Compare multiline strings in bash variables

Question

I have a script with two multiline strings. I'd like to know if they are equal. I haven't found a way to do this, because while comparing is easy, passing the value of the variables to the comparison thingamajig isn't. I haven't had success piping it to diff, but it could be my ineptitude (many of the things I've tried resulted in 'File name too long' errors). A workaround would be to replace newlines by some rare character, but it meets much the same problem. Any ideas?

Hi! That gives me `syntax error near unexpected token '(': diff — entonio, May 30 '16 at 10:41
I should point out that my bash (3.2.57) doesn't seem to support the ` — entonio, May 30 '16 at 10:51
`I have a script with two multiline strings.` Would you mind posting the script? — sjsam, May 30 '16 at 10:59
You can check question: http://stackoverflow.com/questions/31540902/how-to-check-if-one-file-is-part-of-other — abrzozowski, May 30 '16 at 12:57
`bash` 3.2 absolutely supports process substitution, but not if it is run as `sh`. — chepner, May 30 '16 at 13:22
@Will and @chepner, you were right. Changing to `#!/bin/bash` made @ritesht93's `diff` work. What should I do regarding the question now? — entonio, May 31 '16 at 13:54

score 5 · Accepted Answer · answered May 30 '16 at 12:33

5

This might be helpful:

var=$(echo -e "this\nis \na\nstring" | md5sum)
var2=$(echo -e "this\nis not\na\nstring" | md5sum)
if [[ $var == $var2 ]] ; then echo true; else echo false; fi

answered May 30 '16 at 12:33

Michael Vehrs

3,202
10
10

3

1. You better use ${} in stead of $(). 2. You don't need to use md5sum... you can just compare the strings "as is" – I-V May 30 '16 at 13:18
1

@I-V `${...}` cannot be used in place of `$(...)`. You are right about `md5sum`, though. – chepner May 30 '16 at 13:23
Why not {}? (I really want to understand :) ) – I-V May 30 '16 at 13:30
1

@I-V `${var}` is variable expansion, `$(date)` is command substitution. – Michael Vehrs May 30 '16 at 14:54
Yes, this can probably be done directly. However, the OP mentions a problem passing the multi-line strings to a comparison. What is that problem exactly? – Michael Vehrs May 30 '16 at 15:09
I'm accepting this one as it seems to work with `#!/bin/sh`. Without that restriction, plain diff works, as discussed in the question comments. – entonio May 31 '16 at 15:52

score 4 · Answer 2 · answered Dec 01 '18 at 11:25

Working with bats and bats-assert I sued the solution from @dimo414 (upvote his answer)

@test "craft a word object" {
  touch "$JSON_FILE"
  entry=$(cat <<-MOT
  {
    "key": "bonjour",
    "label": "bonjour",
    "video": "video/bonjour.webm"
  },
MOT
)

  run add_word "bonjour"

  content=$(cat $JSON_FILE)
  assert_equal "$entry" "$content"
}

score 3 · Answer 3 · edited Jan 28 '20 at 08:35

Even if you're using sh you absolutely can compare multiline strings. In particular there's no need to hash the strings with md5sum or another similar mechanism.

Demo:

$ cat /tmp/multiline.sh
#!/bin/sh

var='this
is
a
string'

var2='this
is not
a
string'

[ "$var" = "$var" ]   && echo SUCCESS || echo FAILURE
[ "$var" != "$var2" ] && echo SUCCESS || echo FAILURE

$ /tmp/multiline.sh
SUCCESS
SUCCESS

In bash you can (and generally should) use [[ ... ]] instead of [ ... ], but the latter still supports multiline strings.

Compare multiline strings in bash variables

3 Answers3