Manipulating texts: ECHO and CAT

 ECHO

$ echo line one > myfile
$ echo line two >> myfile
$ echo line three >> myfile
">" to create the file
">>" to add a line to an existing file

$ echo $VARIABLE
Displays the current contents of $VARIABLE

CAT

$ cat << EOF > myfile
> line one
> line two
> line three
> EOF
$

Note that "EOF" is not a command, but a variable chosen at your leisure.

Alternatively:

$ cat > myfile
> line one
> line two
> line three
[CTRL+D]

$ cat FILE1 FILE2
Concatenates the two files and display the output

$ cat FILE1 FILE2 > NEWFILE
Concatenates the two files and save the output into NEWFILE

$ cat FILE >> EXISTINGFILE
Append FILE to the end of EXISTINGFILE

$ head -15 FILE
Displays the first 15 lines of a file. Predefined: 10

$ tail -15 FILE
Displays the last 15 lines of a file. Predefined: 10

zat, zecho, zless, zmore, zgrep, zdiff
Same as the related commands, but for compressed files