mjhjmtu

Why can the last command line in the shell script (uptime below) be executed successfully without the newline at the end of the file?

For example:

root@jcdb:/tmp# 
root@jcdb:/tmp# 
root@jcdb:/tmp# cat dp
date
pwd
uptimeroot@jcdb:/tmp# 
root@jcdb:/tmp# 
root@jcdb:/tmp# 
root@jcdb:/tmp# ./dp
Wed Jun 6 16:27:26 EAT 2018
/tmp
 4:27pm up 1 day, 17:25, 2 users, load average: 0.39, 0.40, 0.41
root@jcdb:/tmp# 
root@jcdb:/tmp#

When the shell reads the shell script, its scanning code will accept the last word at the end of the script even if there is no terminating newline.

This is the effect of the first rule in the POSIX standard regarding how the shell should recognize tokens in its input:

If the end of input is recognized, the current token (if any) shall be delimited.

The fact that it say that the current token is delimited here means that the last word in the script, along with the rest of that non-terminated line, will be accepted as input to the interpreter rather than causing an error or putting the shell into an undefined state.

In comments, a reference is made to the vi editor. A standard vi editor is allowed to edit a file whose last line is not terminated by a newline. It will, however, terminate the last line of the buffer with a newline when writing it to file (except for when saving an empty file, in which case adding a newline is explicitly not allowed). This is also true for a standard ex editor:

INPUT FILES

Input files shall be text files or files that would be text files except for an incomplete last line that is not longer than LINE_MAX-1 bytes in length and contains no NUL characters. By default, any incomplete last line shall be treated as if it had a trailing <newline>. The editing of other forms of files may optionally be allowed by ex implementations.

The vim editor has a setting, eol, which determines whether it should preserve the non-terminated last line or not. The default behaviour is to terminate the last line. See :help eol in vim.

The input to the standard ed editor is supposed to be a text file, which means that what happens if one is editing a file whose last line in not properly terminated (which makes it not a text file) is technically unspecified. On OpenBSD, the ed editor will say "newline appended" when opening a non-terminated text file.

INPUT FILES

The input files shall be text files.