What's the purpose of “true” in bash “if sudo true; then”

I've hacked together this script which tests if the user has superuser privileges and if they don't it asks for them. I'm trying to negate the second if statement so that I can remove the following two lines (the echo "password ok" and the else on the following line)

# Root user only

if [[ "$EUID" != 0 ]]; then

    sudo -k # make sure to ask for password on next sudo

    if sudo true; then

        echo "Password ok"

    else

        echo "Aborting script"

        exit 1

    fi

fi



echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement? If so, how do I invert the test that line? Here is what I've tried

if sudo false; then  

if sudo true == false; then

if [!(sudo true)]; then

asked 16 hours ago

Matt Parkins

1796

New contributor

1

If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?

– Matt Parkins
16 hours ago

2

Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?

– Jeff Schaller♦
16 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
16 hours ago

1

@ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.

– Matt Parkins
15 hours ago

1

In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2

– Toby Speight
13 hours ago

|
show 2 more comments

# Root user only

if [[ "$EUID" != 0 ]]; then

    sudo -k # make sure to ask for password on next sudo

    if sudo true; then

        echo "Password ok"

    else

        echo "Aborting script"

        exit 1

    fi

fi



echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement? If so, how do I invert the test that line? Here is what I've tried

if sudo false; then  

if sudo true == false; then

if [!(sudo true)]; then

asked 16 hours ago

Matt Parkins

1796

New contributor

1

If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?

– Matt Parkins
16 hours ago

2

Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?

– Jeff Schaller♦
16 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
16 hours ago

1

@ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.

– Matt Parkins
15 hours ago

1

In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2

– Toby Speight
13 hours ago

|
show 2 more comments

# Root user only

if [[ "$EUID" != 0 ]]; then

    sudo -k # make sure to ask for password on next sudo

    if sudo true; then

        echo "Password ok"

    else

        echo "Aborting script"

        exit 1

    fi

fi



echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement? If so, how do I invert the test that line? Here is what I've tried

if sudo false; then  

if sudo true == false; then

if [!(sudo true)]; then

asked 16 hours ago

Matt Parkins

1796

New contributor

# Root user only

if [[ "$EUID" != 0 ]]; then

    sudo -k # make sure to ask for password on next sudo

    if sudo true; then

        echo "Password ok"

    else

        echo "Aborting script"

        exit 1

    fi

fi



echo "do my ops"

Is the purpose of "true" on the fourth line just a null-statement? If so, how do I invert the test that line? Here is what I've tried

if sudo false; then  

if sudo true == false; then

if [!(sudo true)]; then

bash sudo

asked 16 hours ago

Matt Parkins

1796

New contributor

asked 16 hours ago

Matt Parkins

1796

New contributor

asked 16 hours ago

Matt Parkins

1796

New contributor

asked 16 hours ago

Matt Parkins

1796

asked 16 hours ago

Matt Parkins

1796

New contributor

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

1

If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?

– Matt Parkins
16 hours ago

2

Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?

– Jeff Schaller♦
16 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
16 hours ago

1

@ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.

– Matt Parkins
15 hours ago

1

In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2

– Toby Speight
13 hours ago

|
show 2 more comments

1

If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?

– Matt Parkins
16 hours ago

2

Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?

– Jeff Schaller♦
16 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
16 hours ago

1

@ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.

– Matt Parkins
15 hours ago

1

In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2

– Toby Speight
13 hours ago

If you're going to downvote the question, please could you explain what I can do to improve the question? Is this not the right overflow site?

– Matt Parkins
16 hours ago

Not sure where the downvotes are coming from; are you trying to negate the statement or nullify the statement? You say you want to get rid of both the true/echo and the "else", so what's your final objective?

– Jeff Schaller♦
16 hours ago

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
16 hours ago

@ctrl-alt-delor following that link it seems all I need do is put an exclamation mark in front of the sudo and then I can remove both the echo and the else, and now i've tested it, that works, thanks.

– Matt Parkins
15 hours ago

In passing, you probably ought to redirect the error message to the error stream: echo "Aborting script" >&2

– Toby Speight
13 hours ago

|
show 2 more comments

2 Answers
2

active

oldest

votes

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true

echo $? # 0

false

echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

answered 16 hours ago

JShorthouse

37316

New contributor

7

As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.

– IMSoP
11 hours ago

11

PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully

– IMSoP
11 hours ago

add a comment |

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

answered 8 hours ago

Mehrdad

1,25641533

Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.

– Sergiy Kolodyazhnyy
3 hours ago

1

Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)

– Mehrdad
3 hours ago

The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.

– mckenzm
2 hours ago

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
});

}
});

Matt Parkins 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%2f508756%2fwhats-the-purpose-of-true-in-bash-if-sudo-true-then%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

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true

echo $? # 0

false

echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

answered 16 hours ago

JShorthouse

37316

New contributor

7

As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.

– IMSoP
11 hours ago

11

PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully

– IMSoP
11 hours ago

add a comment |

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true

echo $? # 0

false

echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

answered 16 hours ago

JShorthouse

37316

New contributor

7

As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.

– IMSoP
11 hours ago

11

PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully

– IMSoP
11 hours ago

add a comment |

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true

echo $? # 0

false

echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

answered 16 hours ago

JShorthouse

37316

New contributor

true in bash isn't a keyword, it's a program that instantly exits with a successful exit code.
Likewise, false is a program that exits with an unsuccessful exit code.

You can try this out by running both programs from your terminal, and then reading the $? variable, which contains the exit code of the last program;

true

echo $? # 0

false

echo $? #1

if sudo true isn't equivalent to if sudo == true. if sudo true is running the true program using sudo, and checking the exit code.

Therefore:

if sudo false; then is running the program false as sudo. The return will always be false.

if sudo true == false will run the program true with the arguments == and false using sudo. This obviously isn't want you intended.

if [!(sudo true)] is invalid syntax.

What you are probably looking for is

if ! sudo true;

answered 16 hours ago

JShorthouse

37316

New contributor

answered 16 hours ago

JShorthouse

37316

New contributor

answered 16 hours ago

JShorthouse

37316

answered 16 hours ago

JShorthouse

37316

New contributor

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

7

As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.

– IMSoP
11 hours ago

11

PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully

– IMSoP
11 hours ago

add a comment |

7

As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.

– IMSoP
11 hours ago

11

PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully

– IMSoP
11 hours ago

As a slightly pedantic correction to the first sentence: in bash, true and false are "builtins", commands interpreted directly by the shell; in Unix-like systems in general, they also exist as standalone programs on the file system. The difference doesn't matter that much here, but as far as I know, sudo true will be running the standalone program, not the bash builtin.

– IMSoP
11 hours ago

PS: my favourite summaries of true and false are the titles on their man pages: true - do nothing, successfully and false - do nothing, unsuccessfully

– IMSoP
11 hours ago

add a comment |

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

answered 8 hours ago

Mehrdad

1,25641533

Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.

– Sergiy Kolodyazhnyy
3 hours ago

1

Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)

– Mehrdad
3 hours ago

The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.

– mckenzm
2 hours ago

add a comment |

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

answered 8 hours ago

Mehrdad

1,25641533

Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.

– Sergiy Kolodyazhnyy
3 hours ago

1

Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)

– Mehrdad
3 hours ago

The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.

– mckenzm
2 hours ago

add a comment |

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

answered 8 hours ago

Mehrdad

1,25641533

I feel like the accepted answer didn't actually answer your question?

The purpose of doing this is to check that you can actually sudo.

How this check is performed is via the true program as explained in the accepted answer.

answered 8 hours ago

Mehrdad

1,25641533

answered 8 hours ago

Mehrdad

1,25641533

answered 8 hours ago

Mehrdad

1,25641533

answered 8 hours ago

Mehrdad

1,25641533

Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.

– Sergiy Kolodyazhnyy
3 hours ago

1

Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)

– Mehrdad
3 hours ago

The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.

– mckenzm
2 hours ago

add a comment |

Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.

– Sergiy Kolodyazhnyy
3 hours ago

1

Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)

– Mehrdad
3 hours ago

The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.

– mckenzm
2 hours ago

Essentially, this. Alternative could be to check if the user is in sudoers group, but just running sudo true is also easy enough of a way, although a bit hacky.

– Sergiy Kolodyazhnyy
3 hours ago

Note that this also requires sudo to be installed, so the checks wouldn't be entirely equivalent... you could be in the sudoers group and still not be able to sudo. (This can especially come up if you're e.g. writing a script to setup a new image from a minimal rootfs tarball, which might not have sudo.)

– Mehrdad
3 hours ago

The OP wants to drop out if you cannot sudo, and get rid of two lines rather than have an else following a noop.

– mckenzm
2 hours ago

add a comment |

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

draft saved

draft discarded

Matt Parkins 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

Hcfyk