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Re: Escaped percent sign in shell escape (bang) command (ed 1.16)

From: Andrew L. Moore
Subject: Re: Escaped percent sign in shell escape (bang) command (ed 1.16)
Date: Sat, 8 Jan 2022 20:36:03 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.1

On 1/8/22 17:50, Antonio Diaz Diaz wrote:
$ printf '%s\n' 'r !date +"% modified on \%F"' ',p' Q | ed -s fil1
date +"fil1 modified on %F"
fil1 modified on 2022-01-08

$ ed -V
GNU ed 1.18-rc1

There are a couple of issues that I'd encourage you to explore:

1) Though POSIX doesn't specify ed's behavior when commands are read
   from a pipe, historically, errors do not cause ed to exit.
   Nonetheless, errors should be reported, even with option -s, which
POSIX is very explicit about, i.e., from https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ed.html:

           Suppress the writing of byte counts by e, E, r, and
           w commands and of the '!' prompt after a !command.

   In your example above, if the file `fil1' does not exist, an error
   is reported without option -s.  So an error should also be reported
   with option -s.

   For what it's worth, Illumos ed also suppresses error reporting
   with option -s.  But Illumos ed also prints a backslash and fails
   to assign `fil1' as the default filename.

2) Backslash-escape processing of filenames should be restricted to a
   leading exclamation mark, if any, or eliminated altogether. The
   intent of backslash escape processing was to support reading files
   whose names begin with an exclamation mark without forking a shell.
   But systemd, in particular, likes to generate files and directories
with backslashes (see, for example, https://www.freedesktop.org/software/systemd/man/systemd-escape.html#). This is problematic for
  commands like `systemctl edit' if your editor is set to ed.

   To illustrate the problem:

       ed -p '*' '\x2dhyphen\x2dprefixed\x2dfile'
       \x2dhyphen\x2dprefixed\x2dfile: No such file or directory
       x2dhyphenx2dprefixedx2dfile   <-- backslashes have been stripped!

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