Why Does `Runtime.exec(String)` Sometimes Fail to Execute Commands Correctly?

Why Does Runtime.exec(String) Behave Differently for Certain Commands?

Runtime.exec(String) allows the execution of commands in the environment of the underlying operating system. However, while it works seamlessly for some commands, issues arise with others.

Why the Difference?

The key difference stems from the fact that the commands are not executed in a shell environment. The shell provides essential services that are not carried out by Runtime.exec(String), leading to errors.

When Does Failure Occur?

Commands fail when they rely on shell features, such as:

• Correct splitting of quotes and spaces: Runtime.exec(String) naively splits on spaces, which can break up arguments like "My File.txt" into separate filenames.
• Expanding globs and wildcards: The shell rewrites patterns like *.doc into the actual files. Runtime.exec(String) doesn't perform this expansion.
• Handling pipes and redirections: The shell sets up output redirection, which Runtime.exec(String) does not handle.
• Expanding variables and commands: The shell replaces shell variables and commands before executing the command. Runtime.exec(String) passes them literally.

Solutions

There are two approaches to handling this situation:

Delegating to a Shell (Simple but Sloppy):

Passing commands to a shell (e.g., using Runtime.exec(String[])):

String myCommand = "cp -R '" + myFile + "' $HOME 2> errorlog";
Runtime.getRuntime().exec(new String[] { "bash", "-c", myCommand });

Taking on Shell Responsibilities (Secure and Robust):

Using ProcessBuilder to handle shell-like tasks, such as variable expansion, redirection, and word splitting:

String myFile = "some filename.txt";
ProcessBuilder builder = new ProcessBuilder(
    "cp", "-R", myFile,
    System.getenv("HOME")
);
builder.redirectError(ProcessBuilder.Redirect.to(new File("errorlog")));
builder.start();

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