Running Shell Commands in PHP Using exec and shell_exec

Learn how to execute Linux shell commands in PHP using exec() and shell_exec(). Run system commands, capture output, and automate tasks securely.

Introduction

PHP can interact with the system shell to execute Linux or Windows commands, making it a powerful tool for automating server tasks, running scripts, and managing files.

With PHP, you can:

• Run shell commands dynamically using exec() and shell_exec()
• Capture command output and process it in PHP
• Execute background processes without blocking execution
• Securely run shell scripts with controlled access

This guide covers:

• Understanding exec() vs shell_exec()
• Running simple and complex shell commands in PHP
• Handling command output and errors
• Executing background tasks using PHP
• Securing PHP shell execution to avoid security risks

1. Difference Between exec() and shell_exec() in PHP

PHP provides two main functions to execute shell commands:

Function	Description	Returns
exec($command, $output, $return_var)	Executes a command, captures output in an array	Output array or last line
shell_exec($command)	Runs a command and returns entire output as a string	String output

Which One to Use?

• Use exec() when you need structured array-based output.
• Use shell_exec() when you need the entire command output as a string.

2. Running Basic Shell Commands in PHP

Example 1: Using exec() to List Files in a Directory

$output = [];
$return_var = 0;
exec('ls -la', $output, $return_var);

echo "<pre>";
print_r($output);
echo "</pre>";

What This Does:

• Executes the ls -la command in Linux
• Stores the output as an array
• Displays directory listing in PHP

Example 2: Using shell_exec() to Get System Uptime

$output = shell_exec('uptime');
echo "<pre>$output</pre>";

What This Does:

• Runs the uptime command
• Displays server uptime information

3. Capturing and Processing Command Output in PHP

To capture command output and return status, use exec().

Example: Checking Disk Usage

exec('df -h', $output, $return_var);

if ($return_var === 0) {
    echo "Command executed successfully!<br>";
    echo "<pre>" . implode("\n", $output) . "</pre>";
} else {
    echo "Error executing command!";
}

Why Use This?

• Allows error handling for failed commands
• Provides structured output for processing

4. Running Background Processes Without Blocking PHP

If a command needs to run in the background, append & at the end.

Example: Running a Shell Script in the Background

exec('bash backup_script.sh > /dev/null 2>&1 &');
echo "Backup started in the background.";

What This Does:

• Runs backup_script.sh without blocking PHP execution
• Redirects output to /dev/null to prevent PHP from waiting

5. Handling Errors and Command Failures in PHP

To detect errors when executing shell commands, use:

$output = shell_exec('invalid_command 2>&1');
echo "<pre>$output</pre>";

What This Does:

• Captures error messages from failed commands

For structured error handling, use exec():

exec('invalid_command 2>&1', $output, $return_var);

if ($return_var !== 0) {
    echo "Error: " . implode("\n", $output);
} else {
    echo "Command executed successfully!";
}

6. Running PHP Scripts from the Shell

To run a PHP script from the terminal, use:

php myscript.php

To execute a PHP script from another PHP script via shell:

$output = shell_exec('php another_script.php');
echo "<pre>$output</pre>";

This allows one PHP script to call another script dynamically.

7. Executing Bash Scripts from PHP

To run a Bash script from PHP:

$output = shell_exec('bash myscript.sh');
echo "<pre>$output</pre>";

To pass arguments:

$arg1 = "backup";
$output = shell_exec("bash myscript.sh $arg1");
echo "<pre>$output</pre>";

Why Use This?

• Enables integration of PHP with shell automation
• Allows dynamic execution of server scripts

8. Securely Running Shell Commands in PHP

Running shell commands can be dangerous if not handled properly.

Avoid User Input in Shell Commands

Never execute user-provided input directly in exec() or shell_exec().

Unsafe Example (DO NOT USE):

$filename = $_GET['file'];  
exec("rm -rf $filename"); // Dangerous, allows remote execution!

Safe Example (Use Escaping):

$filename = escapeshellarg($_GET['file']);  
exec("rm -rf $filename"); // Secured against command injection

Disable Shell Execution for Security

If shell execution is not needed, disable it in php.ini:

disable_functions = exec, shell_exec, system, passthru

Use sudo Only When Necessary

To execute privileged commands securely, configure sudoers for PHP.

sudo visudo

Add:

www-data ALL=(ALL) NOPASSWD: /path/to/script.sh

Then, execute in PHP:

$output = shell_exec('sudo bash /path/to/script.sh');
echo "<pre>$output</pre>";

9. Running Commands on Remote Servers Using SSH in PHP

To execute shell commands on a remote server, use SSH:

$server = 'user@192.168.1.100';
$command = 'ls -la';

$output = shell_exec("ssh $server '$command'");
echo "<pre>$output</pre>";

To use password authentication, install sshpass and modify:

$output = shell_exec("sshpass -p 'password' ssh user@192.168.1.100 'ls -la'");

Why Use This?

• Enables remote server management from PHP
• Useful for deployments, backups, and remote administration

Best Practices for Running Shell Commands in PHP

• Use exec() for structured output, shell_exec() for full output
• Escape user input with escapeshellarg() to prevent security risks
• Avoid running shell commands as root unless necessary
• Run background processes (&) for long-running tasks
• Securely execute commands with sudoers configuration

Conclusion

With exec() and shell_exec(), PHP can execute system commands, manage files, run background tasks, and automate server administration.

This guide covered:

• Running shell commands in PHP
• Capturing and handling command output
• Executing background processes and Bash scripts
• Implementing security best practices

By implementing these techniques, PHP applications can interact with system shells effectively and securely.