e2undo:tldr:45a8d

The command you mentioned is: e2undo -z ${filename-e2undo} ${path-to-undo_file} ${-dev-sdXN}.

Here is an explanation of each part of the command:

• e2undo: This is the name of the command or program being executed. It is likely a utility for managing ext2/3/4 filesystem undo logs.
• -z: This is an option or flag provided to the e2undo command. The specific meaning of this flag depends on the implementation of the e2undo tool, but usually, it signifies some kind of compression or decompression. It could be used to compress the output or the undo file being created, for example.
• ${filename-e2undo}: This is a variable that is being used to define the name of the undo file. It is using a parameter expansion syntax where if the variable filename is not set or empty, it defaults to the value "e2undo". The actual filename will depend on the value assigned to the filename variable.
• ${path-to-undo_file}: This is another variable that represents the path or directory where the undo file will be created. The value of this variable will depend on how it is defined or set.
• ${-dev-sdXN}: This seems like a placeholder for specifying a device or partition on which the undo file will be created. The exact value will depend on the specific device or partition you want to target. The "sdXN" part suggests it is referring to a specific device node and partition, where "sd" represents a SCSI disk, "X" refers to the letter assigned to the disk (such as "a" or "b"), and "N" indicates the partition number.

In summary, the given command is likely used to invoke the e2undo program with certain options and parameters, such as compressing the output, specifying the name and path of the undo file, and selecting a specific device or partition for the undo operation. The actual values of the variables and options will vary based on the specific usage context.