Grep with Line Numbers – Output Neighboring Lines

You can write the output to a file (with strace -o asd.out) and then grep it:

From strace manual:

-o filename Write  the  trace  output to the file filename rather than 
to stderr.  Use filename.pid if -ff is used. If the argument begins with
`|' or with `!' then the rest of the argument is treated as a command
and all output is piped to it. This is convenient for piping the
debugging output to a program without affecting the redirections of 
executed programs.

Using awk rather than grep:

awk '/FOO/ { if (matching) printf("\n"); matching = 1 }
     /^$/  { if (matching) printf("\n"); matching = 0 }
     matching' file

A version that enumerates the matches:

awk 'function flush_print_maybe() {
         if (matching) printf("Match %d\n%s\n\n", ++n, buf)
         buf = ""
     }
     /FOO/ { flush_print_maybe(); matching = 1 }
     /^$/  { flush_print_maybe(); matching = 0 }
     matching { buf = (buf == "" ? $0 : buf ORS $0) }
     END   { flush_print_maybe() }' file

Both awk programs uses a very simple "state machine" to determine if it's currently matching or not matching. A match of the pattern FOO will cause it to enter the matching state, and a match of the pattern ^$ (an empty line) will cause it to enter the non-matching state.

Output of empty lines between matching sets of data happens at state transitions from matching (either into matching or into non-matching).

The first program prints any line when in the matching state.

The second program collects lines in a buf variable when in a matching state. It flushes (empties) this after possibly printing it (depending on the state), together with a Match N label at state transitions (when the first program would output an empty line).

Output of this last program on the sample data:

Match 1
this line contains FOO
this line is not blank

Match 2
This line also contains FOO

Match 3
This line contains FOO too
Not blank
Also not blank

Match 4
FOO!
Yet more random text

Match 5
FOO!