How Can I Reliably Execute Shell Commands with Java's Runtime?

Manipulating Shell Behavior in Java Runtime

When using Java's Runtime.exec(String) utility, discrepancies can arise between executing certain shell commands. To understand these anomalies, we must delve into the mechanics of shell operations.

Understanding Shell Responsibilities

Shells perform crucial tasks that support command execution, including:

• Quote and Space Handling: Shells ensure accurate parsing of arguments, preventing spaces from breaking apart strings like "My File.txt."
• Glob Expansion: Globs (*, ?) are expanded into specific filenames before execution.
• Redirection Management: Shells manage pipes (|) and redirections (>) for command output and input.
• Variable and Command Expansion: Variables like $HOME can be expanded, and commands like $(pwd) can be executed and interpolated into the command string.

Causes of Execution Failures

Shell-dependent commands may fail when invoked through Runtime.exec(String) because:

• Lack of Shell Functionality: Runtime.exec(String) executes commands without invoking a shell, which means it lacks the shell's processing capabilities.

Workarounds

Simple But Imperfect:

Using Runtime.exec(String[]) and passing the command to a shell interpreter allows for direct command execution:

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

Robust and Secure:

For increased security and robustness, handle shell responsibilities explicitly using ProcessBuilder:

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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