shell_exec() not returning error when mysqldump cannot create file

Question

I want to backup the complete database before having a php script playing around with its tables so I thought about using mysqldump.

I tried to run the mysqldump command with shell_exec() and to check if it was successfull like this:

$command = 'mysqldump -u username -ppassword database > /path/database.sql';
$result = shell_exec($command);

if(!is_null($result)) {
  echo 'Error during backup';
else {
  echo 'Database saved';
  // rest of the php script to modify the database (create/drop tables etc)
}

But I noticed that $result is alwas null, even when I give a path where the file cannot be written (no existing directory, or something that should return a permission denied on the shell prompt).

What am I missing?

Iam Zesh · Accepted Answer · 2013-02-20 13:59:51Z

3

This worked for me:

$command = 'mysqldump -u username -ppassword database > /path/database.sql';
system($command, $output);

if($output != 0) {
  echo 'Error during backup';
else {
  echo 'Database saved';
  // rest of the php script to modify the database (create/drop tables etc)
}

answered Feb 20, 2013 at 13:59

Iam Zesh

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2 Comments

Álvaro González Over a year ago

It's worth noting that system()'s second argument is not the script output, it's the return value. Names aside, your code is fine but I thought I should clarify that to avoid confusion to future readers.

Tegos Over a year ago

Hi. You have syntax error on line 6 (missing bracket).

Álvaro González · Accepted Answer · 2013-02-14 10:51:50Z

1

From the manual:

Return Values
The output from the executed command or NULL if an error occurred

However, you are redirecting the script's output to a file:

mysqldump -u username -ppassword database > /path/database.sql
                                          ^^^^^^^^^^^^^^^^^^^^

It's possible (I don't really know) that mysqldump generates standard return codes (where 0 means OK and anything else means error). If that's the case, you should be using that mechanism, though you need a different PHP function:

string exec ( string $command [, array &$output [, int &$return_var ]] )
                                                   ^^^^^^^^^^^^^^^^

answered Feb 14, 2013 at 10:51

Álvaro González

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2 Comments

Iam Zesh Over a year ago

Thanks for your answer. I didn't thought that redirecting the script's output could be the source of the problem. I tried with exec but $output was somehow inconsistent and not reliable and stackoverflow.com/a/10828773 oriented me to system().

Álvaro González Over a year ago

@IamZesh - Please note I highlighted $return_var, not $output. And of course whether that will work depends on whether mysqldump generates standard return values. I see from your answer that it does.

Mohammad H. · Accepted Answer · 2014-06-30 11:00:18Z

1

You should remove all spaces between -u and -p and varialbes also use "exec()"

$command = "mysqldump -u$username -p$password database > /home/yourusername/public_html/database.sql";
exec($command,$result, $output);

if($output != 0) {
  echo 'Error during backup';
}else {
  echo 'Database saved';
  // rest of the php script to modify the database (create/drop tables etc)
}

answered Jun 30, 2014 at 11:00

Mohammad H.

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