.

Say you want to filter the word GNU from the /usr/share/licenses/GPL-2 file. That translates to:

while read LINE; do
  [[ $LINE == *GNU* ]] && echo $LINE
done </usr/share/licenses/GPL-2

If you want two or more words, such as both GNU and License, you can use the more advanced pattern matching, enabled by default via the extglob shell option:

while read LINE; do
  [[ $LINE == *@(GNU|License)* ]] && echo $LINE
done </usr/share/licenses/GPL-2

You can, of course, use the more classic and less compact version of the above:

while read LINE; do
  [[ $LINE == *GNU* || $LINE == *License* ]] && echo $LINE
done </usr/share/licenses/GPL-2

Or, you can stop after the first match:

while read LINE; do
  [[ $LINE == *GNU* || $LINE == *License* ]] && echo $LINE && break
done </usr/share/licenses/GPL-2

Or after 4 matches:

while read LINE && (( INDEX++ < 4 )); do
  [[ $LINE == *GNU* || $LINE == *License* ]] && echo $LINE
done </usr/share/licenses/GPL-2

The =~ operator enables the use of regular expressions. Back to our first example, the regular expression version would read:

while read LINE; do
  [[ $LINE =~ .*GNU.* ]] && echo $LINE
done </usr/share/licenses/GPL-2

So, you see, you don’t have to bother grep for simple stuff. Next time you want to filter content, do it with a simple bash loop.