Ytdyklly

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.

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.

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.