removing backslash with tr - bash

So Im removing special characters from filenames and replacing with spaces. I have all working apart from files with single backslashes contained therein.
Note these files are created in the Finder on OS X
old_name="testing\this\folder"
new_name=$(echo $old_name | tr '<>:\\#%|?*' ' ');
This results in new_name being "testing hisolder"
How can I just removed the backslashes and not the preceding character?

This results in new_name being "testing hisolder"
This string looks like the result of echo -e "testing\this\folder", because \t and \f are actually replaced with the tabulation and form feed control characters.
Maybe you have an alias like alias echo='echo -e', or maybe the implementation of echo in your version of the shell interprets backslash escapes:
POSIX does not require support for any options, and says that the
behavior of ‘echo’ is implementation-defined if any STRING contains a
backslash or if the first argument is ‘-n’. Portable programs can use
the ‘printf’ command if they need to omit trailing newlines or output
control characters or backslashes.
(from the info page)
So you should use printf instead of echo in new software. In particular, echo $old_name should be replaced with printf %s "$old_name".
There is a good explanation in this discussion, for instance.
No need for printf
As #mklement0 suggested, you can avoid the pipe by means of the Bash here string:
tr '<>:\\#%|?*' ' ' <<<"$old_name"

Ruslan's excellent answer explains why your command may not be working for you and offers a robust, portable solution.
tl;dr:
You probably ran your code with sh rather than bash (even though on macOS sh is Bash in disguise), or you had shell option xpg_echo explicitly turned on.
Use printf instead of echo for portability.
In Bash, with the default options and using the echo builtin, your command should work as-is (except that you should double-quote $old_name for robustness), because echo by default does not expand escape sequences such as \t in its operands.
However, Bash's echo can be made to expand control-character escape sequences:
explicitly, by executing shopt -s xpg_echo
implicitly, if you run Bash as sh or with the --posix option (which, among other options and behavior changes, activates xpg_echo)
Thus, your symptom may have been caused by running your code from a script with shebang line #!/bin/sh, for instance.
However, if you're targeting sh, i.e., if you're writing a portable script, then echo should be avoided altogether for the very reason that its behavior differs across shells and platforms - see Ruslan's printf solution.
As an aside: perhaps a more robust approach to your tr command is a whitelisting approach: stating only the characters that are explicitly allowed in your result, and excluding other with the -C option:
old_name='testing\this\folder'
new_name=$(printf '%s' "$old_name" | tr -C '[:alnum:]_-' ' ')
That way, any characters that aren't either letters, numbers, _, or - are replaced with a space.

With Bash, you can use parameter expansion:
$ old_name="testing\this\folder"
$ new_name=${old_name//[<>:\\#%|?*]/ }
$ echo $new_name
testing this folder
For more, please refer to the Bash manual on shell parameter expansion.

I think your test case is missing proper escaping for \, so you're not really testing the case of a backslash contained in a string.
This worked for me:
old_name='testing\\this\\folder'
new_name=$(echo $old_name | tr '<>:\\#%|?*' ' ');
echo $new_name
# testing this folder

Related

Why does sh interpret these commands differently to zsh?

Context: I am trying to get the names of fonts installed on my Linux machine. The name is between two colons. When I run this script using zsh, it works. When I switch to sh it breaks. It probably deletes \n characters, because the output is all one line.
cmd="$(fc-list)"
names="$(echo $cmd | grep -oP "(?<=: ).*?(?=:)")"
echo $names
Which command causes the issues? Is it not POSIX compliant?
Why does sh interpret these commands differently to zsh?
Because they are different shell the rules are different.
Which command causes the issues?
echo $cmd
and
echo $names
Is it not POSIX compliant?
The code, by itself, is valid POSIX code. The behavior of zsh shell that doesn't do word splitting is not compliant with POSIX.
In POSIX shell unquoted expansions undergo word splitting. Because default IFS is space tab and newline, these characters are erased when creating words from the result of expansion and passed as separate words to echo which outputs them on one line. To disable word splitting (and filename expansion), you need to quote the expansion. Also prefer printf to echo.
cmd="$(fc-list)"
names="$(printf "%s\n" "$cmd" | grep -oP "(?<=: ).*?(?=:)")"
printf "%s\n" "$names"

How to escape bash parameter expansion (when a "!" followed by letters is a parameter) [duplicate]

This question already has answers here:
echo "#!" fails -- "event not found"
(5 answers)
Closed 7 years ago.
I am attempting to parse the output of a VNC server startup event and have run into a problem in parsing using sed in a command substitution. Specifically, the remote VNC server is started in a manner such as the following:
address1="user1#lxplus.cern.ch"
VNCServerResponse="$(ssh "${address1}" 'vncserver' 2>&1)"
The standard error output produced in this startup event is then to be parsed in order to extract the server and display information. At this point the content of the variable VNCServerResponse is something such as the following:
New 'lxplus0186.cern.ch:1 (user1)' desktop is lxplus0186.cern.ch:1
Starting applications specified in /afs/cern.ch/user/u/user1/.vnc/xstartup
Log file is /afs/cern.ch/user/u/user1/.vnc/lxplus0186.cern.ch:1.log
This output can be parsed in the following way in order to extract the server and display information:
echo "${VNCServerResponse}" | sed '/New.*desktop.*is/!d' \
| awk -F" desktop is " '{print $2}'
The result is something such as the following:
lxplus0186.cern.ch:1
What I want to do is use this parsing in a command substitution something like the following:
VNCServerAndDisplayNumber="$(echo "${VNCServerResponse}" \
| sed '/New.*desktop.*is/!d' | awk -F" desktop is " '{print $2}')"
On attempting to do this, I am presented with the following error:
bash: !d': event not found
I am not sure how to address this. It appears to be a problem in the way sed is being used in the command substitution. I would appreciate guidance.
Bash history expansion is a very odd corner in the bash command line parser, and you are clearly running into an unexpected history expansion, which is explained below. However, any sort of history expansion in a script is unexpected, because normally history expansion is not enabled in scripts; not even scripts run with the source (or .) builtin.
How history expansion is enabled (or disabled)
There are two shell options which control history expansion:
set -o history: Required for the history to be recorded.
set -H (or set -o histexpand): Additionally required for history expansion to be enabled.
Both of these options must be set for history expansion to be recognized. (I found the manual unclear on this interaction, but it's logical enough.)
According to the bash manual, these options are unset for non-interactive shells, so if you want to enable history expansion in a script (and I cannot imagine a reason you would want this), you would need to set both of them:
set -o history -o histexpand
The situation for scripts run with source is more complicated (and what I'm about to say only applies to bash v4, and since it's undocumented in might change in the future). [Note 3]
History recording (and consequently expansion) is turned off in source'd scripts, but through an internal flag which, as far as I know, is not made visible. It certainly does not appear in $SHELLOPTS. Since a sourced script runs in the current bash context, it shares the current execution environment, including shell options. So in the execution of a sourced script initiated from an interactive session, you'll see both history and histexpand in $SHELLOPTS, but no history expansion will take place. In order to enable it, you need to:
set -o history
which is not a no-op because it has the side-effect of resetting the internal flag which suppresses history recording. Setting the histexpand shell option does not have this side-effect.
In short, I'm not sure how you managed to enable history expansion in a script (if, indeed, the misbehaving command was in a script and not in an interactive shell), but you might want to consider not doing so, unless you have a really good reason.
How history expansion is parsed
The bash implementation of history expansion is designed to work with readline, so that it can be performed during command input. (By default this function is bound to Meta-^; generally Meta is ESC, but you can customize that as well.) However, it is also performed immediately after each line is input, before any bash parsing is performed.
By default, the history expansion character is !, and -- as mostly documented -- that will trigger history expansion except:
when it is followed by whitespace or =
if the shell option extglob is set, and it is followed by ( [Note 1]
if it appears in a single-quoted string
if it is preceded by a \ [Note 2 and see below]
if it is preceded by $ or ${ [Note 1]
if it is preceded by [ [Note 1]
(As of bash v4.3) if it is the last character in a double-quoted string.
The immediate issue here is the precise interpretation of the third case, an ! appearing inside of a single-quoted string. Normally, bash starts a new quoting context for a command substitution ($(...) or the deprecated backtick notation). For example:
$ s=SUBSTITUTED
$ # The interior single quotes are just characters
$ echo "'Echoing $s'"
'Echoing SUBSTITUTED'
$ # The interior single quotes are single quotes
$ echo "$(echo 'Echoing $s')"
Echoing $s
However, the history expansion scanner isn't that intelligent. It keeps track of quotes, but not of command substitution. So as far as it is concerned, both of the single quotes in the above example are double-quoted single quotes, which is to say ordinary characters. So history expansion occurs in both of them:
# A no-op to indicated history expansion
$ HIST() { :; }
# Single-quoted strings inhibit history expansion
$ HIST
$ echo '!!'
!!
# Double-quoted strings allow history expansion
$ HIST
$ echo "'!!'"
echo "'HIST'"
'HIST'
# ... and it applies also to interior command substitution.
$ HIST
$ echo "$(echo '!!')"
echo "$(echo 'HIST')"
HIST
So if you have a perfectly normal command like sed '/foo/!d' file, where you would expect the single-quotes to protect you from history-expansion, and you put it inside a double-quoted command substitution:
result="$(sed '/foo/!d' file)"
you suddenly find that the ! is a history expansion character. Worse, you can't fix this by backslash escaping the exclamation point, because although "\!" inhibits history expansion, it doesn't remove the backslash:
$ echo "\!"
\!
In this particular example -- and the one in the OP -- the double quotes are completely unnecessary, because the right-hand side of a variable assignment does not undergo either filename expansion nor word splitting. However, there are other contexts in which removing the double quotes would change the semantics:
# Undesired history expansion
printf "The answer is '%s'\n" "$(sed '/foo/!d' file)"
# Undesired word splitting
printf "The answer is '%s'\n" $(sed '/foo/!d' file)
In this case, the best solution is probably to put the sed argument in a variable
# Works
sed_prog='/foo/!d'
printf "The answer is '%s'\n" "$(sed "$sed_prog" file)"
(The quotes around $sed_prog were not necessary in this case but usually they would be, and they do no harm.)
Notes:
The inhibition of history expansion when the following character is some form of open parenthesis only works if there is a corresponding close parenthesis in the rest of the string. However, it doesn't have to really match the open parenthesis. For example:
# No matching close parenthesis
$ echo "!("
bash: !: event not found
# The matching close parenthesis has nothing to do with the open
$ echo "!(" ")"
!( )
# An actual extended glob: files whose names don't start with a
$ echo "!(a*)"
b
As indicated in the bash manual, a history-expansion character is treated as an ordinary character if immediately preceded by a backslash. This is literally true; it doesn't matter whether the backslash will later be considered an escape character or not:
$ echo \!
!
$ echo \\!
\!
$ echo \\\!
\!
\ also inhibits history expansion inside double quotes, but \! is not a valid escape sequence inside the double quoted string, so the backslash is not removed:
$ echo "\!"
\!
$ echo "\\!"
\!
$ echo "\\\!"
\\!
I'm referring to the source code for bash v4.2 as I write this, so any undocumented behaviour may be completely different as of v4.3.
The problem is that within double quotes, bash is trying to expand !d before passing it to the subshell. You can get around this problem by removing the double quotes but I would also propose a simplification to your script:
VNCServerAndDisplayNumber=$(echo "$VNCServerResponse" | awk '/desktop/ {print $NF}')
This simply prints the last field on the line containing the word "desktop".
On a newer bash, you can use a herestring rather than piping an echo:
VNCServerAndDisplayNumber=$(awk '/desktop/ {print $NF}' <<<"$VNCServerResponse")
Don't wrap the $(...) command substitution in double quotes. You are asking the shell to perform evaluation on the contents of the quotes and are hitting the history substitution expansion feature. Drop the quotes and you stop telling the shell to do that and you won't hit that problem.
And yes, dropping those quotes is safe on that assignment line even if the output may contain spaces or newlines or whatever. Assignments of that sort are not going to split on those the way command substitution or variable evaluation will on a normal shell execution line.
Alternatively, disable history expansion in your shell/script before you run that. (It should be off when running a script by default I believe anyway.)
This only happens when history expansion is enabled, which it normally isn't and definitely shouldn't be for scripts.
Rather than trying to work around it, figure out why history expansion is enabled and what to do so it isn't.
If you're executing your script with . foo or source foo, use ./foo instead.
If you're writing this as a function in .bashrc or similar, consider making it a separate script.
If your script (or BASH_ENV) explicitly does set -H, don't.
Quote it with '' or \ or disable history expansion with set +H or shopt -u -o histexpand. See History Expansion.

How to source a dotenv (.env) file in dash?

There are a lot of examples here how to source a dotenv file in bash but has anyone one a method that achieves the same with dash (which is the default shell for minimal Debian installations)?
The solution should look like this:
$ some foo my-command-using-env-vars
e.g.
$ env $(cat .env) my-command-using-env-vars
And it is important that the solution supports multiline values with spaces like:
SSH_PRIVATE_KEY="-----BEGIN RSA PRIVATE KEY-----\nfoo\nbar\baz"
and special characters like hash within quotes:
SPECIAL="foo#bar"
It seems that your problem is not so much that you're using dash but that you want to support \n escapes. The following works in dash and supports \n escapes:
eval "$(echo $(cat .env))" my-command-using-env-vars
That's because unlike in bash the built-in echo in dash supports backslash escapes even without the -e option. The following works in both bash and dash, provided that the non-built-in, system echo supports the -e option:
eval "$(env echo -e $(cat .env))" my-command-using-env-vars
Note that both approaches will also handle other backslash escapes (either POSIX or GNU), possibly in a different way than you expect.
Some technical notes:
$(cat .env)
performs Field Splitting, converting any newline in file .env into spaces.
"$(env echo -e ...)"
expands backslash escapes regardless of the current shell by invoking echo -e via env. The double quotes disable field splitting, so that newlines are preserved.

How to address error "bash: !d': event not found" in Bash command substitution [duplicate]

This question already has answers here:
echo "#!" fails -- "event not found"
(5 answers)
Closed 7 years ago.
I am attempting to parse the output of a VNC server startup event and have run into a problem in parsing using sed in a command substitution. Specifically, the remote VNC server is started in a manner such as the following:
address1="user1#lxplus.cern.ch"
VNCServerResponse="$(ssh "${address1}" 'vncserver' 2>&1)"
The standard error output produced in this startup event is then to be parsed in order to extract the server and display information. At this point the content of the variable VNCServerResponse is something such as the following:
New 'lxplus0186.cern.ch:1 (user1)' desktop is lxplus0186.cern.ch:1
Starting applications specified in /afs/cern.ch/user/u/user1/.vnc/xstartup
Log file is /afs/cern.ch/user/u/user1/.vnc/lxplus0186.cern.ch:1.log
This output can be parsed in the following way in order to extract the server and display information:
echo "${VNCServerResponse}" | sed '/New.*desktop.*is/!d' \
| awk -F" desktop is " '{print $2}'
The result is something such as the following:
lxplus0186.cern.ch:1
What I want to do is use this parsing in a command substitution something like the following:
VNCServerAndDisplayNumber="$(echo "${VNCServerResponse}" \
| sed '/New.*desktop.*is/!d' | awk -F" desktop is " '{print $2}')"
On attempting to do this, I am presented with the following error:
bash: !d': event not found
I am not sure how to address this. It appears to be a problem in the way sed is being used in the command substitution. I would appreciate guidance.
Bash history expansion is a very odd corner in the bash command line parser, and you are clearly running into an unexpected history expansion, which is explained below. However, any sort of history expansion in a script is unexpected, because normally history expansion is not enabled in scripts; not even scripts run with the source (or .) builtin.
How history expansion is enabled (or disabled)
There are two shell options which control history expansion:
set -o history: Required for the history to be recorded.
set -H (or set -o histexpand): Additionally required for history expansion to be enabled.
Both of these options must be set for history expansion to be recognized. (I found the manual unclear on this interaction, but it's logical enough.)
According to the bash manual, these options are unset for non-interactive shells, so if you want to enable history expansion in a script (and I cannot imagine a reason you would want this), you would need to set both of them:
set -o history -o histexpand
The situation for scripts run with source is more complicated (and what I'm about to say only applies to bash v4, and since it's undocumented in might change in the future). [Note 3]
History recording (and consequently expansion) is turned off in source'd scripts, but through an internal flag which, as far as I know, is not made visible. It certainly does not appear in $SHELLOPTS. Since a sourced script runs in the current bash context, it shares the current execution environment, including shell options. So in the execution of a sourced script initiated from an interactive session, you'll see both history and histexpand in $SHELLOPTS, but no history expansion will take place. In order to enable it, you need to:
set -o history
which is not a no-op because it has the side-effect of resetting the internal flag which suppresses history recording. Setting the histexpand shell option does not have this side-effect.
In short, I'm not sure how you managed to enable history expansion in a script (if, indeed, the misbehaving command was in a script and not in an interactive shell), but you might want to consider not doing so, unless you have a really good reason.
How history expansion is parsed
The bash implementation of history expansion is designed to work with readline, so that it can be performed during command input. (By default this function is bound to Meta-^; generally Meta is ESC, but you can customize that as well.) However, it is also performed immediately after each line is input, before any bash parsing is performed.
By default, the history expansion character is !, and -- as mostly documented -- that will trigger history expansion except:
when it is followed by whitespace or =
if the shell option extglob is set, and it is followed by ( [Note 1]
if it appears in a single-quoted string
if it is preceded by a \ [Note 2 and see below]
if it is preceded by $ or ${ [Note 1]
if it is preceded by [ [Note 1]
(As of bash v4.3) if it is the last character in a double-quoted string.
The immediate issue here is the precise interpretation of the third case, an ! appearing inside of a single-quoted string. Normally, bash starts a new quoting context for a command substitution ($(...) or the deprecated backtick notation). For example:
$ s=SUBSTITUTED
$ # The interior single quotes are just characters
$ echo "'Echoing $s'"
'Echoing SUBSTITUTED'
$ # The interior single quotes are single quotes
$ echo "$(echo 'Echoing $s')"
Echoing $s
However, the history expansion scanner isn't that intelligent. It keeps track of quotes, but not of command substitution. So as far as it is concerned, both of the single quotes in the above example are double-quoted single quotes, which is to say ordinary characters. So history expansion occurs in both of them:
# A no-op to indicated history expansion
$ HIST() { :; }
# Single-quoted strings inhibit history expansion
$ HIST
$ echo '!!'
!!
# Double-quoted strings allow history expansion
$ HIST
$ echo "'!!'"
echo "'HIST'"
'HIST'
# ... and it applies also to interior command substitution.
$ HIST
$ echo "$(echo '!!')"
echo "$(echo 'HIST')"
HIST
So if you have a perfectly normal command like sed '/foo/!d' file, where you would expect the single-quotes to protect you from history-expansion, and you put it inside a double-quoted command substitution:
result="$(sed '/foo/!d' file)"
you suddenly find that the ! is a history expansion character. Worse, you can't fix this by backslash escaping the exclamation point, because although "\!" inhibits history expansion, it doesn't remove the backslash:
$ echo "\!"
\!
In this particular example -- and the one in the OP -- the double quotes are completely unnecessary, because the right-hand side of a variable assignment does not undergo either filename expansion nor word splitting. However, there are other contexts in which removing the double quotes would change the semantics:
# Undesired history expansion
printf "The answer is '%s'\n" "$(sed '/foo/!d' file)"
# Undesired word splitting
printf "The answer is '%s'\n" $(sed '/foo/!d' file)
In this case, the best solution is probably to put the sed argument in a variable
# Works
sed_prog='/foo/!d'
printf "The answer is '%s'\n" "$(sed "$sed_prog" file)"
(The quotes around $sed_prog were not necessary in this case but usually they would be, and they do no harm.)
Notes:
The inhibition of history expansion when the following character is some form of open parenthesis only works if there is a corresponding close parenthesis in the rest of the string. However, it doesn't have to really match the open parenthesis. For example:
# No matching close parenthesis
$ echo "!("
bash: !: event not found
# The matching close parenthesis has nothing to do with the open
$ echo "!(" ")"
!( )
# An actual extended glob: files whose names don't start with a
$ echo "!(a*)"
b
As indicated in the bash manual, a history-expansion character is treated as an ordinary character if immediately preceded by a backslash. This is literally true; it doesn't matter whether the backslash will later be considered an escape character or not:
$ echo \!
!
$ echo \\!
\!
$ echo \\\!
\!
\ also inhibits history expansion inside double quotes, but \! is not a valid escape sequence inside the double quoted string, so the backslash is not removed:
$ echo "\!"
\!
$ echo "\\!"
\!
$ echo "\\\!"
\\!
I'm referring to the source code for bash v4.2 as I write this, so any undocumented behaviour may be completely different as of v4.3.
The problem is that within double quotes, bash is trying to expand !d before passing it to the subshell. You can get around this problem by removing the double quotes but I would also propose a simplification to your script:
VNCServerAndDisplayNumber=$(echo "$VNCServerResponse" | awk '/desktop/ {print $NF}')
This simply prints the last field on the line containing the word "desktop".
On a newer bash, you can use a herestring rather than piping an echo:
VNCServerAndDisplayNumber=$(awk '/desktop/ {print $NF}' <<<"$VNCServerResponse")
Don't wrap the $(...) command substitution in double quotes. You are asking the shell to perform evaluation on the contents of the quotes and are hitting the history substitution expansion feature. Drop the quotes and you stop telling the shell to do that and you won't hit that problem.
And yes, dropping those quotes is safe on that assignment line even if the output may contain spaces or newlines or whatever. Assignments of that sort are not going to split on those the way command substitution or variable evaluation will on a normal shell execution line.
Alternatively, disable history expansion in your shell/script before you run that. (It should be off when running a script by default I believe anyway.)
This only happens when history expansion is enabled, which it normally isn't and definitely shouldn't be for scripts.
Rather than trying to work around it, figure out why history expansion is enabled and what to do so it isn't.
If you're executing your script with . foo or source foo, use ./foo instead.
If you're writing this as a function in .bashrc or similar, consider making it a separate script.
If your script (or BASH_ENV) explicitly does set -H, don't.
Quote it with '' or \ or disable history expansion with set +H or shopt -u -o histexpand. See History Expansion.

How to escape history expansion exclamation mark ! inside a double quoted string?

EDIT: the command substitution is not necessary for the surprising behavior, although it is the most common use case. The same question applies to just echo "'!b'"
b=a
# Enable history substitution.
# This option is on by default on interactive shells.
set -H
echo '!b'
# Output: '!b'
# OK. Escaped by single quotes.
echo $(echo '!b')
# Output: '!b'
# OK. Escaped by single quotes.
echo "$(echo '$b')"
# Output: '$b'
# OK. Escaped by single quotes.
echo "$(echo '!b')"
# Output: history expands
# BAD!! WHY??
In the last example, what is the best way to escape the !?
Why was it not escaped even if I used single quotes, while echo "$(echo '$b')" was? What is the difference between ! and $?
Why was does echo $(echo '!b') (no quotes) work? (pointed by #MBlanc).
I would prefer to do this without:
set +H as I would need set -H afterwards to maintain shell state
backslash escapes, because I need one for every ! and it has to be outside the quotes:
echo "$(echo '\!a')"
# '\!a'.
# No good.
echo "$(echo 'a '\!' b '\!' c')"
# a ! b ! c
# Good, but verbose.
echo $(echo '!b') (no quotes), because the command could return spaces.
Version:
bash --version | head -n1
# GNU bash, version 4.2.25(1)-release (i686-pc-linux-gnu)
In your last example,
echo "$(echo '!b')"
the exclamation point is not single-quoted. Because history expansion occurs so early in the parsing process, the single quotes are just part of the double-quoted string; the parser hasn't recognized the command substitution yet to establish a new context where the single quotes would be quoting operators.
To fix, you'll have to temporarily turn off history expansion:
set +H
echo "$(echo '!b')"
set -H
This was repeatedly reported as a bug, most recently against bash 4.3 in 2014, for behavior going back to bash 3.
There was some discussion whether this constituted a bug or expected but perhaps undesirable behavior; it seems the consensus has been that, however you want to characterize the behavior, it shouldn't be allowed to continue.
It's fixed in bash 4.4, echo "$(echo '!b')" doesn't expand, echo "'!b'" does, which I regard as proper behavior because the single quotes are shell syntax markers in the first example and not in the second.
If History Expansion is enabled, you can only echo the ! character if it is put in single quotes, escaped or if followed by a whitespace character, carriage return, or =.
From man bash:
Only backslash (\) and single quotes can quote the history
expansion character.
Several characters inhibit history expansion if found immediately fol-
lowing the history expansion character, even if it is unquoted: space,
tab, newline, carriage return, and =.
I believe the key word here is “Only”. The examples provided in the question only consider the outer most quotes being double quotes.
Sometimes you need to make a small addition to a big command pipe
The OP's "Good, but verbose" example is actually pretty awesome for many cases.
Please forgive the contrived example. The whole reason I need such a solution is that I have a lot of distracting, nested code. But, it boils down to: I must do a !d in sed within a double quoted bash command expansion.
This works
$ ifconfig | sed '/inet/!d'
inet 127.0.0.1 netmask 0xff000000
…
This does not
$ echo "$(ifconfig | sed '/inet/!d')"
-bash: !d': event not found
This is a simplest compromise
$ echo "$(ifconfig | sed '/inet/'\!'d')"
inet 127.0.0.1 netmask 0xff000000
…
Using the compromise allows me to insert a few characters into the existing code and produce a Pull Request that anyone can understand… even though resulting code is more difficult to understand. If I did a complete refactor, the code reviewers would have a much more challenging time verifying it. And of course this bash has no unit tests.

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