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Old 04-04-2015, 09:19 AM   #1
Registered: Dec 2008
Location: east anglia
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The behaviour of single & double quotes in writing & running aliases?

This isn't a problem, more a how or a why:

I have always written an alias (alias ln="ln -sv") which changes the instruction for a "hard" link into one for a "soft". This worked fine until I needed a hard link, so, as part of the alias, I decided to give myself a reminder of what I was doing. After a bit of a struggle, I ended up with this (which I stuffed into my .bash_aliases file):

alias ln="echo -e 'Symbolic (soft) Link (use \033[1;36mhln\033[0m for Hard Link)\nuse \033[1;36m-f\033[0m for force'; ln -sv "

and it works:

Symbolic (soft) Link (use hln for Hard Link )
use -f for force
ln: missing file operand
Try 'ln --help' for more information.

(OK, no operand was supplied here, but the stuff we're talking about works fine).

Recently, however, I needed to look at the construction of the alias, ran "alias ln" & got:

alias ln='echo -e '\''Symbolic (soft) Link (use \033[1;36mhln\033[0m for Hard Link)\nuse \033[1;36m-f\033[0m for force'\''; ln -sv'

Now, I can (sort of) understand the logic of what's going on here, but not entirely - viz:

..1...'...............Open 'protected' mode (was double quote)
........echo -e....Command: invoke "echo -e"
..2...'...............matches 1: Closes 'protected' mode
........\.............invoke newline(\)? Or is this simply an scape character? (Yes it is see
..3...'...............Reopen 'protected' mode (or an escaped single quote - if so why?)
..4...'...............0pens 'protected' mode for "message" code
........Symbolic (soft) Link (use \033[1;36mhln\033[0m for Hard
........Link)\nuse \033[1;36m-f\033[0m for force
..5...'...............5 matches 4 (closes message code),
........\.............invoke newline(\) - or see above
..6...'...............6 matches 3 & closes the section started @ 'reopen'
..7...'...............Reopens 'protected' mode
........; ln -sv......New command[ ; ], creates link
..8...'...............8 matches 7 - the end (or is it?)

I would guess that the "alias" command has some built built in function that does all this - I have saved the the modified version as an alias & it works fine - but although I have read the excellent post:

I don't understand why the system has both double & single quotes. I have always felt that as long as the command line was explained & demonstrated in simple language it was easier to work with that than any point & click system. I grew up with CPM, DOS, Qemm, Desqview, WP5.1, DbII and these were great programs with great (especially WP51) manuals - but, ahh! - those days have gone

Any suggestions gratefully received

Old 04-04-2015, 10:08 AM   #2
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You are not parsing that output correctly. The 4-character sequence "'\''" is what must be done to represent a literal single quote character embedded within a single-quoted string. Within a single-quoted string, all characters except single quote are taken literally, so that leaves no way to escape that literal single quote. So, the sequence is
1.  '      end the previous single-quoted string
2.  \'     insert an escaped single quote
3.  '      re-open the single-quoted string
The output is formatted with a single-quoted string simply because that's how aliases are always displayed, regardless of how you wrote them or if some other form might be simpler to read. What you wrote was parsed into an internal form. The particulars of how it got into that form are not entirely preserved.
1 members found this post helpful.
Old 04-05-2015, 03:01 PM   #3
Registered: Dec 2008
Location: east anglia
Distribution: SuSE, antiX
Posts: 38

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@rknichols - That's great, I'd almost got there but not quite. Thanks a lot!



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