3

I'm trying to debug an existing program and found an if condition without $ symbol prefixed to it.

Values are:

dt_val=1234
prev_dt_val=1234

If condition goes like:

if [ dt_val -eq prev_dt_val ]
then
  echo "Equal"
else
  echo "Not equal"
fi

Result:
Equal

Anyone throw some light on how the condition is working fine without a $ symbol?
Shouldn't that be..?

[ $dt_val -eq $prev_dt_val ]

The same condition fails when comparing string values. Does that mean, this condition does not require a $ symbol for number?

Additional info:
Comparing Strings with == as suggested:

dt_val="abcd"
prev_dt_val="abcd"

if [ dt_val == prev_dt_val ]
> then
> echo Equal
> else
> echo Not equal
> fi
Not equal

Shell Info:

echo $SHELL
/usr/bin/ksh
Version M-11/16/88f

Wondering why there's no error either.

Improve this question
6
  • It might help illustrate the situation better if you demonstrate unequal values comparing (successfully) as unequal, without $ Commented Jan 15, 2019 at 13:55
  • Really similar, but talks about [[ instead of [ -- unix.stackexchange.com/q/244685/117549 Commented Jan 15, 2019 at 14:02
  • If I was to guess, ksh is using the same code as [[ and is performing variable expansion "for" you. Commented Jan 15, 2019 at 14:22
  • Could you split the question into separate examples of string versus numeric values for testing? Using -eq would be wrong for strings (use = or != for strings). Commented Jan 15, 2019 at 14:41
  • 1
    The [ is a built-in command, and it may treat the -eq test as a proper arithmetic context. $ is not needed on variables in an arithmetic context. Commented Jan 15, 2019 at 16:14

2 Answers 2

Reset to default
5

In ksh, the builtin [ takes the operands of -eq as in an arithmetic context, just like Bash does for [[ and -eq. And in an arithmetic context, variables don't need the $ sign.

$ ksh -c 'a=1 b=1; if [ a -eq b ]; then echo equal; else echo different; fi'
equal
$ ksh -c 'a=1 b=2; if [ a -eq b ]; then echo equal; else echo different; fi'
different

or even:

$ ksh -c 'a=2 b=8; if [ a*4 -eq b ]; then echo equal; else echo different; fi'
equal
$ ksh -c 'a=2 b=9; if [ a*4 -eq b ]; then echo equal; else echo different; fi'
different

(However, the * still globs, so a*4 should be quoted there.)

That's ksh93, ksh --version shows sh (AT&T Research) 93u+ 2012-08-01, it's from Debian's package (ksh, package version 93u+20120801-3.1). I get the same result with Debian's mksh, so I suppose ksh88 is close enough here.

For comparison, in Bash that gives an error:

$ bash -c 'a=1 b=1; if [ a -eq b ]; then echo equal; else echo different; fi'
bash: line 0: [: a: integer expression expected
different

(it prints different, since [ returns a falsy value on error).

With [[ it works:

$ bash -c 'a=1 b=1; if [[ a -eq b ]]; then echo equal; else echo different; fi'
equal

Zsh is like Bash here, [ errors on a -eq b, [[ works.

Improve this answer
0

Firstly, the script is not evaluatling the variables as it's missing the $ in front of the variable names, like you also discovered.

Secondly, it's outputting "Equal" because it's using the numeric compare operator -eq in the test ([) command. As it's passed two non-numerical strings as parameters, they both evaluate to the same numeric value and hence the "Equal" string is output.

Note that at least by bash version complains:

$ if [ 0 -eq bla ]; then echo yes; fi
-bash: [: bla: integer expression expected

My dash version also complains:

$ if [ 0 -eq bla ]; then echo yes; fi
dash: 1: [: Illegal number: bla

I'm interested what shell you are using that doesn't complain.

Even if there are variables present with the names of those strings I get the same result.

Improve this answer
9
  • but if you set dt_val=5, the test fails... Commented Jan 15, 2019 at 13:55
  • I'm using ksh. It gives me the error when i'm comparing string values such as "abcd" ksh: abcd: 0403-009 The specified number is not valid for this command Commented Jan 15, 2019 at 14:11
  • @JeffSchaller, No, it's not failing for me. I get the result as Equal Commented Jan 15, 2019 at 14:12
  • dt_val=5 prev_dt_val=6 if [ dt_val -eq prev_dt_val ] then echo equal else echo not equal fi not equal Commented Jan 15, 2019 at 14:15
  • @SanthoshRam what version of ksh? Commented Jan 15, 2019 at 14:23

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.