The find command is a powerful utility available on Unix-like operating systems used for searching files and directories based on specified criteria. While it provides great flexibility, running this command can sometimes result in unwanted error messages such as "permission denied." These messages typically arise when the user does not have read access to certain directories or files during the search process. In scenarios where you wish to suppress these specific errors without losing other potential error information, knowing how to effectively manage standard output and error streams is crucial.
Understanding Standard Streams
Unix-like systems handle three primary data streams: standard input (stdin), standard output (stdout), and standard error (stderr). By default:
stdout(file descriptor 1) captures the regular command output.stderr(file descriptor 2) captures any error messages generated by commands.
When using the find command, both regular results and permission denied errors can appear on stdout or be redirected to stderr, depending on how you structure your command pipeline.
Suppressing "Permission Denied" Errors
To exclude "permission denied" messages while executing the find command, follow these steps:
Method 1: Redirect All Errors to /dev/null
The simplest approach is to redirect all error messages to /dev/null, effectively silencing them:
find . 2> /dev/null > files_and_folders
Here:
2>redirects standard error (stderr) to/dev/null.- The regular output of
findis redirected to a file namedfiles_and_folders.
Note: This method suppresses all errors, not just "permission denied."
Method 2: Filter Specific Errors
If you prefer to keep other potential errors while only filtering out "permission denied" messages, employ the following technique using pipes and grep:
find . 2>&1 | grep -v 'Permission denied' > files_and_folders
Explanation:
2>&1: Combines both standard output and standard error streams.- The combined stream is piped to
grep, which filters out lines containing "Permission denied." - Only the desired output (non-filtered) is then redirected to
files_and_folders.
Method 3: Advanced Stream Management
For a more controlled approach that distinctly handles errors, consider this complex redirection:
find . 2>&1 > files_and_folders | grep -v 'Permission denied' >&2
Detailed Breakdown:
find . 2>&1: Redirects both standard output and error to be piped into the subsequent command.- The main stream is directed to
files_and_folders. grepprocesses the content, excluding lines with "Permission denied."- The filtered result (now as an error) is sent back to
stderrusing>&2.
This technique ensures that only non-error outputs are captured in your file while maintaining other possible errors visible on standard error.
Considerations
While these methods are generally applicable across Unix-like systems, remember that specific versions of the find command (e.g., GNU find) may offer additional options for error handling. Always check the documentation available to you with man find or similar commands if you need version-specific functionality.
Moreover, when scripting, always ensure error messages are directed correctly so that logs and debugging processes remain effective. Properly managing output streams is key to creating robust scripts in Unix environments.
Conclusion
Mastering output redirection and filtering techniques allows for greater control over the command-line tools we use daily. Whether you’re automating tasks or simply organizing directory contents, suppressing unwanted "permission denied" messages can enhance both usability and clarity in your outputs.