The steps you need to take to complete this recipe are as follows:

1. First, you will create a view to clip the river geometries for each country using the ST_Intersection and ST_Intersects functions. Name the view rivers_clipped_by_country:

       postgis_cookbook=> CREATE VIEW chp03.rivers_clipped_by_country AS 
         SELECT r.name, c.iso2, ST_Intersection(r.the_geom,
           c.the_geom)::geometry(Geometry,4326) AS the_geom
         FROM chp03.countries AS c 
         JOIN chp03.rivers AS r 
         ON ST_Intersects(r.the_geom, c.the_geom); 

2. Create a directory named rivers as follows:

      mkdir working/chp03/rivers

3. Create the following scripts to export a rivers shapefile for each country.

The following is the Linux version (name it export_rivers.sh):

        #!/bin/bash 
        for f in `ogrinfo PG:"dbname='postgis_cookbook' user='me' 
        password='mypassword'" -sql "SELECT DISTINCT(iso2) 
        FROM chp03.countries ORDER BY iso2" | grep iso2 | awk '{print $4}'` 
        do 
          echo "Exporting river shapefile for $f country..." 
          ogr2ogr rivers/rivers_$f.shp PG:"dbname='postgis_cookbook' 
          user='me' password='mypassword'" 
          -sql "SELECT * FROM chp03.rivers_clipped_by_country 
          WHERE iso2 = '$f'" 
        done 

The following is the Windows version (name it export_rivers.bat):

        FOR /F "tokens=*" %%f IN ('ogrinfo 
        PG:"dbname=postgis_cookbook user=me password=password" 
        -sql "SELECT DISTINCT(iso2) FROM chp03.countries 
         ORDER BY iso2" ^| grep iso2 ^| gawk "{print $4}"') DO ( 
           echo "Exporting river shapefile for %%f country..." 
           ogr2ogr rivers/rivers_%%f.shp PG:"dbname='postgis_cookbook' 
           user='me' password='password'" 

           -sql "SELECT * FROM chp03.rivers_clipped_by_country 
           WHERE iso2 = '%%f'" 
        )

4. In Windows, run the batch file:

      C:\export_rivers.bat

5. Now, run the following script (in Linux or macOS, you need to assign execute permissions to the script before running the shell file):

      $ chmod 775 export_rivers.sh
      $ ./export_rivers.sh
      Exporting river shapefile for AD country...
      Exporting river shapefile for AE country...
      ...
      Exporting river shapefile for ZM country...
      Exporting river shapefile for ZW country...

6. Check the output with ogrinfo or a desktop GIS. The following screenshot shows how the output looks in QGIS; we have added the original PostGIS chp03.rivers layer and a couple of the generated shapefiles: