colcrt Command in Linux

---

---

The colcrt command in Linux is a utility that provides an ability to process text with formatting that is typically not supported by simple display terminals, such as Cathode Ray Tube (CRT) displays. This command is particularly useful when dealing with the output of text processors like nroff or troff, which may include various formatting such as underlining and overstriking that CRT displays cannot render.

The colcrt command in Linux is a utility used to process text output from other commands or programs that use overstriking for formatting purposes, such as nroff. It's particularly useful for viewing formatted documents on terminals that do not support these features.

Table of Contents

Here is an extensive tutorial on how to use the colcrt command with examples to illustrate its capabilities −

• Understanding colcrt Command
• Install colcrt Command
• How to Use colcrt Command in Linux?
• Examples of colcrt Command in Linux
• Alternatives of colcrt Command

Understanding colcrt Command

Essentially, colcrt filters the output to make it suitable for CRT displays by providing virtual half-line and reverse line feed sequences. This is especially helpful for terminals where overstriking is destructive. The command also modifies underlining by changing it to dashes and placing them on new lines between the normal output lines.

Options like --no-underlining can suppress all underlining, which is useful for previewing boxed tables from tbl. The -2 or --half-lines option can be used to print all half-lines, effectively double spacing the output. This command is part of the util-linux package and has been around since 3.0BSD, making it a longstanding tool in the Unix and Linux communities.

Install colcrt Command

To install the colcrt command in Linux, you will need to have the util-linux package which includes this utility. The colcrt command is useful for filtering nroff output for CRT previewing, providing virtual half-line and reverse line feed sequences for terminals without such capabilities.

For Ubuntu-based systems

You could use the command and download the util-linux package from the Linux Kernel Archive. Once you have the package, you can use your distribution's package manager to install it.

sudo apt-get install util-linux

Remember to check the manual page for colcrt to understand its options and usage to get the most out of this tool. If you encounter any issues or need further assistance, the Linux community is always a great resource for help and additional information.

How to Use colcrt Command in Linux?

The colcrt command in Linux is used to process text output from utilities that do not support CRT displays. It provides several options to modify the output for better readability on such displays.

Here's the syntax for the colcrt command −

colcrt [OPTION]... [FILE]...

This is an optional FILE argument specifying the file(s) containing the text you want to format. If no file is specified, colcrt reads from standard input. Here's a detailed look at the options available for the colcrt command −

Note − The colcrt command also handles virtual half-line and reverse line feed sequences for terminals that lack such capabilities, placing half-line characters and underlining (converted to dashes) on new lines between the normal output lines.

Examples of colcrt Command in Linux

Here are some examples of the colcrt command in Linux showcasing different functionalities −

Basic Usage (No Underlining)

The colcrt command in Linux is a utility that is used to process text with carriage-return and line-feed characters, making it suitable for display on terminals.

The simplest form of the colcrt command can be invoked by just specifying the file name −

colcrt data.txt

This displays the content of filename on the terminal, with unsupported formatting removed for better readability on CRT displays.

Print Half-Lines

By default, colcrt minimizes the space in the output by suppressing empty lines. The --half-lines option forces the output to include all half-lines, effectively double-spacing the output. This is particularly helpful when dealing with superscripts and subscripts that might be lost on a line printer −

colcrt --half-lines data.txt

Displays Help Text of Command's Syntax

It is particularly useful for viewing the output of nroff and other text formatting commands which include backspaces and carriage returns. Displays help text and exits. This option provides a quick reference for the command's syntax and options −

colcrt --help

Check Version of colcrt Command

Outputs the version information of the colcrt command and exits. The colcrt command also has a history of improvements and bug fixes. It originated in 2.39.3 and has since been included in various Linux distributions as part of the util-linux package −

colcrt -V 

Note − It's important to note that colcrt has some limitations, such as the inability to back up more than 102 lines and the trimming of lines to 132 characters. Additionally, general overstriking is lost, except for specific cases like '|' overstruck with '-' or underline becoming '+'.

Suppressing Underlining

If a document contains underlining, colcrt can remove it to prevent clutter on the display that doesn't support this feature. The - option tells colcrt to suppress underlining −

colcrt - data.txt

Double Spacing

The -2 option can be used to add an empty line after each line of the document, effectively double-spacing the output. This is particularly useful for documents that contain superscripts and subscripts, which might overlap in single-spaced text −

colcrt -2 data.txt

Combining with Other Commands

A typical use case for colcrt would be to filter the output of nroff for CRT previewing, which can then be piped into a pager like more for easy viewing. colcrt is often used in a pipeline with other commands −

colcrt - data.txt | more

This sequence of commands formats the table, processes it for typesetting, removes unsupported formatting, and then paginates the output for easy viewing.

Manual Page of colcrt Command

For those looking to delve deeper into the colcrt command and its capabilities, the Linux manual pages provide comprehensive information and examples that can be accessed using the man command or by visiting the online Linux Kernel Archive −

Whether you're a seasoned system administrator or a curious Linux enthusiast, understanding the colcrt command and its options can be a valuable addition to your command-line toolkit.

Alternatives of colcrt Command

However, as technology evolves, alternatives become necessary to accommodate new features or more modern interfaces.

col command

One such alternative is the col command, which can also handle reverse line feeds and provide similar formatting capabilities. Another option is ul, which translates underlining into a different character, typically an asterisk (*), for terminals that cannot display underlines.

groff and less Command

For more advanced text processing, tools like groff and less can offer additional functionality, such as pagination and horizontal scrolling, which might be useful for viewing complex documents on modern terminals.

These alternatives not only replicate the functionalities of older commands but also introduce new features to enhance the user experience on contemporary display devices.

Conclusion

The colcrt command is a testament to the versatility of Linux in handling text processing for various types of displays and printers. It ensures that even with the limitations of older technology, the output remains legible and structured. For more detailed information and options, you can refer to the colcrt manual page or other online resources.

Understanding and utilizing commands like colcrt can significantly enhance text processing and display in Linux, especially when dealing with legacy systems or specific formatting requirements. It's one of the many tools that make Linux a powerful operating system for a wide range of applications.