I just encountered my first odd "quirk" of Perl in a long time. I can't seem to find any documentation of it online and I don't know what Perl is doing.
Let's just cut to the chase:
my $name = "Steve";
my $v = "$name's name is $name";
The value of $v would turn out to be " name is Steve". I use vim and the first $name turns an odd color (green instead of the normal teal). So I know this is a known feature, but I don't understand what is happening.
For those who might want to suggest alternatives, I know I can do either of the following without any problems:
$v = $name . "'s name is $name";
$v = "${name}'s name is $name";
I just am curious as to what Perl is doing in the first case I gave and what its uses are.
Quoting perlmod,
The old package delimiter was a single quote, but double colon is now
the preferred delimiter, in part because it's more readable to humans,
and in part because it's more readable to emacs macros. It also makes
C++ programmers feel like they know what's going on--as opposed to
using the single quote as separator, which was there to make Ada
programmers feel like they knew what was going on. Because the
old-fashioned syntax is still supported for backwards compatibility,
if you try to use a string like "This is $owner's house", you'll be
accessing $owner::s; that is, the $s variable in package owner,
which is probably not what you meant. Use braces to disambiguate, as
in "This is ${owner}'s house".
You could also use "This is $owner\'s house".
Related
Before you go marking this as a duplicate hear me out
My question;
A: has different requirements than all the others (which are basically "whats an escape character?") Including: not having to use an external file to enter in parameters to functions
B: questions the existence of this mess rather than accepting 'no' or 'its complicated' as the answer
C: understands that there are escape characters that already exist and ways to work around that
D: comes from a different skill level and isn't a 2-7 years old question
E: requires the use of quotes rather than something like [ because quotes are the only thing that works with spaced strings
Also before ya'll say I didn't try stuff
I read these (all of it including comments and such):
Batch character escaping
http://www.robvanderwoude.com/escapechars.php
https://blogs.msdn.microsoft.com/oldnewthing/20091029-00/?p=16213
using batch echo with special characters
Escape angle brackets in a Windows command prompt
Pass, escape and recognize Special Character in Windows Batch File
I didn't understand that all fully, because to fully understand all that I'd have to be much better at batch but here is what I gleaned:
So I understand theres a whole table of escape sequences, that ^ is the most used one, that you can use Delayed Expansion to do this task so that the characters don't get parsed immediately, just 'expanded' at runtime, but then that the Enable Delayed expansion thing doesn't always work because with pipe characters, the other files/things being piped to/from don't inherit the expansion status so
A: you have to enable it there too
B: that forces you to use that expansion
C: it requires multiple escape characters for each parsing pass of the CLI which apparently is hard to determine and ugly to look at.
This all seems rather ridiculous, why wasn't there some sort of creation to set a string of odd inputs to literal rather than process the characters. Why wasn't it just a simple flag upon some super duper special character (think alt character) that would almost never appear unless you set the font to wingdings. Why does each parsing pass of pipe characters remove the escape characters? That just makes everything insane because the user now has to know how many times that string is used. Why hasn't a tool been developed to auto scan through odd inputs and auto escape them? We have a table of the rules, is it really that hard? Has it been done already? What would it require that's 'hard'?
Down the rabbit hole we go
How did I get here you ask? Well it all started when I made a simple trimming function and happened upon one of the biggest problems in batch, escaping characters when receiving inputs. The problem is alot of my inputs to my trimming function had quotes. Quotes are escaped by using "" in place of " so something like
::SETUP
::parenthesis is just to deliniate where it goes, it isn't
::actually in
::the code
set "var=(stuff goes here)"
call :TrimFunc "%var%",var
:TrimFunc
::the + are just to display the spacing otherwise I can't tell
echo beginning param1 is +%~1+
::code goes here for func
gotoEOF
::END SETUP
::NOTE the + characters aren't part of the actual value, just the
::display when I run this function
set "var=""a"""
::got +"a"+
will work but
set "var="a "
::got +"a+
::expected +"a +
set "var="a ""
::got +"a+
::expected +"a "+
set "var="a " "
::got +"a+
::expected +"a " +
set "var="a"
::got +"a",var+
::expected +"a+
will not work as expected. oddly,
set "var="a""
::got +"a"+
seemes to work despite not being escaped fully. Adding any spaces seems to disrupt this edge case.
Oddly enough I've tried doing:
set 'var="a"'
::got ++
::expected +"a"+
But I have no idea what changing ' to " actually does when its the one that contains the argument (not the ones that are supposed to be literal).
To see what would happen and
What I want:
Surely there must be some sort of universal escape character thing such that I can do this (assume the special character was *)
set *var=""something " "" " """*
call :TrimFunc "%var%",var
echo +%~1+
would net me
+""something " "" " """+
with no problems. In fact, why can't I have some universal escape character that can just be used to take in all the other characters inside it literally instead of the command line trying to process them? Perhaps I'm thinking about this wrong but this seems to be a recurring problem with weird inputs all over. I just want my vairbales, pipes and strings and all that to just STAY LITERAL WHEN THEY'RE SUPPOSED TO. I just want a way to have any input and not have any weird output, it should just treat everything literally untill I want it not to because it was enclosed by the mystical super special character that I just invented in my mind.
::noob rant
I don't see why this was never a thing. Whats preventing the makers of this highly useful language from simply creating a flag and some character that is never used to be the supremo escape character. Why do we need like 10 different ways of escaping characters? I should be able to programatically escape inputs if necessary, it should NEVER be the users job to escape their inputs, thats absolutely ridiculous and probably a violation of every good coding standard in existence
::END noob rant
Anyways. I'd be happy to be enlightened as to why the above is or isn't a thing. I just want to be able to use quotes IN A STRING (kinda important) And I can't comprehend why its not as simple as having one giant "treat these things as literals" flag that ALWAYS JUST WORKS(tm).
By the way, In case you're wondering why my function takes in the name of the variable it's writing to, I couldn't figure out how to get labels inside labels working without using delayed expansion. Using that means the variable I'm making is local not global so I use the name of the global variable to basically set it (using it's name) to the local value on return like this:
endlocal & set "%~2=%String%"
Feel free to yell at me on various things because I am 99% certain I'm doing something horribly syntactically wrong, have some bad misunderstandings or am simply way to naive to truly understand the complexity of this problem but to me it seems amazingly superfluous.
Why can't the last quote be used like the special character it is but any preceeding ones are taken literally (maybe depending upon a flag)
for example
set "var="a ""
why doesn't the ending two quotes act specially and the ones in between act literally? Can the CLI not tell where the line ends? Can it not tell the difference between the first and last quotes and the ones in between? This seems simple to implement to me.
As long as I can echo things properly and save their literal value from parameter to variable I'm happy.
Firstly, I'm don't really understand why you will want to use set "var=something" instead of set var=something, it doesn't seems to have difference.
Secondly, hope that helps, which I have (recently? IDK.) invented a method to deal with the annoying quotes. Hope this batch inspires or helps you to do sth similar.
#echo off
title check for length of string
color de
mode con: cols=90 lines=25
goto t
:t
set str=error
set /p str=Please enter four characters:
set len=0
goto sl
:sl
call set this=%%str:~%len%%%
if not "%this%" == "" (set /a len+=1 & rem debug" == "" (set /a len+=1
goto sl)
if not "%len%" == "4" (set n= not) else (set n=)
echo This is%n% a four character string.
pause >nul
exit
Which in your case:
if not "%var%" == "" (call :TrimFunc "%var%",var & rem debug" == "" (call :TrimFunc "%var%,var)
)
Hope that helps. Add oil~ (My computer doesn't support delayexpansion with unknown reason. Therefore, most of the codes are a bit clumsy.)
P.S.: If you are simply removing text and not replacing text, why not use set var=%var:string=%? If the string required is a variable too, then you can try this: call set var=%%var:%string%=%%
Writing a small script in bash (MacOS in fact) and I want to use find, with multiple sources. Not normally a problem, but the list of source directories to search is held as a string in a variable. Again, not normally a problem, but some of them contain spaces in their name.
I can construct the full command string and if entered directly at the command prompt (copy and paste in fact) it works as required and expected. But when I try and run it within the script, it flunks out on the spaces in the name and I have been unable to get around this.
I cannot quote the entire source string as that is then just seen as one single item which of course does not exist. I escape each space with a backslash within the string held in the variable and it is simply lost. If I use double backslash, they both remain in place and again it fails. Any method of quoting I have tried is basically ignored, the quotes are seen as normal characters and splitting is done at each space.
I have so far only been able to use eval on the whole command string to get it to work but I felt there ought to be a better solution than this.
Ironically, if I use AppleScript I CAN create a suitable command string and run it perfectly with doShellScript (ok, that's using JXA, but it's the same with actual AppleScript). However, I have so far been unable to find the correct escape mechanism just in a bash script, without resorting to eval.
Anyone suggest a solution to this?
If possible, don't store all paths in one string. An array is safer and more convenient:
paths=("first path" "second path" "and so on")
find "${paths[#]}"
The find command will expand to
find "first path" "second path" "and so on"
If you have to use the string and don't want to use eval, split the string into an array:
string="first\ path second\ path and\ so\ on"
read -a paths <<< "$string"
find "${paths[#]}"
Paths inside string should use \ to escape spaces; wraping paths inside"" or '' will not work. eval might be the better option here.
I'm having an extremely hard time figuring out how to echo this:
I keep getting this error:
bash: ![alt: event not found
Using double quotes around it does not work. The using single quotes around it does work, however, I also need to echo strings that have single quotes within them. I wouldn't be able to wrap the string with single quotes then.
Is there a way to echo a string of ANY content?
Thanks.
EDIT: Here is some context. I am making a Markdown renderer that grabs the content of a code editor, then appends every line of the code individually into a text file. I am doing this by doing this:
echo TheLineOfMarkdown > textfile.txt
Unlike in many programing languages, '...' and "..." in Bash do not represent "strings" per se; they quote/escape whatever they contain, but they do not create boundaries that separate arguments. So, for example, these two commands are equivalent:
echo foobar
echo "fo"ob'ar'
So if you need to quote some of an argument with single-quotes, and a different part of the argument has to contain single-quotes — no problem.
For example:
echo '![alt text](https://... "What'"'"'s up, Doc?")'
Another option is to use \, which is similar to '...' except that it only quotes a single character. It can even be used inside double-quotes:
echo "\![alt text](https://... \"What's up, Doc?\")"
For more information, see §3.1.2 "Quoting" in the Bash Reference Manual.
! is annoying. My advice: Use \!.
! invokes history completion, which is also performed inside double-quotes. So you need to single-quote the exclamation mark, but as you say that conflicts with the need to not single-quote other single-quotes.
Remember that you can mix quotes:
$ echo '!'"'"'"'
!'"
(That's just one argument.) But in this case, the backslash is easier to type and quite possibly more readable.
I did use the manual but I am unable to get all the options together to understand what the above code is actually doing.
awk -v v='"' 'BEGIN{FS=OFS=v}{gsub(",","",$2);print }' \
${SOURCE_LOCATION}/TEMP1_$file_name>${SOURCE_FILE_LOCATION}/TEMP2_$file_name
When do we have to use the curly brackets in a code after the '$' and when not to. Please explain. Any help is really appreciated.
This command would remove all the commas in the second field. The field separator being the quote character " (as specified by FS).
For example, the following string:
something "string, with, commas" something "else, here, and more"
would be transformed to:
something "string with commas" something "else, here, and more"
The significance of {} in variable names has been well explained by #Joni.
The input is read from the file ${SOURCE_LOCATION}/TEMP1_$file_name and output is redirected to ${SOURCE_LOCATION}/TEMP2_$file_name.
You must use the curly brackets syntax when a variable name is followed by something that's not part of the variable name but could be confused with it. For example, compare
hello="Hello"
echo $hello_world
with
hello="Hello"
echo ${hello}_world
The first one outputs an empty line (or the value of the shell variable hello_world, if it exists), and the second one outputs Hello_world.
In your case they are not necessary because a slash can never be a part of the variable name. Some people prefer to use the brackets to make it clear where the variable begins and where it ends even when they are not required.
I would like to extract a string as below:
setenv VARIABLE /opt/Application/File/Version22
I want to display only
File/Version22
on screen. It means /opt/Application/ is always the prefix. I want to display the rest of the string. How do I do it in csh?
Regards
Now that you've updated the question (thank you for that), it's clear that you always want to remove the /opt/Application prefix.
The most straightforward way to do that, which will work in any shell, is:
echo $VARIABLE | sed 's|^/opt/Application/||'
(It's usual to use / as a delimiter for replacements like this, but you can use any punctuation character; I'm using | to avoid conflicting with the / characters in the pattern.)
A more tcsh-specific, and possibly more efficient, way to do it is:
echo $VARIABLE:s|/opt/Application/||
It's likely to be more efficient because it's done within the shell and avoids invoking the external sed command. On the other hand, the overhead of executing sed is unlikely to be significant.
Note carefully that the :s syntax is not supported in the original csh. It is supported in tcsh and in some newer versions of csh. If you want to do this portably, just use sed.
C-Shell has the built-in string modifiers that can do this with very little code:
echo $VARIABLE:h:t/$VARIABLE:t
string modifiers:
:h = remove the last directory (aka head)
:t = remove every directory except the last one (aka tail)
I assume that you want to get the 2 last names in a filename:
setenv VARIABLE /opt/Application/File/Version22
set lastfile=`basename $VARIABLE`
set prevpath=`dirname $VARIABLE`
set previousfile=`basename $prevpath`
set file=$previousfile/$lastfile
echo $file