[Semibug] Indented "here documents"

[Jeffrey Marraccini]

Thank you very much, Nick, Michael, and all! The last meeting was very
informative and fun. Nick, this follow up is also fantastic!

On Wed, Aug 22, 2018, 22:53 Nick Holland <nick at holland-consulting.net>
wrote:

> As I mentioned after Michael's presentation on "ed" yesterday, when
> moving my shell scripting tips presentation from whatever it started on
> to whatever I finished using, I missed a slide.
>
> Often forgot when scripting are "here documents" -- done by doing
> something like this:
>
> program <<__EOT
> input for program 1
> input for program 2
> input for program 3
> __EOT
>
> where the three lines between <<__EOT and __EOT are fed to "program".
> "__EOT" can be any unique string that isn't in the body of the text
> being fed to the program...double leading underscores are common, but
> not at all required.
>
> Lots of people know about this...though I know I often forgot about them
> until I started putting together my shell scripting tips presentation.
>
> The problem is, in a neatly formatted script, here documents are often
> ugly -- crammed against the left margin, even though the "program" may
> be indented...so your here document might look like this:
>
> while bla; do
>     program <<__EOT
> input for program
> input for program
> input for program
> __EOT
>     done
> bla bla
>
> Well, turns out as is often the case, Unix coders long ago noticed the
> same problems we thought we discovered, and fixed it long ago.  It turns
> out if you change the << to <<- the shell will strip off leading tabs
> (and ONLY tabs), so your script can now look more like this:
>
> while bla; do
>     program <<-__EOT
>         input for program
>         input for program
>         input for program
>         __EOT
>     done
> bla bla
>
> Now note, both the "here document" text and the end marker are all
> indented with a tab.  Now, the tabs are all stripped, so you can have
> multiple (or inconsistent) tabs if needed/desired.
>
>
> And ... here's a script I used today, slightly sanitized, I ran today.
> A few weeks ago, I updated the /etc/hosts file on all our customers
> systems to fix a long standing and painful set of problems, purging a
> lot of junk and replacing it with consistent data, while not messing
> with the system specific data.  And about two days after completing this
> pain in the *** task, I found out I forgot one entry...times 90 systems.
>
> So ... a simple ed script to see if the machine needs the update, if so
> find where I wanted the new line to be inserted, open up a new line, add
> the new text, exit insert mode, write it, quit.  Move on to the next.
>
> #!/usr/bin/ksh
>
> for H in `/opt/bin/progthatlistssystems`; do
>     echo === $H ===
>     if ! ssh $H "host img-gf.example.org >/dev/null 2>&1"; then
>         echo "must fix $H"
>         ssh -t $H "ed /etc/hosts" <<-__ENDED
>                 /img-sf.example.org/
>                 a
>                 10.3.217.20     img-gf.example.org        img-gf
>                 .
>                 w
>                 q
>                 __ENDED
>     fi
>     sleep 1
> done
>
> Note that I tend to use a 4 space indent.  By the time we got to the ed
> input, we should have been at 12 chars, so there's a TWO tab leading
> indent.
>
> And while I have blundered my way through ed before, I wrote that
> completely from memory from Michael's talk, no man page or book used. :)
>
> Nick.
>
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