Overwrite Line in File with PHP

Question

What is the best way to overwrite a specific line in a file? I basically want to search a file for the string '@parsethis' and overwrite the rest of that line with something else.

Answer 1

If the file isn't too big, the best way would probably be to read the file into an array of lines with file(), search through the array of lines for your string and edit that line, then implode() the array back together and fwrite() it back to the file.

Answer 2

If the file is really big (log files or something like this) and you are willing to sacrifice speed for memory consumption you could open two files and essentially do the trick Jeremy Ruten proposed by using files instead of system memory.

$source='in.txt';
$target='out.txt';

// copy operation
$sh=fopen($source, 'r');
$th=fopen($target, 'w');
while (!feof($sh)) {
    $line=fgets($sh);
    if (strpos($line, '@parsethis')!==false) {
        $line='new line to be inserted' . PHP_EOL;
    }
    fwrite($th, $line);
}

fclose($sh);
fclose($th);

// delete old source file
unlink($source);
// rename target file to source file
rename($target, $source);

Answer 3

Your main problem is the fact that the new line may not be the same length as the old line. If you need to change the length of the line, there is no way out of rewriting at least all of the file after the changed line. The easiest way is to create a new, modified file and then move it over the original. This way there is a complete file available at all times for readers. Use locking to make sure that only one script is modifying the file at once, and since you are going to replace the file, do the locking on a different file. Check out flock().

If you are certain that the new line will be the same length as the old line, you can open the file in read/write mode (use r+ as the second argument to fopen()) and call ftell() to save the position the line starts at each time before you call fgets() to read a line. Once you find the line that you want to overwrite, you can use fseek() to go back to the beginning of the line and fwrite() the new data. One way to force the line to always be the same length is to space pad it out to the maximum possible length.

Answer 4

or if your file isn't too big:

$sample = file_get_contents('sample');
$parsed =preg_replace('#@parsethis.*#', 'REPLACE TO END OF LINE', $sample);

You'll have to choose delimiters '#' that aren't present in the file though.

Answer 5

This is a solution that works for rewriting only one line of a file in place with sed from PHP. My file contains only style vars and is formatted: $styleVarName: styleVarProperty;\n
For this I first add the ":" to the ends of myStyleVarName, and sed replaces the rest of that line with the new property and adds a semicolon. Make sure characters are properly escaped in myStyleVarProp.

$command = "pathToShellScript folder1Name folder2Name myStyleVarName myStyleVarProp";
shell_exec($command);

/* shellScript */
#!/bin/bash
file=/var/www/vhosts/mydomain.com/$1/$2/scss/_variables.scss
str=$3"$4"
sed -i "s/^$3.*/$str;/" $file

Answer 6

If you want to completely replace the contents of one file with the contents of another file you can use this:

rename("./some_path/data.txt", "./some_path/data_backup.txt");
rename("./some_path/new_data.txt", "./some_path/data.txt");

So in the first line you backup the file and in the second line you replace the file with the contents of a new file.

As far as I can tell the rename returns a boolean. True if the rename is successful and false if it fails. One could, therefore, only run the second step if the first step is successful to prevent overwriting the file unless a backup has been made successfully. Check out:

https://www.php.net/manual/en/function.rename.php

Hope that is useful to someone.

Cheers

Adrian

Answer 7

I'd most likely do what Jeremy suggested, but just for an alternate way to do it here is another solution. This has not been tested or used and is for *nix systems.

$cmd = "grep '@parsethis' " . $filename;
$output = system($cmd, $result);
$lines = explode("\n", $result);
// Read the entire file as a string
// Do a str_repalce for each item in $lines with ""