tr:tldr:67ef4

This command is using the tr command utility to convert all lowercase characters to uppercase characters in a file specified by ${filename}.

Here is a breakdown of the command:

• tr: This is the command utility used for character translation.
• "[:lower:]": This is a character class that represents all lowercase letters in the current locale. It specifies the characters to be replaced.
• "[:upper:]": This is another character class that represents all uppercase letters in the current locale. It specifies the characters to be used for replacement.
• < ${filename}: This is input redirection, it sets the standard input stream for the tr command to the contents of the file specified by ${filename}.

So, the command takes the contents of the specified file, finds all lowercase letters, and replaces them with their corresponding uppercase letters. The output is then displayed on the command line.