In /usr/bin/libesmtp-config line 4:
exec_prefix=${prefix}
^---------^ SC2034: exec_prefix appears unused. Verify use (or export if used externally).


In /usr/bin/libesmtp-config line 23:
    exit $1
         ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
    exit "$1"


In /usr/bin/libesmtp-config line 30:
cflags=false
^----^ SC2034: cflags appears unused. Verify use (or export if used externally).


In /usr/bin/libesmtp-config line 31:
libs=false
^--^ SC2034: libs appears unused. Verify use (or export if used externally).


In /usr/bin/libesmtp-config line 35:
    -*=*) optarg=`echo "$1" | sed 's/[-_a-zA-Z0-9]*=//'` ;;
                 ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
    -*=*) optarg=$(echo "$1" | sed 's/[-_a-zA-Z0-9]*=//') ;;


In /usr/bin/libesmtp-config line 45:
	echo $prefix
             ^-----^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
	echo "$prefix"


In /usr/bin/libesmtp-config line 68:
       	echo -I${prefix}/include -pthread
               ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
       	echo -I"${prefix}"/include -pthread


In /usr/bin/libesmtp-config line 72:
       	echo -pthread -L${prefix}/lib/x86_64-linux-gnu -lesmtp 
                        ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
       	echo -pthread -L"${prefix}"/lib/x86_64-linux-gnu -lesmtp 

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- cflags appears unused. Verify use...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...