Citing the Open Group Specification on Shell Command Languages,
specifically this section "2.6.3 Command Substitution" this statement regarding
backtick's deprecation is actually false. Both forms of the command
substitution, backticks (
`..cmd..`) or dollar parens (
$(..cmd..)) are still supported in so far as the specification goes.
Command substitution allows the output of a command to be substituted in place
of the command name itself. Command substitution shall occur when the command
is enclosed as follows:
or (backquoted version):
The shell shall expand the command substitution by executing command in a
subshell environment (see Shell Execution Environment) and replacing the
command substitution (the text of command plus the enclosing
backquotes) with the standard output of the command, removing sequences of one
or more characters at the end of the substitution. Embedded characters before the end of the output shall not be removed; however,
they may be treated as field delimiters and eliminated during field splitting,
depending on the value of IFS and quoting that is in effect. If the output
contains any null bytes, the behavior is unspecified.
Within the backquoted style of command substitution, <backslash> shall retain
its literal meaning, except when followed by: '$', '
\`', 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.
$(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 vs. the backticks. Many of the most reputable sites (including U&L) often state this as well, throughout, so it's sound advice, but it's just that, good advice.
`...` is the legacy syntax required by only the very oldest of
non-POSIX-compatible bourne-shells. There are several reasons to always
This is the older Bourne-compatible form of the command substitution.
$(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 $(...) preferred over `...` (backticks)?.
Because of these inconsistent behaviors, the backquoted variety of command substitution is not recommended for new applications that nest command substitutions or attempt to embed complex scripts.
NOTE: This third excerpt (above) is likely the source of why many would consider the backticks as being deprecated. It shows several situations where backticks simply won't work, but the newer dollar parens method does:
Additionally, the backquoted syntax has historical restrictions on the contents of the embedded command. While the newer "$()" 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.
So it's generally preferable that you use dollar parens vs. backticks but you
aren't actually using something that's been technically deprecated. After reading all this you should have the take away that you're strongly encouraged to use dollar parens unless you have a pressing need to use backticks.