tr:tldr:0e1c7

The command tr -s '${input_characters}' < ${filename} uses the tr command in the Unix shell to perform character translation or deletion.

Here is a breakdown of each part of the command:

• tr is a command used to translate or delete specified characters in the input stream.
• -s is an option that tells tr to squeeze repeated occurrences of a character into a single occurrence. It replaces each sequence of repeated characters with a single occurrence.
• ${input_characters} is a variable that holds the characters you want to squeeze or delete.
• < ${filename} is the input redirection symbol (<) followed by the name of the file (${filename}) from which the input is obtained. This means that the command reads the content of the specified file as input.

In summary, this command will take the content of the input file (${filename}), and it will squeeze repeated occurrences of the characters specified in ${input_characters}.