tac:tldr:39368

This command executes two operations: cat and tac.

1. ${cat filename}: This part uses the cat command to read the contents of the specified file (filename) and passes that output to the next part of the command using pipe (|). The ${} syntax is typically used in shell scripts to refer to the value of a variable, but in this case, it seems unnecessary since it only contains the cat command without any variables.

2. tac: This part of the command is executed after the output from cat is received through the pipe. The tac command is used to reverse the order of the lines in the input. So, it takes the output received from cat and reverses the order of the lines before displaying the result on the terminal.

In summary, the command ${cat filename} | tac reads the contents of the specified file and then prints those contents in reverse order line by line.