GDAL: Convert from UTM Zone to Lat/Lon (EPSG:4326)

1 - Find UTM Zone of source data .sh

$ gdalsrsinfo source.shp

PROJ.4 : '+proj=utm +zone=34 +south +datum=WGS84 +units=m +no_defs '

OGC WKT :
PROJCS["WGS_1984_UTM_Zone_34S",
    GEOGCS["GCS_WGS_1984",
        DATUM["WGS_1984",
            SPHEROID["WGS_84",6378137,298.257223563]],
        PRIMEM["Greenwich",0],
        UNIT["Degree",0.017453292519943295]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",21],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",10000000],
    UNIT["Meter",1]]

2 - Search for EPSG code on epsg.io .md

Go to http://epsg.io

Search for e.g. "WGS 84 34S"

Copy the EPSG:**** code from the search results

3 - Run ogr2ogr .sh

$ ogr2ogr -f GeoJSON target.json source.shp -s_srs EPSG:32734 -t_srs EPSG:4326