Inspect X Colour Names Safely with showrgb

showrgb turns the X colour-name database into plain text so you can read, search or diff it like any other file. The examples use the installed showrgb from x11-xserver-utils version 7.7+10build2; its manual identifies the underlying RGB data format as rgb 1.1.0.

Allow about ten minutes. You need a shell and the package installed. This guide only reads files and writes redirected output where you explicitly choose a destination. It does not change X configuration, install a colour database or require elevated privileges.

1. Confirm the command and package

Start by checking the executable and package version. These are ordinary read-only commands:

$ command -v showrgb
/usr/bin/showrgb
$ dpkg-query -W -f='${Package} ${Version}\n' x11-xserver-utils
x11-xserver-utils 7.7+10build2

The exact package version will vary between distributions. Keep the version beside any bug report or repeatable build note: the installed package determines which database layout and command behaviour you are testing.

2. Print the default database

Run showrgb without an argument:

$ showrgb | head -n 5
255 250 250              snow
248 248 255              ghost white
248 248 255              GhostWhite
245 245 245              white smoke
245 245 245              WhiteSmoke

The command converts the database back into source form and writes it to standard output. Each useful row contains red, green and blue values followed by a colour name. The output can contain more than one spelling for the same RGB triplet, as the paired ghost white/GhostWhite and white smoke/WhiteSmoke entries show.

The manual documents /usr/share/X11/rgb as the default database. Check that path directly if you need to establish what is present on this machine:

$ test -r /usr/share/X11/rgb.txt && echo readable
readable
$ showrgb /usr/share/X11/rgb.txt | cmp -s - <(showrgb)
$ printf 'comparison status: %s\n' "$?"
comparison status: 0

The installed database is commonly stored as rgb.txt, while the manual's default name omits the suffix. The command itself accepts the database name without .txt, .pag or .dir. On this host, passing the text file explicitly produces the same 753 rows as the default invocation.

3. Save a copy without losing the terminal output

Pipe the output to tee when you want to inspect it and save it at the same time:

$ showrgb | tee /tmp/x-rgb.txt | head -n 3
255 250 250              snow
248 248 255              ghost white
248 248 255              GhostWhite

Verify the saved copy independently:

$ wc -l /tmp/x-rgb.txt
753 /tmp/x-rgb.txt
$ sed -n '1,3p' /tmp/x-rgb.txt

This example writes only under /tmp. If you choose a project or shared directory instead, check its permissions and available space first. Do not redirect over an existing reference file unless that replacement is deliberate: a plain > truncates its destination before showrgb has finished.

4. Read a custom database

The optional argument selects another database. Use an explicit temporary copy when testing a change so the system database stays untouched:

$ cp /usr/share/X11/rgb.txt /tmp/my-rgb.txt
$ printf '  1   2   3\tcustom blue\n' >> /tmp/my-rgb.txt
$ showrgb /tmp/my-rgb.txt | tail -n 3
144 238 144              light green
144 238 144              LightGreen
  1   2   3              custom blue

showrgb reads the file and emits the entries in source form; it does not install the file or alter the default database. The appended line demonstrates the format this database expects: three numeric colour components, whitespace, then a name. Preserve the source file if another program depends on it, and use a new path for experiments.

There is no persistent change to undo in this example. When you have finished inspecting the temporary file, remove that specific file through your normal cleanup process. Keep the original /usr/share/X11/rgb.txt intact.

5. Diagnose a database that will not open

A missing or unreadable argument produces an error and a non-zero exit status:

$ showrgb /tmp/showrgb-does-not-exist
showrgb:  unable to open rgb database "/tmp/showrgb-does-not-exist"
$ printf 'exit status: %s\n' "$?"
exit status: 1

The process ID is not involved in this diagnostic. Check the exact path, existence and read permission instead:

$ ls -l /path/to/rgb.txt
$ test -r /path/to/rgb.txt && echo readable

If the file exists but is not at the path you passed, correct the argument rather than adding sudo. Elevated privileges do not repair a typo. Use sudo only when the file's permissions genuinely require it and your system policy permits that access; reading the standard database normally works as an ordinary user.

Remember that the argument is a database path, not a colour name. showrgb navy asks the program to open a file called navy; it does not look up the name. To search the default output, filter after the command instead:

$ showrgb | grep -i '[[:space:]]navy$'
0   0 128              navy