Print all complete lines of file

If all you want to do is add text to the last line, it's very easy with sed. Replace $ (pattern matching at the end of the line) by the text you want to add, only on lines in the range $ (which means the last line).

sed '$ s/$/ and Linux/' <file >file.new &&
mv file.new file

which on Linux can be shortened to

sed -i '$ s/$/ and Linux/' file

If you want to remove the last byte in a file, Linux (more precisely GNU coreutils) offers the truncate command, which makes this very easy.

truncate -s -1 file

A POSIX way to do it is with dd. First determine the file length, then truncate it to one byte less.

length=$(wc -c <file)
dd if=/dev/null of=file obs="$((length-1))" seek=1

Note that both of these unconditionally truncate the last byte of the file. You may want to check that it's a newline first:

length=$(wc -c <file)
if [ "$length" -ne 0 ] && [ -z "$(tail -c -1 <file)" ]; then
  # The file ends with a newline or null
  dd if=/dev/null of=file obs="$((length-1))" seek=1
fi

A file does not end with an End of File character, as the previous answers correctly state. But I think the answers and comments contain some inaccuracies worth pointing out:

• The ASCII character set does not contain an exact EOF character. There are several "end" control characters: End of Text (3), End of Transmission (4), End of Transmission Block (23), End of Medium (25). File Separator (28) maybe comes closest to an EOF character. Code 26 is "Substitute", not EOF.

• Ctrl-D is only associated with terminal input. For example the command cat filea fileb filec > outfile does not involve Ctrl-D. By the way, you can change the terminal EOF character to something else than Ctrl-D using the stty command.

• Strictly speaking, Ctrl-D (or whatever you have changed to) is not an EOF key code. What it does is make the read system call return with what input is available, just like pressing return makes the read system call return a line of characters to the caller. By convention a return value of zero from the read system call (i.e. zero characters read) signals an end of file condition. However, the input file is not closed automatically, and, if the input comes from the terminal, it is not put in an "end of file" state. You can write a program that continues reading from the terminal even after an "end of file" and the read call can return non-zero for the next input line.

• The analogy between the eof and eol characters can be seen if Ctrl-D is pressed when some input has already been written on the line. For example, if you write "abc" and the press Ctrl-D the read call returns, this time with a return value of 3 and with "abc" stored in the buffer passed as argument. Because read does not return 0, this is not interpreted as an EOF condition by the convention above. Similarly, pressing return to makes the read call return with the whole input line (including newline). You can try this out with the cat command: write some characters on the line and press Ctrl-D. You'll see the characters echoed back to you and cat waiting for more input.

• All the above only applies when the terminal is in the "cooked" mode, as opposed to "raw" mode, in which line input processing is minimized. In raw mode a Ctrl-D character really is delivered to the input buffer.