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 10 hours ago

Matt Parkins

1645

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
9 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♦
9 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
9 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
9 hours ago

1

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

– Toby Speight
7 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 10 hours ago

Matt Parkins

1645

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
9 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♦
9 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
9 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
9 hours ago

1

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

– Toby Speight
7 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 10 hours ago

Matt Parkins

1645

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 10 hours ago

Matt Parkins

1645

New contributor

asked 10 hours ago

Matt Parkins

1645

New contributor

asked 10 hours ago

Matt Parkins

1645

New contributor

asked 10 hours ago

Matt Parkins

1645

asked 10 hours ago

Matt Parkins

1645

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
9 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♦
9 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
9 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
9 hours ago

1

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

– Toby Speight
7 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
9 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♦
9 hours ago

1

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
9 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
9 hours ago

1

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

– Toby Speight
7 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
9 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♦
9 hours ago

see stackoverflow.com/q/10552711/537980

– ctrl-alt-delor
9 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
9 hours ago

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

– Toby Speight
7 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 9 hours ago

JShorthouse

32316

New contributor

3

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
5 hours ago

5

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
5 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 2 hours ago

Mehrdad

1,22641533

add a comment |

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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 9 hours ago

JShorthouse

32316

New contributor

3

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
5 hours ago

5

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
5 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 9 hours ago

JShorthouse

32316

New contributor

3

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
5 hours ago

5

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
5 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 9 hours ago

JShorthouse

32316

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 9 hours ago

JShorthouse

32316

New contributor

answered 9 hours ago

JShorthouse

32316

New contributor

answered 9 hours ago

JShorthouse

32316

answered 9 hours ago

JShorthouse

32316

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.

3

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
5 hours ago

5

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
5 hours ago

add a comment |

3

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
5 hours ago

5

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
5 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
5 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
5 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 2 hours ago

Mehrdad

1,22641533

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 2 hours ago

Mehrdad

1,22641533

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 2 hours ago

Mehrdad

1,22641533

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 2 hours ago

Mehrdad

1,22641533

answered 2 hours ago

Mehrdad

1,22641533

answered 2 hours ago

Mehrdad

1,22641533

answered 2 hours ago

Mehrdad

1,22641533

add a comment |

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

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