How to check if a diff command is successful in Unix shell script

Question

In a Unix shell script, I want to check if the output of a diff command is equal to 0 through an if statement. So that I can accordingly give the if case statements.

For example: Abc.sh

#!/bin/bash
cd users
diff s1 d1 > a.txt
wc -l a.txt | awk '{print f1}' > a
echo "value of a is"
cat a
if [ $a == 0 ]
  then
echo "checksums match"
  else
echo "checksums do not match"
fi

Output:

value of a is
0
[: ==: unary operator expected
checksums do not match

Does this answer your question? [Fastest way to tell if two files have the same contents in Unix/Linux?](https://stackoverflow.com/questions/12900538/fastest-way-to-tell-if-two-files-have-the-same-contents-in-unix-linux) — Piotr Siupa, Jul 23 '22 at 11:04
`diff -q s1 d1 &>/dev/null && echo "checksums match" || echo "checksums do not match"` — ufopilot, Jul 24 '22 at 07:47

score 1 · Answer 1 · answered Jul 23 '22 at 11:19

1

Just check diff.

if diff s1 d1 ; then
   echo "checksums match"
else
   echo "checksums do not match"
fi

If you don't want to have output on terminal, you would use cmp.

if cmp -s s1 d2 ; then ...

answered Jul 23 '22 at 11:19

KamilCuk

120,984
8
59
111

score 0 · Accepted Answer · answered Jul 23 '22 at 11:10

0

There are many ways to fix your script, but this is probably the most minimal:

a=$(cat a)
if [ "$a" = "" ]; then

Variables are different from files, so you have to read the output of the file you created.
Variables that potentially contain spaces must be quoted when used as arguments, and strings must be compared against empty rather than 0.

(As suggested in the comment, if you don't need the diff output, you could just use the exit code.)

answered Jul 23 '22 at 11:10

Brett Kail

33,593
2
85
90

Thank you. This partly worked. if [ "$a" == 0 ] then echo "checksums match" else echo "checksums don't match".. this has worked – melony_r Jul 23 '22 at 18:12

RARE Kpop Manifesto · Answer 3 · 2022-07-24T01:38:56.400

UPDATE 1 : quick note on short-circuiting

before u even start to consider which of the comparison tools, check their file sizes via stat command.

if even those don't match, then it's close to statistically impossible for any hashing algorithm to report a matching hash.

One method I use myself is

check file sizes
for matching ones, do a high speed batch run of them over xxhash.
then finally, only for the suggested duplicates from step 2, run them once more via a cryptographically acceptable hash, like Keccak or Shake from SHA3 family, to confirm they're truly duplicates beyond all reasonable doubt

==============================

awk has a useful feature of having NR = 0 if you only have an END block, but nothing got read in, either from files or from pipe.

One can also re-use the same string for both scenarios w/o using a ternary operator … ? … : … by leveraging the 0th-power of any base.

if u don't have too many files, here's a pretty brute force but high speed way to do it all at once.

--just remove the bckgrnd placing bit "... & )"  if u wanna do it sequentially

|

for f1 in "${m2p}" "${m3l}" "${m3m}";              do 
for f2 in "${m3m}" "${m2a}" "${m2p}" "${m3supp}" ; do 

    ( diff "${f1}" "${f2}" |  

      {m,g}awk -F'^$' -v __="${f1}" -v ___="${f2}" '
      END {
              ____=ENVIRON["HOME"] 
          sub(____,"~",__)
          sub(____,"~",___)
      
          print "checksums for \n\t\42"(__)"\42\n\t\b\b\b\band \42"\
                                       (___)"\42 ===> "   \
          substr(" DO NOT match", ((_+= ++_)^++_+!!_)^!NR)"\n" }' &)
      
done ; done | gcat -b


 1  checksums for 
 2      "~/m2map_main.txt"
 3      and "~/m2map_main.txt" ===> match

 4  checksums for 
 5      "~/m3vid_genie26.txt"
 6      and "~/m3vid_genie26.txt" ===> match

 7  checksums for 
 8      "~/m3vid_genie26.txt"
 9      and "~/m2art_main_03.txt" ===>  DO NOT match

10  checksums for 
11      "~/m3vid_genie26.txt"
12      and "~/m3vid_genie25_supp.txt" ===>  DO NOT match

13  checksums for 
14      "~/m23lyricsFLT_05.txt"
15      and "~/m3vid_genie26.txt" ===>  DO NOT match

16  checksums for 
17      "~/m23lyricsFLT_05.txt"
18      and "~/m2art_main_03.txt" ===>  DO NOT match

19  checksums for 
20      "~/m23lyricsFLT_05.txt"
21      and "~/m3vid_genie25_supp.txt" ===>  DO NOT match

22  checksums for 
23      "~/m3vid_genie26.txt"
24      and "~/m2map_main.txt" ===>  DO NOT match

25  checksums for 
26      "~/m2map_main.txt"
27      and "~/m3vid_genie26.txt" ===>  DO NOT match

28  checksums for 
29      "~/m2map_main.txt"
30      and "~/m2art_main_03.txt" ===>  DO NOT match

31  checksums for 
32      "~/m2map_main.txt"
33      and "~/m3vid_genie25_supp.txt" ===>  DO NOT match

34  checksums for 
35      "~/m23lyricsFLT_05.txt"
36      and "~/m2map_main.txt" ===>  DO NOT match

How to check if a diff command is successful in Unix shell script

3 Answers3