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...