Removing a newline character at the end of a file

A simpler solution than the accepted one:

truncate -s -1 <<file>>

From the truncate man page (man truncate):

-s, --size=SIZE
    set or adjust the file size by SIZE
SIZE may also be prefixed by one of the following modifying characters:
    '+' extend by, '-' reduce by, '<' at most, '>' at least, '/' round down
    to multiple of, '%' round up to multiple of.

If you are sure the last character is a new-line, it is very simple:

head -c -1 days.txt

head -c -N means everything except for the last N bytes

Take advantage of the fact that a) the newline character is at the end of the file and b) the character is 1 byte large: use the truncate command to shrink the file by one byte:

# a file with the word "test" in it, with a newline at the end (5 characters total)
$ cat foo 
test

# a hex dump of foo shows the '\n' at the end (0a)
$ xxd -p foo
746573740a

# and `stat` tells us the size of the file: 5 bytes (one for each character)
$ stat -c '%s' foo
5

# so we can use `truncate` to set the file size to 4 bytes instead
$ truncate -s 4 foo

# which will remove the newline at the end
$ xxd -p foo
74657374
$ cat foo
test$ 

You can also roll the sizing and math into a one line command:

truncate -s $(($(stat -c '%s' foo)-1)) foo