Using Colors With the watch Command

1. Introduction

The watch command provides options for displaying colors to make the text more readable.

In this tutorial, we’ll learn several ways to use those options. We’ll first consider the examples of watching various Linux commands. After that, we’ll look at the limitations of the watch command and consider a possible alternative.

2. Use the -c Option

To be able to watch the command while preserving any colors, we need to add the -c option by running the watch -c ‘<command>’.

Let’s look at this command in detail:

• the -c option interprets the color and style sequences
• ‘<command>’  is the command to watch

It’s important to put the command in single quotes as otherwise, some color codes may get mixed up.

For example, let’s consider watching the echo command with the ANSI codes to print Hello World in red color:

$ watch -c 'echo -e "\033[31mHello World\033[0m"'

After running the above, we should be able to watch the Hello World text written in red.

3. Use Specific Program Color Flags

For many commands, we should not only add the -c option, but also ensure specific color flags within the content remain. That’s because when we use watch, programs can detect they’re not writing to a terminal and could strip the color.

Let’s consider two examples where this is the case: the ls command, and the git diff command.

3.1. Watch the ls Command

The ls command allows listing files and directories. To make its output color-formatted, we need to use the –color flag. So, the overall command would have two flags for coloring:

$ watch -c 'ls --color'

Now, we should be able to watch the files and directories in a colored fashion.

3.2. Watch the git diff Command

Similarly, we should add the –color=always flag of the git diff command:

$ watch -c 'git diff --color=always'

Here, the output of this command should be a colored version of the usual git diff command.

4. Replace watch With while

There can be times when a command doesn’t provide color flags. Let’s consider the pygmentize tool which specializes in printing file contents in color. It would be problematic to color-watch its output because it doesn’t provide the color flags we used above.

When there are no specific flags to preserve colors, we can avoid the watch command and replace it with the following while loop, which uses the terminal as its standard output:

$ while sleep <time>; do clear; <command>; done

Here, <time> is the time interval between the command runs, while <command> is the command we want to watch.

For example, to watch the pygmentize <file_to_print> command every second we need to run the following line:

$ while sleep 1; do clear; pygmentize <file_to_print>; done

This shows the colored output of the pygmentize command every second.

5. Conclusion

In this article, we learned how to use the watch command to preserve the output colors. We firstly looked at the echo, the ls, and the git diff commands as examples. Finally, we learned an alternative to the watch command to avoid some of its color-flag limitations.