How to use the command 'unexpand' (with examples)

The unexpand command is a utility from the GNU core utilities package that is used primarily for converting spaces into tab characters in a given text file or input stream. This can be particularly useful when working with files that adhere to specific formatting standards or require reduced file sizes, as tabs typically take up less space than multiple space characters.

Use case 1: Convert blanks in each file to tabs, writing to stdout

Code:

unexpand path/to/file

Motivation:

The primary reason for converting spaces to tabs in a file is often to compact the layout and reduce the overall file size. This is immensely helpful when dealing with large text files where consistency and storage efficiency are required. Using stdout allows us to view the changes immediately in the terminal without altering the original file, enabling a quick review before potentially making any permanent modifications.

Explanation:

• unexpand is the core command used here.
• path/to/file specifies the path to the file whose spaces you wish to convert into tabs.

Example Output:

Assume path/to/file contains:

This is    a    text    with    spaces

After running the command, the output on the terminal (stdout) might look like:

This is  a   text    with    spaces

The space sequences have been partly replaced by tabs, depending on default tab settings.

Use case 2: Convert blanks to tabs, reading from stdout

Code:

unexpand

Motivation:

When working with data streams or piping data between programs, there’s often a need to transform the input on-the-fly without having a static file saved on the disk. This approach allows programs to pass formatted data in a streamlined manner using the pipeline features of the shell.

Explanation:

• unexpand by itself reads from standard input, therefore, allows for dynamic input transformation.
• By executing this command, the program expects data to be piped in or typed into the terminal directly.

Example Output:

Typing directly or piping data:

$ echo -e "This is    a    sentence" | unexpand
This is  a   sentence

Here the spaces have been converted into tabs in the output.

Use case 3: Convert all blanks, instead of just initial blanks

Code:

unexpand -a path/to/file

Motivation:

In certain contexts, it’s crucial that all spaces in a file, not only the leading ones, are preserved and replaced with tabs. This becomes imperative when uniformity is needed across all text for correct alignment or formatting when opening the file in different text editors or environments.

Explanation:

• The -a option tells unexpand to convert all spaces to tabs across the entire input instead of just at the beginning of each line.
• path/to/file indicates the specific file you wish to modify.

Example Output:

Given the file:

This is    a sample        text.

Output after unexpand -a:

This    is  a   sample  text.

All groups of spaces are replaced by tabs.

Use case 4: Convert only leading sequences of blanks (overrides -a)

Code:

unexpand --first-only path/to/file

Motivation:

In some coding standards, only leading spaces need to be converted to tabs for maintaining indentation while leaving spaces within lines untouched. This can be advantageous when editing source code files that must comply with specific indentation requirements but have inline spaces maintained for aesthetic reasons.

Explanation:

• The --first-only option forces unexpand to convert only leading spaces to tabs, ensuring internal line formatting is retained.
• path/to/file directs which file to apply these changes to.

Example Output:

Assuming a file content of:

    Indented line with spaces in between.

Output after running the command:

    Indented line with spaces in between.

Here, only the spaces at the start are replaced by tabs.

Use case 5: Have tabs a certain number of characters apart, not 8 (enables -a)

Code:

unexpand -t number path/to/file

Motivation:

Different systems or coding standards might require custom tab stops. By specifying the number of character spaces per tab stop, unexpand can be customized to match these settings. This ensures compatibility and consistent formatting across diverse platforms, reducing issues arising from handling text with improper tab lengths.

Explanation:

• The -t number option allows setting the width of tab stops. Replace number with the desired tab width.
• Providing just -t without -a automatically enables -a, changing all spaces throughout the file.

Example Output:

Consider a file with:

Words        spaced with exact stops.

When running unexpand -t 4 path/to/file, assuming number was 4:

Words   spaced  with    exact   stops.

Given each tab is set to 4 spaces, the conversion of spaces to tabs reflects this alignment.

Conclusion:

The unexpand command offers a powerful means of transforming spaces into tabs in data processing and file formatting. Each of its options serves distinct purposes: from optimizing file space by reducing character count to formatting code-related files according to specific indentation guidelines. By choosing the appropriate options, users can tailor the conversion process to fit their particular needs seamlessly, ensuring consistency and efficiency in their workflow.