Scripting pinning Subversion Externals

How to control Subversion externals with scripts (bash):

Command Line setting 1 External
cd ./ProjectB/Reuse/A
svn propset svn:externals "Generic -r 19049 http://svnl/ReuseA/something" .

Command Line setting multiple Externals
cd ./ProjectC/Reuse
echo "A -r 16234 http://svnl/ReuseA/something" > svn.externals
echo "B -r 18086 http://svnl/ReuseB/something" >> svn.externals
svn propset svn:externals . -F svn.externals
#Optional
rm svn.externals
svn update
cd -