Command Substitution: $(command) vs `command`

penguinbody · 04-12-2016, 05:20 PM

The LPI Certification In a Nutshell says:

"Another form of command substitution is `command`. The result is the same, except that the back quote (or backtick) syntax has some special rules..."

I followed the reference to the BASH manual for an explanation of the special rules here.

Does that mean the characters after the first \ here can be interpreted into something else as opposed to the literal meaning? Like what? I need some examples for that to be clearer.

>`\$;command`
>`\`;command`
>`\\`;command`

Lastly, what does this statement mean: The first backquote not preceded by a backslash terminates the command substitution.

Shadow_7 · 04-12-2016, 06:14 PM

Your context seems missing a bit. Assuming something is preceding the ">" and it's not just a command prompt symbol. But `` and $() are interchange-able. Except that $() allows for the nesting of commands. The \ char is an escape character. And the ; char is a line separator. With special chars like $ ^ ? and stuff. The non-special characters just get passed as an uninterpreted character less the preceding \.

ondoho · 04-13-2016, 01:27 AM

Quote:

Originally Posted by penguinbody

Another form of command substitution is `command`.

i heard people say that this is "the old way", only kept for backward compatibility or some such.

hazel · 04-13-2016, 01:52 AM

I've noticed that the LFS books, when they need to nest two command substitutions, use backquotes for one and the $() construct for the other. They don't nest $() constructs. I suppose this is for reasons of legibility and comprehensibility.

Backquotes may be "the old way" but a lot of us still use them because it's that's the way we learned to do it.

cnamejj · 04-13-2016, 03:21 AM

I think backticks (meaning ``) for getting the results of a command should be consider "legacy baggage" at this point. And I always use the $() construct instead, even for nested calls. It's so much cleaner and easier to read. And there's no confusion about whether a backtick is opening a command or closing one.

JJJCR · 04-13-2016, 04:10 AM

Example:
#pwd
/usr/my_pics

Current print working directory is /usr/my_pics

#cp `pwd` /home

or

#cp $(pwd) /home

cp $(pwd) /home or cp `pwd` /home
is equivalent to:
cp /usr/my_pics /home

which copies my_pics folder to /home folder.

Backtick runs the contents of the enclosed string, and used it as a parameter to the other command on the line, in this case the "cp" command.

I think to save bytes use backtick, for clarity purposes use $().

BW-userx · 04-13-2016, 05:49 AM

Code:

cp -rv /usr/my_pics ~/

Would work too. The -rv is just in case directories are present within the working directory, and output to term to see what's going on. If /home is actually meant to mean/home/user. In real world situations. What logical reasons would one have to put files in just the /home directory minus a user directory or subdirectory?

Quote:

>`\$;command`
>`\`;command`
>`\\`;command`

Lastly, what does this statement mean: The first backquote not preceded by a backslash terminates the command substitution.

to eliminate the termination just add another ` because they have to have an opening and closing, or matching pairs. I think they where just giving you a "matter of fact".

Weapon S · 04-13-2016, 11:19 AM

Quote:

Originally Posted by penguinbody

The LPI Certification In a Nutshell says:
Lastly, what does this statement mean: The first backquote not preceded by a backslash terminates the command substitution.

That's just an escape sequence. A backslash "escapes" the character. If you want to run a command containing a backtick (in this case) you will have to tell Bash to not interpret that backtick as the ending of command substitution clause. So you make the backtick literal with a backslash, making it lose its special meaning of ending your clause.

http://wiki.bash-hackers.org/syntax/expansion/cmdsubst

ondoho · 04-14-2016, 01:29 AM

Quote:

Originally Posted by JJJCR

Example:
#pwd
/usr/my_pics

Current print working directory is /usr/my_pics

#cp `pwd` /home

or

#cp $(pwd) /home

cp $(pwd) /home or cp `pwd` /home
is equivalent to:
cp /usr/my_pics /home

which copies my_pics folder to /home folder.

Backtick runs the contents of the enclosed string, and used it as a parameter to the other command on the line, in this case the "cp" command.

I think to save bytes use backtick, for clarity purposes use $().

that's not a good example.
it does not save any bytes either way.
also it will not work like this (cp throws a warning and doesn't copy anything, at least on my version).
i'd do it like this:

Code:

cp -r . /home

but you're likely to not have permission to copy anything to /home, i guess it really should be

Code:

cp -r . "$HOME"

and to "save bytes":

Code:

cp -r . ~

also, you usually don't need a command to get the present working directory, there's an env.variable called "$PWD", so this is equivalent:

Code:

cp -r "$PWD" "$HOME"

jpollard · 04-15-2016, 07:00 AM

Backtics are for portability. Even now, not all systems use bash unless the admins have installed it. Many have ksh/csh/sh (The Bourne shell) which don't use the $() construct.

A few of us even think bash is a bit bloated with things that shouldn't be there...