[SOLVED] Detecting Whitespaces in Shell Scripts (Bash,Korn etc.)

metallica1973 · 10-13-2011, 10:18 AM

I cring when I say this but when using notepad in windows, if you have content in the file such as a list of ip address and you hit "control a" on your keyboard, it will show the whitespaces in blue. Is there a way in linux to detect unnecessary white spaces in a file that could cause issues when creating shell scripts? I ran into this before from a previous post:

http://www.linuxquestions.org/questions/programming-9/expect-function-help-using-bash-906440/

Any tricks?

Regards

tronayne · 10-13-2011, 10:39 AM

Here's a bunch of ways to do that in vim: http://vim.wikia.com/wiki/Highlight_unwanted_spaces.

Hope this helps some.

unSpawn · 10-13-2011, 10:51 AM

[EDIT]Apparently I did read the OP wrong so the text below doesn't apply, sorry.[/EDIT]

in BASH you could for instance:

Code:

 ~]$ whitespace() { [ "${1}" = "${1// /}" ] || echo "Got whitespace"; }
 ~]$ whitespace "RIP Dennis Ritchie"
Got whitespace

or

Code:

 ~]$ hasSpace() { A="$1"; B=(${A}); [ ${#B} -eq ${#A} ] || echo "Got whitespace"; }
 ~]$ hasSpace "RIP Dennis Ritchie"
Got whitespace

or

Code:

 ~]$ A="RIP Dennis Ritchie"; IFS=' '; set -- $A; echo $(( $#-1 ))
2

or really lame

Code:

 ~]$ hasSpace() { echo "$1"|grep -q "[[:blank:]]" && echo "Got whitespace"; }
 ~]$ hasSpace "RIP Dennis Ritchie"
Got whitespace

Telengard · 10-13-2011, 10:57 AM

Quote:

Originally Posted by metallica1973

"control a" on your keyboard

Ctrl-a is the Windows shortcut for select all (not just spaces). It works in most Windows applications.

On Linux, Keyboard shortcuts may do different things, or nothing at all, in different applications. Ctrl-a seems to work pretty much the same in Kate on Kubuntu as it does on Windows. In Emacs it moves the cursor to the beginning of the current line (ie the most recent character directly preceded by a newline). Different distros may do things differently too, so YMMV.

Without knowing anything about the application you're using to compose your shell scripts it is a little impossible to guess how you are getting extra spaces, or how to detect them. If you really like Microsoft's Notepad, you might look into using something like Notepad++.

HTH

metallica1973 · 10-13-2011, 11:38 AM

Awesome,

thanks

grail · 10-13-2011, 12:07 PM

I would also question the use of ctrl-a. It will highlight all text and has nothing to do with white space at all in Windows.

David the H. · 10-14-2011, 08:56 AM

Extra whitespace around and after commands in shell scripts isn't generally a problem anyway. Your shell just ignores it. They normally only cause errors inside certain syntactical elements, such as with the infamous [ command. Anyone with experience can easily spot these problem spaces.

The only place I can think of where a space would cause problems, and not be easily noticed, is when you use backslashes to split a command onto multiple lines. e.g.:

Code:

this is a very long command line with many arguments and so I am breaking \
it up onto two lines with a backslash.

The backslash escapes the non-printing newline character at the end of the line, so that the shell treats it as normal whitespace (i.e. it ignores it) instead of a command terminator. If you had a space character after it, then it would escape that instead, and the newline itself would still be "live".

A bigger problem is if you generate a script or input text file using Notepad or a similar Windows program, and then try to run it on a *nix box. Since Windows uses dos crlf line-endings, there will be extra non-printing carriage-return characters at the end of each line, which will likely cause syntax errors.

Edit: I just checked out the thread above, and am reminded that the terminating block of a here document is another potential pitfall location.

Also, another way to see non-printing characters in a file is to run it through cat -A. It doesn't highlight spaces directly in any way, but it does highlight newlines, so there will be a gap between the final alphabetic character and the end of the line, if any exist there.