In /usr/bin/xzdiff line 46:
  s/'\''/'\''\\'\'''\''/g
              ^-- SC1003: Want to escape a single quote? echo 'This is how it'\''s done'.
                      ^-- SC2016: Expressions don't expand in single quotes, use double quotes for that.


In /usr/bin/xzdiff line 55:
    -*\'*) cmp="$cmp '"`printf '%sX\n' "$1" | sed "$escape"`;;
                       ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
    -*\'*) cmp="$cmp '"$(printf '%sX\n' "$1" | sed "$escape");;


In /usr/bin/xzdiff line 64:
  test "X$file" = X- || <"$file" || exit 2
                        ^------^ SC2188: This redirection doesn't have a command. Move to its command (or use 'true' as no-op).


In /usr/bin/xzdiff line 88:
      FILE=`expr "X$1" : 'X\(.*\)[-.][abglmoxzZ2]*$'`;;
           ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
      FILE=$(expr "X$1" : 'X\(.*\)[-.][abglmoxzZ2]*$');;


In /usr/bin/xzdiff line 90:
      FILE=`expr "X$1" : 'X\(.*[-.]t\)[abglx]z$'`ar;;
           ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
      FILE=$(expr "X$1" : 'X\(.*[-.]t\)[abglx]z$')ar;;


In /usr/bin/xzdiff line 92:
      FILE=`expr "X$1" : 'X\(.*[-.]t\)bz2$'`ar;;
           ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
      FILE=$(expr "X$1" : 'X\(.*[-.]t\)bz2$')ar;;


In /usr/bin/xzdiff line 94:
      FILE=`expr "X$1" : 'X\(.*[-.]t\)zo$'`ar;;
      ^--^ SC2034: FILE appears unused. Verify use (or export if used externally).
           ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
      FILE=$(expr "X$1" : 'X\(.*[-.]t\)zo$')ar;;


In /usr/bin/xzdiff line 136:
            F=`expr "/$2" : '.*/\(.*\)[-.][ablmotxz2]*$'` || F=$prog
              ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
            F=$(expr "/$2" : '.*/\(.*\)[-.][ablmotxz2]*$') || F=$prog


In /usr/bin/xzdiff line 148:
              tmp=`mktemp -d -t "$prog.XXXXXXXXXX"` || exit 2
                  ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean: 
              tmp=$(mktemp -d -t "$prog.XXXXXXXXXX") || exit 2

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- FILE appears unused. Verify use (...
  https://www.shellcheck.net/wiki/SC2188 -- This redirection doesn't have a c...
  https://www.shellcheck.net/wiki/SC1003 -- Want to escape a single quote? ec...