Strava

's/^ *//' replaces any spaces at the beginning of the line with nothing, essentially stripping off leading whitespace.

^ matches the beginning of the line
the name space character matches at least one space
space then kleene star (*) matches any number (including 0) spaces
replace it with nothing.

's/^ +//' would be cleaner (match beginning of line and then 1 or more spaces) but I've been bitten by lack of + support in the dim past and so I naturally avoid it.

As for indentation, something like xmllint will do a better job, but for hackiness you could also just do something along the lines of:-

cat a.gpx | tr -d '\n' | sed -e 's/> *</>\n</g' | perl -e '$in=0; while( <> ) { if( /<[^\/].*<\// ) { print " "x$i.$_; } elsif( /<[^\/]/ ) { print " "x$i.$_; $i++; } elsif( /<\// ) { $i--; print " "x$i.$_; } }'

The sed separates tags onto a line by themselves and the perl does some rudimentary indentation.

Expect the above to blow up (especially for strings that contain embedded > or < characters). It's 5 minutes' work whilst distracted on a conference call.