Question: I would like to *grep* (or equivalent) what is written inside my terminal after a

```
$ gfortran -Wall file.f90

```

I thought a simple

```
$ gfortran -Wall file.f90 | grep something

```

would work, but it's not the case. It seems like `gfortran` is not actually writing in the standard output like `cat`.

Would you have an advice to "redirect" the messages of gcc or gfortran so that I can grep them?

Answer: 
Try:

```
gfortran -Wall file.f90 2>&1 | grep something

```

`grep` would operate on `STDOUT` so you need to redirect the `STDERR`.
Error messages are usually written to standard error (hence the name), not standard output. If you want to pipe it, you need to redirect it first:

```
gfortran -Wall file.f90 2>&1 | grep something

```