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[solved] why does "diff" always return "1"?!?

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bash script

--- Code: ---function check_result()
   local ans="$?" # get the exit status from a terminated program
   local answ

   if [ "$ans" == "0" ] # 0=success, 1=failure
          answ="failure (ans=$ans)"
   echo "$answ"

echo "AAAA"  >  A
cp A B
echo "BBBB"  >> B

diff -Naur A B

#       -N, --new-file: treat absent files as empty
diff -Naur A C    # C doesn't exist

--- End code ---

--- Code: ------ A   2021-09-25 13:52:21.012000000 -0000
+++ B   2021-09-25 13:52:21.012000000 -0000
@@ -1 +1,2 @@
failure (ans=1)
failure (ans=1)
--- A   2021-09-25 13:52:21.012000000 -0000
+++ C   1970-01-01 00:00:00.000000000 -0000
@@ -1 +0,0 @@
failure (ans=1)
failure (ans=1)

--- End code ---

--- Code: ---NAME
       GNU diff - compare files line by line

       diff [OPTION]... FILES

       Compare FILES line by line.

       Mandatory arguments to long options are mandatory for short options too.

              output a normal diff (the default)

       -q, --brief
              report only when files differ

       -s, --report-identical-files
              report when two files are the same

       -c, -C NUM, --context[=NUM]
              output NUM (default 3) lines of copied context

       -u, -U NUM, --unified[=NUM]
              output NUM (default 3) lines of unified context

       -e, --ed
              output an ed script

       -n, --rcs
              output an RCS format diff

       -y, --side-by-side
              output in two columns

       -W, --width=NUM
              output only the left column of common lines

              do not output common lines

       -p, --show-c-function
              show which C function each change is in

       -F, --show-function-line=RE
              show the most recent line matching RE

       --label LABEL
              use LABEL instead of file name and timestamp (can be repeated)

       -t, --expand-tabs
              expand tabs to spaces in output

       -T, --initial-tab
              make tabs line up by prepending a tab

              tab stops every NUM (default 8) print columns

              suppress space or tab before empty output lines

       -l, --paginate
              pass output through 'pr' to paginate it

       -r, --recursive
              recursively compare any subdirectories found

              don't follow symbolic links

       -N, --new-file
              treat absent files as empty

              treat absent first files as empty

              ignore case when comparing file names

              consider case when comparing file names

       -x, --exclude=PAT
              exclude files that match PAT

       -X, --exclude-from=FILE
              exclude files that match any pattern in FILE

       -S, --starting-file=FILE
              start with FILE when comparing directories

              compare FILE1 to all operands; FILE1 can be a directory

              compare all operands to FILE2; FILE2 can be a directory

       -i, --ignore-case
              ignore case differences in file contents

       -E, --ignore-tab-expansion
              ignore changes due to tab expansion

       -Z, --ignore-trailing-space
              ignore white space at line end

       -b, --ignore-space-change
              ignore changes in the amount of white space

       -w, --ignore-all-space
              ignore all white space

       -B, --ignore-blank-lines
              ignore changes where lines are all blank

       -I, --ignore-matching-lines=RE
              ignore changes where all lines match RE

       -a, --text
              treat all files as text

              strip trailing carriage return on input

       -D, --ifdef=NAME
              output merged file with '#ifdef NAME' diffs

              format GTYPE input groups with GFMT

              format all input lines with LFMT

              format LTYPE input lines with LFMT

              These format options provide fine-grained control over the output

              of diff, generalizing -D/--ifdef.

       LTYPE is 'old', 'new', or 'unchanged'.
              GTYPE is LTYPE or 'changed'.

              GFMT (only) may contain:

       %<     lines from FILE1

       %>     lines from FILE2

       %=     lines common to FILE1 and FILE2

              printf-style spec for LETTER

              LETTERs are as follows for new group, lower case for old group:

       F      first line number

       L      last line number

       N      number of lines = L-F+1

       E      F-1

       M      L+1

              if A equals B then T else E

              LFMT (only) may contain:

       %L     contents of line

       %l     contents of line, excluding any trailing newline

              printf-style spec for input line number

              Both GFMT and LFMT may contain:

       %%     %

       %c'C'  the single character C

              the character with octal code OOO

       C      the character C (other characters represent themselves)

       -d, --minimal
              try hard to find a smaller set of changes

              keep NUM lines of the common prefix and suffix

              assume large files and many scattered small changes

              colorize the output; WHEN can be 'never', 'always', or 'auto' (the default)

              the colors to use when --color is active; PALETTE is a colon-separated list of terminfo capabilities

       --help display this help and exit

       -v, --version
              output version information and exit

       FILES  are  'FILE1  FILE2'  or 'DIR1 DIR2' or 'DIR FILE' or 'FILE DIR'.  If --from-file or --to-file is given,
       there are no restrictions on FILE(s).  If a FILE is '-', read standard input.  Exit status is 0 if inputs  are
       the same, 1 if different, 2 if trouble.

--- End code ---

I am working on a bash-script, and I am confused by the exit status of the tool "diff", because it always return "1", which should mean FAILURE and others programs return "0" to say "success".

So why does it always return "1" ?!? :-//

Maybe read TFM that you posted above? :P

I haven't looked at your commands but you are doing this the complicated way.

--- Code: ---if diff [options] A B
   echo "No differences"
   echo "Differences"
--- End code ---

diff behaves exactly as described in the man page your posted above.

not on my shells  ???

0 = match
1 = mismatch
2 = error



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