Have backticks (i.e. `cmd`) in *sh shells been deprecated?

There are two different meanings of "deprecated."

be deprecated: (chiefly of a software feature) be usable but regarded as obsolete and best avoided, typically due to having been superseded.

—New Oxford American Dictionary

By this definition backticks are deprecated.

Deprecated status may also indicate the feature will be removed in the future.

—Wikipedia

By this definition backticks are not deprecated.

Still supported:

Citing the Open Group Specification on Shell Command Languages, specifically section "2.6.3 Command Substitution," it can be seen that both forms of command substitution, backticks (`..cmd..`) or dollar parens ($(..cmd..)) are still supported insofar as the specification goes.

excerpt

Command substitution allows the output of a command to be substituted in place of the command name itself. Command substitution shall occur when the command is enclosed as follows:
          $(command)

          or (backquoted version):

          `command`
The shell shall expand the command substitution by executing command in a subshell environment (see Shell Execution Environment) and replacing the command substitution (the text of command plus the enclosing $() or backquotes) with the standard output of the command, removing sequences of one or more <newline> characters at the end of the substitution. Embedded <newline> characters before the end of the output shall not be removed; however, they may be treated as field delimiters and eliminated during field splitting, depending on the value of IFS and quoting that is in effect. If the output contains any null bytes, the behavior is unspecified.

Within the backquoted style of command substitution, <backslash> shall retain its literal meaning, except when followed by: '$', '\`', or <backslash>. The search for the matching backquote shall be satisfied by the first unquoted non-escaped backquote; during this search, if a non-escaped backquote is encountered within a shell comment, a here-document, an embedded command substitution of the $(command) form, or a quoted string, undefined results occur. A single-quoted or double-quoted string that begins, but does not end, within the "`...`" sequence produces undefined results.

With the $(command) form, all characters following the open parenthesis to the matching closing parenthesis constitute the command. Any valid shell script can be used for command, except a script consisting solely of re-directions which produces unspecified results.

So then why does everyone say that backticks have been deprecated?

Because most of the use cases should be making use of the dollar parens form instead of backticks. (Deprecated in the first sense above.) Many of the most reputable sites (including U&L) often state this as well, throughout, so it's sound advice. This advice should not be confused with some non-existent plan to remove support for backticks from shells.

BashFAQ #082 - Why is $(...) preferred over `...` (backticks)?

excerpt

`...` is the legacy syntax required by only the very oldest of non-POSIX-compatible bourne-shells. There are several reasons to always prefer the $(...) syntax:

...
Bash Hackers Wiki - Obsolete and deprecated syntax

excerpt

This is the older Bourne-compatible form of the command substitution. Both the `COMMANDS` and $(COMMANDS) syntaxes are specified by POSIX, but the latter is greatly preferred, though the former is unfortunately still very prevalent in scripts. New-style command substitutions are widely implemented by every modern shell (and then some). The only reason for using backticks is for compatibility with a real Bourne shell (like Heirloom). Backtick command substitutions require special escaping when nested, and examples found in the wild are improperly quoted more often than not. See: Why is $(...) preferred over `...` (backticks)?.
POSIX standard rationale

excerpt

Because of these inconsistent behaviors, the backquoted variety of command substitution is not recommended for new applications that nest command substitutions or attempt to embed complex scripts.

NOTE: This third excerpt (above) goes on to show several situations where backticks simply won't work, but the newer dollar parens method does, beginning with the following paragraph:

Additionally, the backquoted syntax has historical restrictions on the contents of the embedded command. While the newer "$()" form can process any kind of valid embedded script, the backquoted form cannot handle some valid scripts that include backquotes.

If you continue reading that section the failures are highlighted showing how they would fail using backticks, but do work using the newer dollar parens notation.

Conclusions

So it's preferable that you use dollar parens instead of backticks but you aren't actually using something that's been technically "deprecated" as in "this will stop working entirely at some planned point."

After reading all this you should have the take away that you're strongly encouraged to use dollar parens unless you specifically require compatibility with a real original non-POSIX Bourne shell.

It's not deprecated, but the backticks (`...`) is the legacy syntax required by only the very oldest of non-POSIX-compatible bourne-shells and $(...) is POSIX and more preferred for several reasons:

Backslashes (\) inside backticks are handled in a non-obvious manner:

$ echo "`echo \\a`" "$(echo \\a)"
a \a
$ echo "`echo \\\\a`" "$(echo \\\\a)"
\a \\a
# Note that this is true for *single quotes* too!
$ foo=`echo '\\'`; bar=$(echo '\\'); echo "foo is $foo, bar is $bar" 
foo is \, bar is \\

Nested quoting inside $() is far more convenient:
```
echo "x is $(sed ... <<<"$y")"
```
instead of:
```
echo "x is `sed ... <<<\"$y\"`"
```
or writing something like:
```
IPs_inna_string=`awk "/\`cat /etc/myname\`/"'{print $1}' /etc/hosts`
```
because $() uses an entirely new context for quoting

which is not portable as Bourne and Korn shells would require these backslashes, while Bash and dash don't.
Syntax for nesting command substitutions is easier:
```
x=$(grep "$(dirname "$path")" file)
```
than:
```
x=`grep "\`dirname \"$path\"\`" file`
```
because $() enforces an entirely new context for quoting, so each command substitution is protected and can be treated on its own without special concern over quoting and escaping. When using backticks, it gets uglier and uglier after two and above levels.

Few more examples:
```
echo `echo `ls``      # INCORRECT
echo `echo \`ls\``    # CORRECT
echo $(echo $(ls))    # CORRECT
```
It solves a problem of inconsistent behavior when using backquotes:
- echo '\$x' outputs \$x
- echo `echo '\$x'` outputs $x
- echo $(echo '\$x') outputs \$x

Backticks syntax has historical restrictions on the contents of the embedded command and cannot handle some valid scripts that include backquotes, while the newer $() form can process any kind of valid embedded script.

For example, these otherwise valid embedded scripts do not work in the left column, but do work on the right^IEEE:

echo `                         echo $(
cat <<\eof                     cat <<\eof
a here-doc with `              a here-doc with )
eof                            eof
`                              )


echo `                         echo $(
echo abc # a comment with `    echo abc # a comment with )
`                              )


echo `                         echo $(
echo '`'                       echo ')'
`                              )

Therefore the syntax for $-prefixed command substitution should be the preferred method, because it is visually clear with clean syntax (improves human and machine readability), it is nestable and intuitive, its inner parsing is separate, and it is also more consistent (with all other expansions that are parsed from within double-quotes) where backticks are the only exception and ` character is easily camouflaged when adjacent to " making it even more difficult to read, especially with small or unusual fonts.

Source: Why is $(...) preferred over `...` (backticks)? at BashFAQ

Have backticks (i.e. `cmd`) in *sh shells been deprecated?

Still supported:

So then why does everyone say that backticks have been deprecated?

Conclusions

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