4. ldd - print shared library dependencies
$ ldd /bin/ls
linux-gate.so.1 => (0xffffe000)
librt.so.1 => /lib/tls/i686/cmov/librt.so.1 (0xb7f13000)
libacl.so.1 => /lib/libacl.so.1 (0xb7f0d000)
libselinux.so.1 => /lib/libselinux.so.1 (0xb7ef9000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0xb7dc4000)
libpthread.so.0 => /lib/tls/i686/cmov/libpthread.so.0 (0xb7db1000)
/lib/ld-linux.so.2 (0xb7f22000)
libattr.so.1 => /lib/libattr.so.1 (0xb7dad000)
libdl.so.2 => /lib/tls/i686/cmov/libdl.so.2 (0xb7da9000)
libsepol.so.1 => /lib/libsepol.so.1 (0xb7d6c000)
$