[SOLVED] Bash integer comparision

marcoubuntu · 04-09-2018, 06:55 AM

I really can't understand why if I do integer comparision like this:

$ [ 1 == 1]

or

$ [ 1 -eq 1 ]

then

echo $?

I get 0 that is false instead of 1 that is true!

What I'm missing?

schneidz · 04-09-2018, 07:01 AM

$? returns the return value of the previous command (0 means success; any other value would indicate some sort of error).

BW-userx · 04-09-2018, 07:06 AM

you are mixing your comparison operators in case you didn't know.
http://tldp.org/LDP/abs/html/comparison-ops.html
and, your understanding of what 0 and 1 indicate is backwards.

to keep it simple
0 = false, no errors were encountered.
1 = true, errors were encountered.

average_user · 04-09-2018, 02:38 PM

Quote:

Originally Posted by marcoubuntu

I really can't understand why if I do integer comparision like this:

$ [ 1 == 1]

It should be:

Code:

[ 1 == 1 ]

Quote:

Originally Posted by marcoubuntu

or

$ [ 1 -eq 1 ]

then

echo $?

I get 0 that is false instead of 1 that is true!

What I'm missing?

See what man bash says:

Code:

EXIT STATUS   
The exit status of an executed command is the value returned by the
waitpid system call or equivalent function.  Exit statuses fall
between 0 and 255, though, as explained below, the shell may
use values above 125 spe- cially.  Exit statuses from shell
builtins and compound commands are also limited to this
range. Under certain circumstances, the shell will use special
values to indicate specific failure modes.

For the shell's purposes, a command which exits with a zero exit
status has succeeded.  An exit status of zero indicates success.  A
non-zero exit status indicates failure.  When a command terminates on
a fatal signal N, bash uses the value of 128+N as the exit status.

MadeInGermany · 04-09-2018, 03:51 PM

In fact the shell has inverse logic for practical reason.

Code:

true; echo $?
false; echo $?

keefaz · 04-09-2018, 04:48 PM

Exit status is an integer value returned by a function, not true or false value from operator

If you want true or false value

Code:

(( x = 1 == 1 ))
echo $x

marcoubuntu · 04-11-2018, 03:57 AM

ok I understood, so I can even do:

Code:

x=$(expr 1 == 1 )
echo $x

or

Code:

INPUT='string1'
LABEL=$(expr "${INPUT}" == "string1")
echo "$LABEL"

result

Code:

Thank you

keefaz · 04-11-2018, 06:45 AM

Yes, but with expr you use an external program which is not necessary for integer comparision

Code:

x=$(( 1 == 1 ))
echo $x

MadeInGermany · 04-11-2018, 12:01 PM

$(( )) is for arithmetics i.e. numbers only.
While expr takes strings.

With strings you can use the internal [ ] command or the more robust [[ ]] compound, and set the return value explicitly:

Code:

INPUT='string1'
if [[ $INPUT == "string1" ]]; then LABEL=1; else LABEL=0; fi
echo "$LABEL"

keefaz · 04-11-2018, 03:13 PM

You can also write a function that inverts $? value

Code:

error_test() {
  (( $? != 0 ))
}

INPUT='string1'
[[ $INPUT == "string1" ]]; error_test
echo $?

But I wouldn't use this