Exit statuses of comparisons in test constructs

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).

edited 9 hours ago

asked 12 hours ago

Elegance

183

New contributor

I didn't know about the space being necessary. What I meant with "odd" is "weird if you read as pseudo code, ignoring the quirks of the language". Thanks. @ilkkachu

– Elegance
11 hours ago

1

The only way I can get [ $rc=0 ] to fail with rc=1 is to set IFS to 1 as well. That would cause both tests to error out and set $? to 2 (in bash).

– Kusalananda
11 hours ago

@ilkkachu There was a typo. Of course I can't ignore them, but if I knew them, I wouldn't have to ask. That's exactly the point of the question.

– Elegance
9 hours ago

@Elegance, ok, good, thanks. And yes, you're right, you wouldn't have to ask if you knew. It's just that even a typo like that can send the readers off in the wrong direction, looking for some really weird edge case that could explain the result. (The shell can be a bit quirky sometimes so there might have been a remote possibility of an edge case where both would return 1...)

– ilkkachu
9 hours ago

@ilkkachu Thank you for the helpful feedback.

– Elegance
9 hours ago

add a comment |

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).

edited 9 hours ago

asked 12 hours ago

Elegance

183

New contributor

I didn't know about the space being necessary. What I meant with "odd" is "weird if you read as pseudo code, ignoring the quirks of the language". Thanks. @ilkkachu

– Elegance
11 hours ago

1

The only way I can get [ $rc=0 ] to fail with rc=1 is to set IFS to 1 as well. That would cause both tests to error out and set $? to 2 (in bash).

– Kusalananda
11 hours ago

@ilkkachu There was a typo. Of course I can't ignore them, but if I knew them, I wouldn't have to ask. That's exactly the point of the question.

– Elegance
9 hours ago

@Elegance, ok, good, thanks. And yes, you're right, you wouldn't have to ask if you knew. It's just that even a typo like that can send the readers off in the wrong direction, looking for some really weird edge case that could explain the result. (The shell can be a bit quirky sometimes so there might have been a remote possibility of an edge case where both would return 1...)

– ilkkachu
9 hours ago

@ilkkachu Thank you for the helpful feedback.

– Elegance
9 hours ago

add a comment |

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).

edited 9 hours ago

asked 12 hours ago

Elegance

183

New contributor

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).

test control-flow

edited 9 hours ago

asked 12 hours ago

Elegance

183

New contributor

edited 9 hours ago

asked 12 hours ago

Elegance

183

New contributor

edited 9 hours ago

asked 12 hours ago

Elegance

183

New contributor

asked 12 hours ago

Elegance

183

asked 12 hours ago

Elegance

183

New contributor

Elegance is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

I didn't know about the space being necessary. What I meant with "odd" is "weird if you read as pseudo code, ignoring the quirks of the language". Thanks. @ilkkachu

– Elegance
11 hours ago

1

The only way I can get [ $rc=0 ] to fail with rc=1 is to set IFS to 1 as well. That would cause both tests to error out and set $? to 2 (in bash).

– Kusalananda
11 hours ago

@ilkkachu There was a typo. Of course I can't ignore them, but if I knew them, I wouldn't have to ask. That's exactly the point of the question.

– Elegance
9 hours ago

@Elegance, ok, good, thanks. And yes, you're right, you wouldn't have to ask if you knew. It's just that even a typo like that can send the readers off in the wrong direction, looking for some really weird edge case that could explain the result. (The shell can be a bit quirky sometimes so there might have been a remote possibility of an edge case where both would return 1...)

– ilkkachu
9 hours ago

@ilkkachu Thank you for the helpful feedback.

– Elegance
9 hours ago

add a comment |

I didn't know about the space being necessary. What I meant with "odd" is "weird if you read as pseudo code, ignoring the quirks of the language". Thanks. @ilkkachu

– Elegance
11 hours ago

1

The only way I can get [ $rc=0 ] to fail with rc=1 is to set IFS to 1 as well. That would cause both tests to error out and set $? to 2 (in bash).

– Kusalananda
11 hours ago

@ilkkachu There was a typo. Of course I can't ignore them, but if I knew them, I wouldn't have to ask. That's exactly the point of the question.

– Elegance
9 hours ago

@Elegance, ok, good, thanks. And yes, you're right, you wouldn't have to ask if you knew. It's just that even a typo like that can send the readers off in the wrong direction, looking for some really weird edge case that could explain the result. (The shell can be a bit quirky sometimes so there might have been a remote possibility of an edge case where both would return 1...)

– ilkkachu
9 hours ago

@ilkkachu Thank you for the helpful feedback.

– Elegance
9 hours ago

I didn't know about the space being necessary. What I meant with "odd" is "weird if you read as pseudo code, ignoring the quirks of the language". Thanks. @ilkkachu

– Elegance
11 hours ago

The only way I can get [ $rc=0 ] to fail with rc=1 is to set IFS to 1 as well. That would cause both tests to error out and set $? to 2 (in bash).

– Kusalananda
11 hours ago

@ilkkachu There was a typo. Of course I can't ignore them, but if I knew them, I wouldn't have to ask. That's exactly the point of the question.

– Elegance
9 hours ago

@Elegance, ok, good, thanks. And yes, you're right, you wouldn't have to ask if you knew. It's just that even a typo like that can send the readers off in the wrong direction, looking for some really weird edge case that could explain the result. (The shell can be a bit quirky sometimes so there might have been a remote possibility of an edge case where both would return 1...)

– ilkkachu
9 hours ago

@ilkkachu Thank you for the helpful feedback.

– Elegance
9 hours ago

add a comment |

2 Answers
2

active

oldest

votes

-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

edited 10 hours ago

answered 12 hours ago

Jesse_b

13.2k23369

I'm just a bit confused about what is meant by numeric and string comparisons. Let's say I use = to compare numbers. When is it not going to not yield the same result as a numeric comparison?

– Elegance
11 hours ago

Any examples with strings that have nothing but digits, and don't have leading zeros?

– Elegance
11 hours ago

1

@Elegance The point is that with -eq, the left and right hand side are compared as integers, not as strings. The test would evaluate the strings as integers (probably by passing them through strtol() or some such C function) before carrying out the comparison.

– Kusalananda
11 hours ago

1

@Elegance In a bash test with [[ ... -eq ... ]] you could even have arithmetic calculations on either side, as in [[ 1+1 -eq 2 ]]. Obviously, [[ 1+1 == 2 ]] would not be true.

– Kusalananda
11 hours ago

@Jesse_b You shared the same link twice. I take it the first one is incorrect.

– Elegance
9 hours ago

|
show 1 more comment

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

answered 12 hours ago

freiheitsnetz

112

add a comment |

Your Answer

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "106"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});

}
});

Elegance is a new contributor. Be nice, and check out our Code of Conduct.

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f504979%2fexit-statuses-of-comparisons-in-test-constructs%23new-answer', 'question_page');
}
);

Post as a guest

Name

Required, but never shown

2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

-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

edited 10 hours ago

answered 12 hours ago

Jesse_b

13.2k23369

I'm just a bit confused about what is meant by numeric and string comparisons. Let's say I use = to compare numbers. When is it not going to not yield the same result as a numeric comparison?

– Elegance
11 hours ago

Any examples with strings that have nothing but digits, and don't have leading zeros?

– Elegance
11 hours ago

1

@Elegance The point is that with -eq, the left and right hand side are compared as integers, not as strings. The test would evaluate the strings as integers (probably by passing them through strtol() or some such C function) before carrying out the comparison.

– Kusalananda
11 hours ago

1

@Elegance In a bash test with [[ ... -eq ... ]] you could even have arithmetic calculations on either side, as in [[ 1+1 -eq 2 ]]. Obviously, [[ 1+1 == 2 ]] would not be true.

– Kusalananda
11 hours ago

@Jesse_b You shared the same link twice. I take it the first one is incorrect.

– Elegance
9 hours ago

|
show 1 more comment

-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

edited 10 hours ago

answered 12 hours ago

Jesse_b

13.2k23369

I'm just a bit confused about what is meant by numeric and string comparisons. Let's say I use = to compare numbers. When is it not going to not yield the same result as a numeric comparison?

– Elegance
11 hours ago

Any examples with strings that have nothing but digits, and don't have leading zeros?

– Elegance
11 hours ago

1

@Elegance The point is that with -eq, the left and right hand side are compared as integers, not as strings. The test would evaluate the strings as integers (probably by passing them through strtol() or some such C function) before carrying out the comparison.

– Kusalananda
11 hours ago

1

@Elegance In a bash test with [[ ... -eq ... ]] you could even have arithmetic calculations on either side, as in [[ 1+1 -eq 2 ]]. Obviously, [[ 1+1 == 2 ]] would not be true.

– Kusalananda
11 hours ago

@Jesse_b You shared the same link twice. I take it the first one is incorrect.

– Elegance
9 hours ago

|
show 1 more comment

-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

edited 10 hours ago

answered 12 hours ago

Jesse_b

13.2k23369

-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

edited 10 hours ago

answered 12 hours ago

Jesse_b

13.2k23369

edited 10 hours ago

answered 12 hours ago

Jesse_b

13.2k23369

answered 12 hours ago

Jesse_b

13.2k23369

answered 12 hours ago

Jesse_b

13.2k23369

I'm just a bit confused about what is meant by numeric and string comparisons. Let's say I use = to compare numbers. When is it not going to not yield the same result as a numeric comparison?

– Elegance
11 hours ago

Any examples with strings that have nothing but digits, and don't have leading zeros?

– Elegance
11 hours ago

1

@Elegance The point is that with -eq, the left and right hand side are compared as integers, not as strings. The test would evaluate the strings as integers (probably by passing them through strtol() or some such C function) before carrying out the comparison.

– Kusalananda
11 hours ago

1

@Elegance In a bash test with [[ ... -eq ... ]] you could even have arithmetic calculations on either side, as in [[ 1+1 -eq 2 ]]. Obviously, [[ 1+1 == 2 ]] would not be true.

– Kusalananda
11 hours ago

@Jesse_b You shared the same link twice. I take it the first one is incorrect.

– Elegance
9 hours ago

|
show 1 more comment

I'm just a bit confused about what is meant by numeric and string comparisons. Let's say I use = to compare numbers. When is it not going to not yield the same result as a numeric comparison?

– Elegance
11 hours ago

Any examples with strings that have nothing but digits, and don't have leading zeros?

– Elegance
11 hours ago

1

@Elegance The point is that with -eq, the left and right hand side are compared as integers, not as strings. The test would evaluate the strings as integers (probably by passing them through strtol() or some such C function) before carrying out the comparison.

– Kusalananda
11 hours ago

1

@Elegance In a bash test with [[ ... -eq ... ]] you could even have arithmetic calculations on either side, as in [[ 1+1 -eq 2 ]]. Obviously, [[ 1+1 == 2 ]] would not be true.

– Kusalananda
11 hours ago

@Jesse_b You shared the same link twice. I take it the first one is incorrect.

– Elegance
9 hours ago

I'm just a bit confused about what is meant by numeric and string comparisons. Let's say I use = to compare numbers. When is it not going to not yield the same result as a numeric comparison?

– Elegance
11 hours ago

Any examples with strings that have nothing but digits, and don't have leading zeros?

– Elegance
11 hours ago

@Elegance The point is that with -eq, the left and right hand side are compared as integers, not as strings. The test would evaluate the strings as integers (probably by passing them through strtol() or some such C function) before carrying out the comparison.

– Kusalananda
11 hours ago

@Elegance In a bash test with [[ ... -eq ... ]] you could even have arithmetic calculations on either side, as in [[ 1+1 -eq 2 ]]. Obviously, [[ 1+1 == 2 ]] would not be true.

– Kusalananda
11 hours ago

@Jesse_b You shared the same link twice. I take it the first one is incorrect.

– Elegance
9 hours ago

|
show 1 more comment

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

answered 12 hours ago

freiheitsnetz

112

add a comment |

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

answered 12 hours ago

freiheitsnetz

112

add a comment |

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

answered 12 hours ago

freiheitsnetz

112

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

answered 12 hours ago

freiheitsnetz

112

answered 12 hours ago

freiheitsnetz

112

answered 12 hours ago

freiheitsnetz

112

answered 12 hours ago

freiheitsnetz

112

add a comment |

Elegance is a new contributor. Be nice, and check out our Code of Conduct.

draft saved

draft discarded

Elegance is a new contributor. Be nice, and check out our Code of Conduct.

Thanks for contributing an answer to Unix & Linux Stack Exchange!

Please be sure to answer the question. Provide details and share your research!

But avoid …

Asking for help, clarification, or responding to other answers.

Making statements based on opinion; back them up with references or personal experience.

To learn more, see our tips on writing great answers.

draft saved

draft discarded

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Post as a guest

Name

Required, but never shown

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Sign up or log in

StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});

Post as a guest

Name

Required, but never shown

Name

Required, but never shown

Name

Required, but never shown

This page is only for reference, If you need detailed information, please check here

搜尋此網誌

Cfxtrjtrk