Use double quotes instead. Like:
"#transcode{vcodec=h264,fps=15,venc=x264{preset=fast,tune=zerolatency,keyint=30,bframes=0,ref=1,level=30,profile=baseline,hrd=cbr,crf=20,ratetol=1.0,vbv-maxrate=1200,vbv-bufsize=1200,lookahead=0}}:standard{access=file,mux=ps,dst=${capture_output_file}}'