Ggtcf

I was writing some "if then" statements and found what seemed to me an odd behavior. Upon investigation I realized that it boiled down to the exit code of the comparison I was making. I illustrate my findings in the following code snippet.

As you can see

rc=1
[ $rc -eq 0 ]
es_num=$?
[ $rc=0 ]
es_str=$?
echo "es_num is $es_num"
echo "es_str is $es_str"

Outputs

es_num is 1
es_str is 0

Is there any documentation, preferably from the POSIX standards, that talks about the difference in the exit statuses of -eq and = in a test construct?

What should I be aware of when writing conditional statements? What are some best practices regarding this?

Portable code is preferable to Bash code (which I'm using).

-eq

True if the integers n1 and n2 are algebraically equal; otherwise, false.

test

=

True if the strings s1 and s2 are identical; otherwise, false.

test

So -eq compares integers and = compares strings (which will also work with some limited integer cases).

You do have a syntax issue though, it should be:

[ "$rc" = 0 ]

And not

[ $rc=0 ]

[ "$rc" = 0 ] should exit with 1 because rc does not equal 0

[ $rc=0 ] should actually exit with 0 because it's likely going to be treated as a string and the presence of a string within the [ test construct will evaluate to true

With the sh [ test there are a few differences:

# leading 0
$ [ 01 -eq 1 ]; echo $?
0
# adjacent whitespace
$ [ ' 1' -eq 1 ]; echo $?
0
# negative 0 vs positive 0
$ [ 0 -eq -0 ]; echo $?
0

However with the bash [[ test there are a large number of differences (Including the ones mentioned above):

# base 8
$ [[ 032 -eq 26 ]]; echo $?
0
# Arithmetic expressions
$ [[ 1*6+32/15*2-1 -eq 9 ]]; echo $?
0
# Base 64
$ [[ 64#Hello_world -eq -5506400892957379251 ]]; echo $?
0

For numeric comparisons you have to use -eq whereas = is for string comparisons (as from your variable naming you already seem to know).

One of the best introductions on the test aka [ command I know is The Unix Shell's Humble If