The tee command reads from standard input and writes to both standard output and one or more files1. Therefore, the command myapp | tee file1.log will send the output of the program myapp to both the terminal and the file file1.log. The other commands will either fail to write to both standard output and file, or write the wrong output. For example, the command cat < myapp | cat > file1.log will try to read the file myapp as input and write it to the file file1.log, but it will not display anything on the terminal. The command myapp 0>&1 | cat > file1.log will redirect the standard input of myapp to its standard output, and then pipe it to the file file1.log, but it will not display anything on the terminal. The command myapp | cat > file1.log will pipe the output of myapp to the file file1.log, but it will not display anything on the terminal. The command tee myapp file1.log will try to read from standard input and write to both the file myapp and the file file1.log, but it will not execute the program myapp. References:

[LPI Linux Essentials - 1.3 Basic Editing]

[LPI Linux Essentials - 1.4 I/O Redirection]

[LPI Linux Essentials - 1.5 Manage Simple Partitions and Filesystems]