Diff several files, true if all not equal

With GNU diff, pass one of the files as an argument to --from-file and any number of others as operand:

$ diff -q --from-file file1 file2 file3 file4; echo $?
0
$ echo >>file3
$ diff -q --from-file file1 file2 file3 file4; echo $?
Files file1 and file3 differ
1

You can use globbing as usual. For example, if the current directory contains file1, file2, file3 and file4, the following example compares file2, file3 and file4 to file1.

$ diff -q --from-file file*; echo $?

Note that the “from file” must be a regular file, not a pipe, because diff will read it multiple times.

How about:

md5sum * | awk 'BEGIN{rc=1}NR>1&&$1!=last{rc=0}{last=$1}END{exit rc}'

Calculates the MD5 value for each file, then compares each entry with the next, if any are different, then return a zero (true) exit status. This would be much shorter if it returned false if different:

md5sum * | awk 'NR>1&&$1!=last{exit 1}{last=$1}'

There is no need to sort since we are just checking if any are different.

The following code should be fairly self explanatory. $# is the number of file arguments, and shift just consumes them one at a time. Uses cmp -s for silent byte-wise comparison.

#!/bin/sh
# diffseveral

if [ $# -lt 2 ]; then
    printf '%s\n' "Usage: $0 file1 file2 [files ...]" >&2
    exit 2
fi

oldfile="$1"
shift

while [ $# -gt 0 ]; do
    newfile="$1"
    if ! cmp -s "$oldfile" "$newfile"; then
         echo 'Files differ.'
         exit 1;
    fi

    shift
done

echo 'All files identical.'
exit 0