Unlocking the Mystery of Dashed Filenames in Linux


Written by - Deepak Prasad

In this tutorial guide I will give you an overview on dashed filename and it's usage with examples to create, open, read, access, find and remove such files and directories whose name starts with dash (-) or double dash (--) as per Posix Standards

 

What is single dash (-) and double dash (--) in Posix?

As per Posix Guidelines

  • All options should be preceded by the '-' delimiter character.
  • So all the options which is provided to any tool such as the most used option --help or -h should start with single dash (-)
  • For utilities that use operands to represent files to be opened for either reading or writing, the '-' operand should be used to mean only standard input (or standard output when it is clear from context that an output file is being specified) or a file named -
  • The first -- argument that is not an option-argument should be accepted as a delimiter indicating the end of options. Any following arguments should be treated as operands, even if they begin with the '-' character.

 

How to create dashed filename and directories?

In the Linux environment, creating files and directories with dashes, especially at the beginning or end of the name, is possible, but it comes with its own set of peculiarities. Let’s delve into the methodologies and potential pitfalls of this practice.

Using touch:

The touch command is frequently used to create empty files in Linux. To create a file with a dash at the beginning or end, you'd typically enclose the name in quotes:

touch -- "-filename" # starts with a dash
touch "filename-" # ends with a dash

When creating a filename starting with dash then you have to pass -- argument which indicates the end of command options. After --, all arguments are treated as filenames and not options.

If you don't pass -- then you may get below error:

touch: invalid option -- 'i'
Try 'touch --help' for more information.

Using Redirection:

Redirection is another method to create or write to files. For filenames with dashes, the approach is similar:

echo "content" > "-filename"
echo "content" > "filename-"

Creating Directories with Dashes in Linux

Using mkdir to Create Directories: The mkdir command is used to create directories in Linux. Just like with touch, if you want to create a directory with a dash at the beginning or the end, special care is needed.To create a directory named -dirName:

mkdir -- -dirName

To create a directory named dirName-:

mkdir dirName-

 

How to open and read dashed filename?

The usual syntax from cat will not work as cat would consider (-) as an STDIN and wait for user INPUT on the screen.

# cat -

You can tweak the cat syntax to check the content of (-)

# cat < -
server1.example.com

The proper way to view content of dashed filename would be again to prefix the path of the file

# cat ./-
Mon May 18 15:02:44 IST 2020
server1.example.com

You can use any supported command such as more, less, tail, head to view the content using the path with filename

# more ./-
Mon May 18 15:02:44 IST 2020
server1.example.com

We can also use double dash (--) combined with cat or other similar commands to view a dashed filename

# echo "some content" > -file
# echo "some more content" > -file2

View the content of the file using (--)

# cat -- -file
some content

# cat -- -file2
some more content

 

How to Rename Dashed Filenames?

Working with dashed filenames in Linux, especially those beginning with a dash, requires particular attention to ensure the commands are executed as intended. Here's a guide on how to rename such files using the mv command.

Standard Rename: For filenames that have dashes but don't begin with them, you use the mv command as you would with any other file:

mv filename- newfilename

Renaming Files Beginning with a Dash: If the filename starts with a dash, it can be confused as an option for the mv command. In this case, you'll have to use a specific format to ensure it's interpreted as a filename:

mv -- -filename newfilename

The -- indicates to the mv command that there are no more options after it, and thus -filename is treated as a file and not an option.

Renaming a filename that starts with a dash to another filename that also starts with a dash can be tricky because command-line utilities might interpret the dashed name as an option. However, you can use the mv command with the ./ prefix to specify that you're referring to a file in the current directory.

For example, let's say you have a file named -oldfile.txt and you want to rename it to -newfile.txt. Here's how you can do it:

mv ./-oldfile.txt ./-newfile.txt

The ./ prefix indicates that the filename follows, ensuring that the file named -oldfile.txt in the current directory is interpreted as a filename and not as an option to the mv command.

 

How to Delete Files with Dashed Filenames?

Managing files with dashes, particularly those that commence with a dash, demands special care in Linux to evade unintended actions. The rm command, commonly used to delete files, can exhibit peculiar behavior with dashed filenames. Here’s a guide to safely remove such files.

The rm Command and its Peculiar Behavior with Dashed Filenames

Standard Deletion: If a file has dashes but doesn't start with one, you use the rm command in the usual manner:

rm filename-

Deleting Files Beginning with a Dash: Files starting with a dash can be perceived as options by the rm command. Hence, the regular deletion method may not work and can even result in an error. For example:

rm -filename

This could be mistaken by the rm command as trying to delete using the -f and -i options, followed by a file named lename, which is not what's intended.

Safely Removing Files with Dashes without Unwanted Consequences

Using the -- Convention: The -- argument indicates that there are no more options after it. It ensures that what follows is treated as a filename, not an option.

rm -- -filename

Using the ./ Prefix: Another method to safely remove files beginning with a dash is to prefix them with ./, which signifies the current directory:

rm ./-filename

 

How to Find Files with Dashed Names?

Finding files with specific patterns, such as dashed names, is a common requirement in Linux. The operating system provides a variety of tools for this purpose, most notably find and grep. Let's look into the methodologies and techniques of locating such files.

Utilizing the find Command

Finding Files with Dashes Anywhere in the Name:

find /path/to/search -type f -name "*-*"

This command searches the directory /path/to/search for all files (-type f) with dashes in their names.

Finding Files Starting with a Dash:

find /path/to/search -type f -name "-*"

Here, the pattern -name "-*" matches files that start with a dash.

Finding Files Ending with a Dash:

find /path/to/search -type f -name "*-"

This command finds files ending with a dash.

Using grep in Conjunction with Other Commands

While find is powerful for file searching, sometimes we might need the help of other commands. grep is a text search utility, and it can be combined with commands like ls for pattern searching.

Listing Files with Dashes and Filtering with grep:

ls /path/to/search | grep '^-'

This command lists all entities in /path/to/search and then filters out only those that start with a dash.

Techniques to Narrow Down Searches for Dashed Filenames

Combining Search Criteria: You can combine different search patterns with find to narrow down your results. For example, to find text files starting with a dash:

find /path/to/search -type f -name "-*.txt"

Limiting Depth of Search: If you only want to search the top directory and not delve into subdirectories, you can limit the depth:

find /path/to/search -maxdepth 1 -type f -name "*-*"

Using grep with Regular Expressions: grep can use extended regular expressions for more complex pattern matching. For example, to find files that have a dash followed by numbers:

ls /path/to/search | grep '^[^-]*-[0-9]\+'

Modified Time: To find dashed filenames modified in the last 7 days:

find /path/to/search -type f -name "*-*" -mtime -7

 

Frequently Asked Questions

Why do some filenames start with a dash?

Filenames starting with a dash can be created either mistakenly (usually a result of command misinterpretation) or intentionally for specific purposes like making the file less noticeable or harder to accidentally modify. However, it's generally not a standard or recommended practice as it can lead to command ambiguities.

How can I prevent accidental creation of files starting with a dash?

Always double-check your commands before execution. When redirecting outputs to a file, ensure there's no space between the redirection operator and the filename. If scripting, include checks to validate filenames.

Why does the command rm -filename not work?

Commands interpret text after a dash as an option. So, rm -filename might be interpreted as trying to delete using the -f option followed by a file named ilename. Use rm -- -filename or rm ./-filename to safely delete such files.

Can I create directories starting with a dash?

Yes, the same principles that apply to dashed filenames also apply to directory names. You can create them using mkdir -- -dirname or mkdir ./-dirname.

Is there a way to list only files that have dashes in their names?

Yes, you can use the find command (find /path -type f -name "*-*") or a combination of ls and grep (ls /path | grep "-").

How do I open a file with a dash at the beginning in a text editor like nano or vim?

Use the -- convention to prevent the editor from interpreting the filename as an option: nano -- -filename or vim -- -filename.

 

Best Practices

Managing files in a Linux environment can be straightforward until unconventional characters like dashes are introduced to filenames. To ensure smooth operations, here are some best practices for dealing with dashed filenames:

1. Naming Conventions to Adopt

  • Descriptive Naming: Use dashes to improve readability. For example, backup-2023-10-05.txt is more readable than backup20231005.txt.
  • Limit Front-Dash Use: Avoid creating filenames that start with a dash unless there's a specific and unavoidable reason. Files that start with a dash can easily be misconstrued as command options.
  • Use Underscores Instead: If you're concerned about compatibility and potential mishaps, consider using underscores (_) as an alternative to dashes. For instance, file_name.txt instead of file-name.txt.

2. Avoiding Names that Conflict with Command Options

  • Awareness of Common Options: Familiarize yourself with commonly used command options (like -v, -f, -r). Avoid naming files after these or, if you must, ensure the file doesn't start with these sequences.
  • Stick to Letters and Numbers Post-Dash: If you need to use dashes, try to make sure that the character immediately after the dash is a letter or a number. This minimizes the chances of it being confused with command options.
  • Test Before Deployment: If you're deploying a script or application, test its behavior with dashed filenames, especially files starting with a dash. This can help you catch potential issues in advance.

3. Encapsulating Filenames in Quotes for Safer Operations

Quote Your Filenames: Whenever referencing a dashed filename in a command, especially in scripts, encapsulate it in quotes. This helps ensure the filename is treated as a single entity.

cat "my-file.txt"

cat "my-file.txt"

Escape the Dash: Another method is to use a backslash (\) before the dash to escape it, which tells the command to interpret the next character literally:

cat my\-file.txt

Use the Full Path: When possible, use the full path or prepend the filename with ./ to specify that it's a file in the current directory. It's a clear way to distinguish between filenames and options.

rm ./-file.txt

 

Summary

Dashed filenames in Linux, while valid, can pose unique challenges, especially when the filename begins with a dash. These filenames can sometimes be interpreted as command options, leading to unexpected behavior. To navigate these challenges, it's crucial to adopt certain best practices, such as being cautious with naming conventions, understanding command conflicts, and encapsulating filenames in quotes.

 

Additional Resources

  • GNU Core Utilities Manual: This covers a range of basic command-line utilities, many of which will interact with filenames.
  • These are the built-in manuals for commands on Linux. Here are some potentially relevant ones:
    • touch: Create a file. Man Page
    • rm: Remove files or directories. Man Page
    • mv: Move or rename files. Man Page
    • find: Search for files in a directory hierarchy. Man Page
  • GNU Bash Manual: This guide covers how Bash (the default shell for many Linux distributions) interprets command-line input, which is essential when dealing with peculiar filenames.
  • Filesystem Hierarchy Standard (FHS): While not directly about dashed filenames, understanding the hierarchy can help in comprehending where and why certain files might exist.

 

Deepak Prasad

He is the founder of GoLinuxCloud and brings over a decade of expertise in Linux, Python, Go, Laravel, DevOps, Kubernetes, Git, Shell scripting, OpenShift, AWS, Networking, and Security. With extensive experience, he excels in various domains, from development to DevOps, Networking, and Security, ensuring robust and efficient solutions for diverse projects. You can reach out to him on his LinkedIn profile or join on Facebook page.

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2 thoughts on “Unlocking the Mystery of Dashed Filenames in Linux”

  1. i should think having a filename that begins with a dash might be avoided. is there a good reason to begin a filename with a dash?
    i am reminded of people who have spaces in their file name, then I am called in to put quotes around it to rename or delete it.

    Reply

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