In /usr/bin/gdiffmk line 23:
CMD=`basename $0`
    ^-----------^ SC2006: Use $(...) notation instead of legacy backticked `...`.
              ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
CMD=$(basename "$0")


In /usr/bin/gdiffmk line 28:
		echo >&2 "${CMD}:  $@"
                                   ^-- SC2145: Argument mixes string and array. Use * or separate argument.


In /usr/bin/gdiffmk line 71:
	for arg
        ^-^ SC2034: arg appears unused. Verify use (or export if used externally).


In /usr/bin/gdiffmk line 76:
	exit ${exitcode}
             ^---------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
	exit "${exitcode}"


In /usr/bin/gdiffmk line 92:
		Exit $1 "File '$2' not found."
                     ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
		Exit "$1" "File '$2' not found."


In /usr/bin/gdiffmk line 96:
		Exit $1 "File '$2' not readable."
                     ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
		Exit "$1" "File '$2' not readable."


In /usr/bin/gdiffmk line 117:
			Exit $1 "File '$2' not created; " \
                             ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
			Exit "$1" "File '$2' not created; " \


In /usr/bin/gdiffmk line 118:
			  "Cannot write directory '`dirname "$2"`'."
                                                   ^------------^ SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
			  "Cannot write directory '$(dirname "$2")'."


In /usr/bin/gdiffmk line 120:
		Exit $1 "File '$2' not writeable."
                     ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
		Exit "$1" "File '$2' not writeable."


In /usr/bin/gdiffmk line 175:
DIFFCMD=diff
^-----^ SC2209: Use var=$(command) to assign output (or quote to assign string).


In /usr/bin/gdiffmk line 176:
SEDCMD=sed
^----^ SC2209: Use var=$(command) to assign output (or quote to assign string).


In /usr/bin/gdiffmk line 184:
		ADDMARK=`RequiresArgument "${OPTION}" "$2"`		&&
                        ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
		ADDMARK=$(RequiresArgument "${OPTION}" "$2")		&&


In /usr/bin/gdiffmk line 188:
		CHANGEMARK=`RequiresArgument "${OPTION}" "$2"`		&&
                           ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
		CHANGEMARK=$(RequiresArgument "${OPTION}" "$2")		&&


In /usr/bin/gdiffmk line 192:
		DELETEMARK=`RequiresArgument "${OPTION}" "$2"`		&&
                           ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
		DELETEMARK=$(RequiresArgument "${OPTION}" "$2")		&&


In /usr/bin/gdiffmk line 199:
		MARK1=`RequiresArgument "${OPTION}" "$2"`		&&
                      ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
		MARK1=$(RequiresArgument "${OPTION}" "$2")		&&


In /usr/bin/gdiffmk line 212:
		SEDCMD=`RequiresArgument "${OPTION}" "$2"`		&&
                       ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
		SEDCMD=$(RequiresArgument "${OPTION}" "$2")		&&


In /usr/bin/gdiffmk line 216:
		DIFFCMD=`RequiresArgument "${OPTION}" "$2"`		&&
                        ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
		DIFFCMD=$(RequiresArgument "${OPTION}" "$2")		&&


In /usr/bin/gdiffmk line 283:
		exec >$3
                      ^-- SC2086: Double quote to prevent globbing and word splitting.

Did you mean: 
		exec >"$3"


In /usr/bin/gdiffmk line 307:
		   a\
                    ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 326:
		   i\
                    ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 338:
		     i\
                      ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 339:
'"${MARK2}"'\
            ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 342:
		     a\
                      ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 351:
		   i\
                    ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 352:
'"${MARK2}"'\
            ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.


In /usr/bin/gdiffmk line 355:
		   a\
                    ^-- SC1004: This backslash+linefeed is literal. Break outside single quotes if you just want to break the line.

For more information:
  https://www.shellcheck.net/wiki/SC2145 -- Argument mixes string and array. ...
  https://www.shellcheck.net/wiki/SC2034 -- arg appears unused. Verify use (o...
  https://www.shellcheck.net/wiki/SC2209 -- Use var=$(command) to assign outp...