(keanu@acissej:~)$ cat blabla 
bridge name     bridge id               STP enabled     interfaces
br549           8000.00004c9f0bd2       no              eth0
                                                        eth1
br555           8000.00004c9f0bd2       no              eth2
                                                        eth3
(keanu@acissej:~)$ IF=eth0; cat blabla | sed -e "1 { d }; /^[a-z]/ { N }; /.*${IF}.*/ { s/^\([a-z0-9]\+\).*/\1/p }; d"
br549
(keanu@acissej:~)$ IF=eth1; cat blabla | sed -e "1 { d }; /^[a-z]/ { N }; /.*${IF}.*/ { s/^\([a-z0-9]\+\).*/\1/p }; d"
br549
(keanu@acissej:~)$ IF=eth2; cat blabla | sed -e "1 { d }; /^[a-z]/ { N }; /.*${IF}.*/ { s/^\([a-z0-9]\+\).*/\1/p }; d"
br555
(keanu@acissej:~)$ IF=eth3; cat blabla | sed -e "1 { d }; /^[a-z]/ { N }; /.*${IF}.*/ { s/^\([a-z0-9]\+\).*/\1/p }; d"
br555