Part of an answer: Every *nix has its own filesystem. Here's some examples. An arrow "->" means "was replaced by". Linux: ext->ext2->ext3
Sun Solaris: FFS->UFS
BSD: FFS
IBM AIX: JFS
HP HP-UX: HFS
SGI IRIX: EFS->XFS
Linux can read most or all of these.
Linux is far more common these days than traditional Unix. But as any old system administrator will tell you, learn to learn, don't learn the system. There are numerous differences between the different Linux distros, differences between Linux and Unix, and differences between each of the Unices. Learning everything about each system is a daunting task, and probably near impossible. Rather than, say, learning all of the different command line switches for "ls" on each system, just know how to access a man page.
Linux is an open system, Unix is not.
what are similarities and differences between linux and unix?
No, Linux IS Unix, so the question makes no sense. Unix is a general term for a class of operating systems. Linux is also a term applied to more than one operating system variant, but all Linux OS are Unix OS. Redhat is one type of Linux. BSD is a type of Unix that is not Linux.
To put it very generically, Linux is an operating system kernel, and UNIX is a certification for operating systems. The UNIX standard evolved from the original Unix system developed at Bell Labs. After Unix System V, it ceased to be developed as a single operating system, and was instead developed by various competing companies, such as Solaris (from Sun Microsystems), AIX (from IBM), HP-UX (from Hewlett-Packard), and IRIX (from Silicon Graphics). UNIX is a specification for baseline interoperability between these systems, even though there are many major architectural differences between them. Linux has never been certified as being a version of UNIX, so it is described as being "Unix-like." A comprehensive list of differences between Linux and "UNIX" isn't possible, because there are several completely different "UNIX" systems.
Command set and command switches. Both are based on the UNIX operating system, so they have the core of the operating system in common.
The 'ls' command is the same in Linux and Unix systems. System Administrators may set up an alias that has the -FC or other options but the command itself with no other parameters (ls) is just the 'ls' command.
Linux is a Unix-like system. This means that it is inspired or influenced by Unix in some shape or form (Linux started off from Minix), but it is not directly derived from Unix. However, BSD is based on Unix, and macOS is indirectly Unix-based because of its mixed heritage with BSD.
Unix came first; Linux is a clone of the Unix Operating System.
In some Unix and Linux systems there is a command called 'dos2unix' that will do the conversion automatically. If there isn't such a utility on your system you can use the 'tr' translate command to do the translation: tr -d '\015' < windows-file > unix-file which is essentially what dos2unix will do.
The 'merge' command is not standard in Unix, so not every system has it. The 'cat' command is standard in every Unix system. The 'cat' command merely echoes the contents of a file. It can be used to put together several files (concatenate). The 'merge' command (at least on Linux) merges changes from an original file to a modified (patched) file. It is considered a 3-way merge utility. Check the 'man' command for both and you will see the differences in the two commands.
unix