sponge:tldr:6b029

The command cat ${filename} | sponge -a ${filename} is a Unix/Linux command that performs the following operations:

1. ${filename} is a placeholder for the name of a file. It can be replaced with an actual file name.

2. cat is a command used to concatenate and display the contents of a file on the standard output (usually the terminal). In this context, it is used to read the contents of the file specified by ${filename}.

3. The | (pipe) symbol is a way to connect the output of one command to the input of another. It redirects the output of cat as the input for the next command (sponge in this case).

4. sponge is a command-line utility that reads data from its input and writes it to a file. In this command, sponge reads the contents provided by cat and writes them back to the same file (specified by ${filename}).

5. The -a option in sponge -a ${filename} tells sponge to append the input data to the file instead of overwriting it completely. This means the contents of the file will be preserved, and the new data read from cat will be added at the end.

In summary, this command reads the contents of the file ${filename}, appends the data from cat (the contents of the file) to the same file, and preserves the existing contents.