Using echo; What is difference between > and >>

When echoing something to a file, >> appends to the file and > overwrites the file.

$ echo foobar > test
$ cat test
foobar
$ echo baz >> test
$ cat test
foobar
baz
$ echo foobar > test
$ cat test
foobar

From the example you posted, a log directory is created and then *.log is put into log/.gitignore so that no log files are committed to git. Since > was used, if a .gitignore file had existed, it would be overwritten with only *.log.

The log directory itself is then added to your local git stage.

On the next line, >> is added so that tmp is appended to the end of the .gitignore file instead of overwriting it. It is then added to the staging area.

> is a redirection operator. < > >| << >> <& >& <<- <> are all redirection operators in the shell command interpreter.

In your examples, essentially > overwrites and >> appends.

See man sh, (from your terminal you can access the manual via man sh).

Redirections
     Redirections are used to change where a command reads its input or sends its output.  In
     general, redirections open, close, or duplicate an existing reference to a file.  The over‐
     all format used for redirection is:

           [n] redir-op file

     where redir-op is one of the redirection operators mentioned previously.  Following is a
     list of the possible redirections.  The [n] is an optional number, as in '3' (not '[3]'),
     that refers to a file descriptor.

           [n]> file   Redirect standard output (or n) to file.

           [n]>| file  Same, but override the -C option.

           [n]>> file  Append standard output (or n) to file.

           [n]< file   Redirect standard input (or n) from file.

           [n1]<&n2    Duplicate standard input (or n1) from file descriptor n2.

           [n]<&-      Close standard input (or n).

           [n1]>&n2    Duplicate standard output (or n1) to n2.

           [n]>&-      Close standard output (or n).

           [n]<> file  Open file for reading and writing on standard input (or n).

     The following redirection is often called a "here-document".

           [n]<< delimiter
                 here-doc-text ...
           delimiter

     All the text on successive lines up to the delimiter is saved away and made available to the
     command on standard input, or file descriptor n if it is specified.  If the delimiter as
     specified on the initial line is quoted, then the here-doc-text is treated literally, other‐
     wise the text is subjected to parameter expansion, command substitution, and arithmetic
     expansion (as described in the section on "Expansions").  If the operator is "<<-" instead
     of "<<", then leading tabs in the here-doc-text are stripped.