How to enter escape character in a positional string argument from the command line?

[Jach Feng]

I have a script using the argparse module. I want to enter the string "step\x0A" as one of its positional arguments. I expect this string has a length of 5, but it gives 8. Obviously the escape character didn't function correctly. How to do it?

--Jach

[Mark Bourne]

Jach Feng wrote:
> I have a script using the argparse module. I want to enter the string "step\x0A" as one of its positional arguments. I expect this string has a length of 5, but it gives 8. Obviously the escape character didn't function correctly. How to do it?

That depends on the command-line shell you're calling your script from.

In bash, you can include a newline in a quoted string:
./your_script 'step
'
(the closing quote is on the next line)

Or if you want to do it on a single line (or use other escape
sequences), you can use e.g.:
./your_script $'step\x0a'
(dollar sign before a single-quoted string which contains escape sequences)

--
Mark.

[Jach Feng]

Mark Bourne 在 2022年12月20日 星期二凌晨4:49:13 [UTC+8] 的信中寫道：

That's really good for Linux user! How about Windows?

[Thomas Passin]

One way is to process the argument after it gets into Python rather than
before. How hard that will be depends on how general you need the
argument to be. For your actual example, the argument comes into Python
as if it were

arg1 = r"step\x0A" # or "step\\x0a"

You can see if there is an "\\x":

pos = arg1.find('\\x') # 4

Replace or use a regex to replace it:

arg1_fixed = arg1.replace('\\x0A', '\n')

Naturally, if "\\x0A" is only a special case and other combinations are
possible, you will need to figure out what you need and do some more
complicated processing.

[Eryk Sun]

On 12/19/22, Jach Feng <jf...@ms4.hinet.net> wrote:
>
> That's really good for Linux user! How about Windows?

In CMD, typing the "^" escape character at the end of a line ignores
the newline and prompts for "more" input. If you press enter again,
you'll get another "more" prompt in which you can write the rest of
the command line. Command-line arguments are separated by spaces, so
you have to start the next line with a space if you want it to be a
new argument. Also, "^" is a literal character when it's in a
double-quoted string, which requires careful use of quotes. For
example:

C:\>py -c "import sys; print(sys.orig_argv[3:])" spam^
More?
More? eggs^
More?
More? " and spam"
['spam\n', 'eggs\n and spam']

The above is easier in PowerShell, which supports entering multiline
strings without having to escape the newline. The second-level prompt
is ">> ". For example:

> py -c "import sys; print(sys.orig_argv[3:])" spam"
>> " eggs"
>> and spam"
['spam\n', 'eggs\n and spam']

[Jach Feng]

Thomas Passin 在 2022年12月20日 星期二上午11:36:41 [UTC+8] 的信中寫道：

That's what I am taking this path under Windows now, the ultimate solution before Windows has shell similar to bash:-)

[Jach Feng]

ery...@gmail.com 在 2022年12月20日 星期二中午12:35:52 [UTC+8] 的信中寫道：

Thanks for the information. No idea Windows CMD can take such a trick to enter "\n" :-)

[Chris Angelico]

On Wed, 21 Dec 2022 at 15:28, Jach Feng <jf...@ms4.hinet.net> wrote:
> That's what I am taking this path under Windows now, the ultimate solution before Windows has shell similar to bash:-)

Technically, Windows DOES have a shell similar to bash. It's called
bash. :) The trouble is, most people use cmd.exe instead.

ChrisA

[Jach Feng]

Chris Angelico 在 2022年12月21日 星期三下午1:02:01 [UTC+8] 的信中寫道：

Really? Where? I can't find it in my Windows 8.1

[Albert-Jan Roskam]

On Dec 21, 2022 06:01, Chris Angelico <ros...@gmail.com> wrote:

On Wed, 21 Dec 2022 at 15:28, Jach Feng <jf...@ms4.hinet.net> wrote:
> That's what I am taking this path under Windows now, the ultimate
solution before Windows has shell similar to bash:-)

Technically, Windows DOES have a shell similar to bash. It's called
bash. :) The trouble is, most people use cmd.exe instead.

=====
I use Git Bash quite a lot: https://gitforwindows.org/
Is that the one you're referring to?

[Lars Liedtke]

Or you could have "native" bash ($SHELL) with WSL. But I assume not everyone is using it.

Cheers

Lars


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Am 21.12.22 um 13:15 schrieb Albert-Jan Roskam:

On Dec 21, 2022 06:01, Chris Angelico <ros...@gmail.com><mailto:ros...@gmail.com> wrote:

[Chris Angelico]

On Wed, 21 Dec 2022 at 23:16, Albert-Jan Roskam <sjeik...@hotmail.com> wrote:
>
>
>
> On Dec 21, 2022 06:01, Chris Angelico <ros...@gmail.com> wrote:

>
> On Wed, 21 Dec 2022 at 15:28, Jach Feng <jf...@ms4.hinet.net> wrote:
> > That's what I am taking this path under Windows now, the ultimate solution before Windows has shell similar to bash:-)
>
> Technically, Windows DOES have a shell similar to bash. It's called
> bash. :) The trouble is, most people use cmd.exe instead.
>
>
> =====
>
> I use Git Bash quite a lot: https://gitforwindows.org/
>
> Is that the one you're referring to?
>

Yeah, that's probably the easiest way to get hold of it.

ChrisA

[Chris Angelico]

Ah, I didn't mean "ships with"; bash for Windows is very much
available, but you'd have to go fetch it. Try the Windows app store
first, and if not, a quick web search will find it. Or install Git for
Windows, which comes with bash.

ChrisA

[Chris Angelico]

On Thu, 22 Dec 2022 at 03:11, Stefan Ram <r...@zedat.fu-berlin.de> wrote:

>
> Lars Liedtke <l...@solute.de> writes:
> >Or you could have "native" bash ($SHELL) with WSL.
>

> In this newsgroup, it would actually be obvious to use Python.

Less obvious than you might think - partly because bash is just so
dang good that it's really really hard to outdo it :) Sure, bash has a
lot of weird and wonky edge cases, but it's an incredibly practical
shell to use.

> When commands are typed manually, this might be a bit verbose,
> though. I mean
>
> os.chdir( r'C:\EXAMPLE' )
>
> versus
>
> CD C:\EXAMPLE

Exactly. What's good for a programming language is often not good for a shell.

> class PythonShell( cmd.Cmd ):
>
> intro = 'Welcome to the Python shell. Type help or ? to list commands.\n'
> prompt = '(Python) '
> file = None
>
> def do_cd( self, arg ):
> 'change directory: CD C:\EXAMPLE'
> os.chdir( *parse( arg ))
>
> def do_bye( self, arg ):
> 'Exit: BYE'
> print( 'Thank you for using the Python Shell!' )
> return True

Sure, you can always create your own shell. But I think you'll find
that, as you start expanding on this, you'll end up leaning more
towards "implementing bash-like and/or cmd-like semantics in Python"
rather than "creating a Python shell". Shells, in general, try to
execute programs as easily and conveniently as possible. Programming
languages try to stay inside themselves and do things, with subprocess
spawning being a much less important task.

Fun challenge: see how much you can do in bash without ever forking to
another program. And by "fun", I mean extremely difficult, and by
"challenge" I really mean "something you might have to do when your
system is utterly hosed and all you have available is one root shell".

It's amazing how far you can go when your hard drive has crashed and
you desperately need to get one crucial login key that you thought you
had saved elsewhere but hadn't.

ChrisA

[gene heskett]

On 12/21/22 11:22, Chris Angelico wrote:
> On Thu, 22 Dec 2022 at 03:11, Stefan Ram <r...@zedat.fu-berlin.de> wrote:
>>
>> Lars Liedtke <l...@solute.de> writes:

>>> Or you could have "native" bash ($SHELL) with WSL.
>>

>> In this newsgroup, it would actually be obvious to use Python.
>
> Less obvious than you might think - partly because bash is just so
> dang good that it's really really hard to outdo it :) Sure, bash has a
> lot of weird and wonky edge cases, but it's an incredibly practical
> shell to use.
>

When you make a statement like that, Chris, you should also note that
every single one of those "wonky edge cases" is documented down to the
last dotted i. Bash's docs will kill a good sized pulp tree, needing
around a ream of paper to print on a duplex printer. I know, I did it
around a decade ago. If you like to write scripts, having a dead tree
copy of the docs at your elbow in incredibly useful. That huge man page
does not cover it like the printed docs do.

Cheers, Gene Heskett.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/>

[Chris Angelico]

On Thu, 22 Dec 2022 at 03:58, gene heskett <ghes...@shentel.net> wrote:
>
> On 12/21/22 11:22, Chris Angelico wrote:
> > On Thu, 22 Dec 2022 at 03:11, Stefan Ram <r...@zedat.fu-berlin.de> wrote:
> >>
> >> Lars Liedtke <l...@solute.de> writes:

> >>> Or you could have "native" bash ($SHELL) with WSL.
> >>

> >> In this newsgroup, it would actually be obvious to use Python.
> >
> > Less obvious than you might think - partly because bash is just so
> > dang good that it's really really hard to outdo it :) Sure, bash has a
> > lot of weird and wonky edge cases, but it's an incredibly practical
> > shell to use.
> >
> When you make a statement like that, Chris, you should also note that
> every single one of those "wonky edge cases" is documented down to the
> last dotted i. Bash's docs will kill a good sized pulp tree, needing
> around a ream of paper to print on a duplex printer. I know, I did it
> around a decade ago. If you like to write scripts, having a dead tree
> copy of the docs at your elbow in incredibly useful. That huge man page
> does not cover it like the printed docs do.
>

Oh yes, absolutely true. Its wonkiness is dependable and consistent;
but it is definitely quirky (look at all the different ways to embed
arguments into things, and the ways that $*, $@, "$*, and "$@" behave
when put into variables). Not usually a problem, but it does sometimes
leave you thinking "wow, wouldn't it be easier to just use something
like Python?". And in the complicated cases, yeah, it can be. But in
the simple cases? Bash rocks.

ChrisA

[Chris Angelico]

On Thu, 22 Dec 2022 at 04:12, Barry <ba...@barrys-emacs.org> wrote:
> I see bash scripts that are 1000’s of line of code at work and its a maintenance nightmare.
>
> Knowing when to make the move from “handy bash script” to “this is a production application” and needs to be python is what I see people miss.
>
> After a certain point in complexity the python code wins on maintenance.
> Personally i set a low bar to move from bash to python.

Agreed 100%. However, how many commands do you type a day at the bash
prompt? *Every one of those* is a simple case that doesn't need
Python. That's what makes bash such a great shell. You can type
simple, convenient commands, and they *just work*. Need something a
bit more complicated? You can do that too. Want to automate a short
sequence of commands? Don't have to translate them into another system
(like Python subprocess calls), just copy/paste them into a shell
script.

Yes, there absolutely is a point beyond which it's better to translate
everything. But the vast number of commands executed every day? Most
of them are fine with bash. And that's why Python will never replace
bash as a shell.

ChrisA

[Barry]

> On 21 Dec 2022, at 17:06, Chris Angelico <ros...@gmail.com> wrote:

I see bash scripts that are 1000’s of line of code at work and its a maintenance nightmare.

Knowing when to make the move from “handy bash script” to “this is a production application” and needs to be python is what I see people miss.

After a certain point in complexity the python code wins on maintenance.
Personally i set a low bar to move from bash to python.

Barry


>
> ChrisA
> --
> https://mail.python.org/mailman/listinfo/python-list
>