Assuming a shapefile called myshapefile.shp, a table mytable in schema xyz, in a PostGIS enabled database called mydb on localhost. The table is owned by user dbuser who has password "secret".
shp2pgsql myshapefile -I xyz.mytable > statements.sql psql -d mydb -h localhost -U dbuser -f statements.sql
This tip and many more can be read in Making Maps Fast.
This is even easier with ogr2ogr:
ogr2ogr -f "PostgreSQL" PG:"host=localhost user=dbuser dbname=mydb password=secret" -lco SCHEMA=xyz myshapefile.shp