I'm getting different outputs when running the program in bash and dash
#!/bin/sh
echo $SHELL
n=1
a=$((++n))
echo $n
Bash:
$ bash shell_test.sh
2
Dash:
$ dash shell_test.sh
1
dash is the Debian Almquist shell and an extreme light-weight version of a full POSIX-compliant shell-implementation of /bin/sh that aims to be as small as possible creating faster bootup times.
Operators such as $((n++)), $((--n)) and similar are features that are not required by POSIX and therefore not implemented.
To see how dash interprets these statements, see Chepner's answer
A nice page explaining how to make your script POSIX compliant, is here.
2.6.4 Arithmetic Expansion: Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its value. The format for arithmetic expansion shall be as follows:
$((expression))
The expression shall be treated as if it were in double-quotes, except that a double-quote inside the expression is not treated specially. The shell shall expand all tokens in the expression for parameter expansion, command substitution, and quote removal.
Next, the shell shall treat this as an arithmetic expression and substitute the value of the expression. The arithmetic expression shall be processed according to the rules given in Arithmetic Precision and Operations, with the following exceptions:
Only signed long integer arithmetic is required.
Only the decimal-constant, octal-constant, and hexadecimal-constant constants specified in the ISO C standard, Section 6.4.4.1 are required to be recognized as constants.
The sizeof() operator and the prefix and postfix ++ and -- operators are not required.
Selection, iteration, and jump statements are not supported.
source: POSIX IEEE Std 1003.1-2017
Prefix ++ is not required by POSIX, and dash doesn't implement it. Instead, it's parsed as two unary + operators:
$ n=1
$ echo $((+(+n)))
1
$ echo $((++n))
1
$ echo $n
1
I'm writing a shell, and am getting unexpected parsing from both bash, dash, and busybox's ash:
echo "`echo a #`"
prints a, however
echo "$(echo a #)"
gives an error about missing a closing ).
How is a comment in a command-substitution parsed according to POSIX?
So, for the commands:
echo "`echo a #`"
and
echo "$(echo a #)"
Will the shell parse the comment as extending to the end of the command substitution, or to the end of the line?
Also, will the shell parse it differently if the command substitutions are not in double quotes?
Finally, are there any other constructs (either in POSIX or bash) where a comment can start inside quotes like this?
According to Posix (Shell&Utilities, §2.6.3), "`echo a #`" is undefined (implying that it should not be used):
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, … undefined results occur. (emphasis added)
However, the $( command substitution marker is terminated by the "first matching )"; the implication (made explicit by examples in the Rationale, Note 1) is that the matching ) cannot be inside of a shell comment, here-doc or quoted string.
The quotes surrounding the command substitution are not relevant in either case (although, of course, "undefined results" could be different in the quoted case, since they are undefined.)
In bash and certain other shells, comments could also be present inside process substitution (eg., <(…)); however, process substitution cannot be quoted.
Notes:
Thanks to #mklement0, who included this link in a comment.
When I went to answer this question, I was going to use the ${} notation, as I've seen so many times on here that it's preferable to backticks.
However, when I tried
joulesFinal=${echo $joules2 \* $cpu | bc}
I got the message
-bash: ${echo $joules * $cpu | bc}: bad substitution
but
joulesFinal=`echo $joules2 \* $cpu | bc`
works fine. So what other changes do I need to make?
The `` is called Command Substitution and is equivalent to $() (parenthesis), while you are using ${} (curly braces).
So all of these expressions are equal and mean "interpret the command placed inside":
joulesFinal=`echo $joules2 \* $cpu | bc`
joulesFinal=$(echo $joules2 \* $cpu | bc)
# v v
# ( instead of { v
# ) instead of }
While ${} expressions are used for variable substitution.
Note, though, that backticks are deprecated, while $() is POSIX compatible, so you should prefer the latter.
From man bash:
Command substitution allows the output of a command to replace the
command name. There are two forms:
$(command)
or
`command`
Also, `` are more difficult to handle, you cannot nest them for example. See comments below and also Why is $(...) preferred over ... (backticks)?.
They behave slightly differently in a specific case:
$ echo "`echo \"test\" `"
test
$ echo "$(echo \"test\" )"
"test"
So backticks silently remove the double quotes.
${} refer to Shell parameter expansion. Manual link:https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
The ‘$’ character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name or symbol to
be expanded may be enclosed in braces, which are optional but serve to
protect the variable to be expanded from characters immediately
following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first ‘}’ not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
FULLPATH=/usr/share/X11/test.conf_d/sk-synaptics.conf
echo ${FULLPATH##*/}
echo ${FILENAME##*.}
First echo will get filename. second will get file extension as per manual ${parameter##word} section.
$(command)
`command`
refer to command substitution.
Bash performs the expansion by executing command in a subshell
environment and replacing the command substitution with the standard
output of the command, with any trailing newlines deleted.
https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html
I know from the manpage from bash that the variable that is null or unset is regarded as zero.
And I guess that non-number should be regarded as zero in arithmetic evaluation.
But without official ruling, it could be ambiguous like the second case of below example.
$ FOO=10
$ echo $((FOO))
10
$ FOO=10.abc
$ echo $((FOO))
bash: 10.abc: syntax error: invalid arithmetic operator (error token is ".abc")
atoi() from C parses the second example as 10.
What's the formal semantic of
parsing non-number in bash's arithmetic evaluation?
The $(( ... )) construct for arithmetic expansion was first introduced in the Korn shell back in the '80s, and then adopted for bash (which originally had a different syntax for arithmetic expansions) and POSIX.
Bash and ksh93 expect well-formed arithmetic expressions inside $(( ... )). They go way beyond atoi() in parsing them.
How the shells handle empty and unset variables is a red herring in this case. It's a sensible convenience that just happens to make sense in the context of the shell.
This question already has answers here:
What is the difference between $(command) and `command` in shell programming?
(6 answers)
Closed last year.
There are two ways to capture the output of command line in bash:
Legacy Bourne shell backticks ``:
var=`command`
$() syntax (which as far as I know is Bash specific, or at least not supported by non-POSIX old shells like original Bourne)
var=$(command)
Is there any benefit to using the second syntax compared to backticks? Or are the two fully 100% equivalent?
The major one is the ability to nest them, commands within commands, without losing your sanity trying to figure out if some form of escaping will work on the backticks.
An example, though somewhat contrived:
deps=$(find /dir -name $(ls -1tr 201112[0-9][0-9]*.txt | tail -1l) -print)
which will give you a list of all files in the /dir directory tree which have the same name as the earliest dated text file from December 2011 (a).
Another example would be something like getting the name (not the full path) of the parent directory:
pax> cd /home/pax/xyzzy/plugh
pax> parent=$(basename $(dirname $PWD))
pax> echo $parent
xyzzy
(a) Now that specific command may not actually work, I haven't tested the functionality. So, if you vote me down for it, you've lost sight of the intent :-) It's meant just as an illustration as to how you can nest, not as a bug-free production-ready snippet.
Suppose you want to find the lib directory corresponding to where gcc is installed. You have a choice:
libdir=$(dirname $(dirname $(which gcc)))/lib
libdir=`dirname \`dirname \\\`which gcc\\\`\``/lib
The first is easier than the second - use the first.
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 rightIEEE:
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
See also:
POSIX standard section "2.6.3 Command Substitution"
POSIX rationale for including the $() syntax
Command Substitution
bash-hackers: command substitution
From man bash:
$(command)
or
`command`
Bash performs the expansion by executing command and replacing the com-
mand substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by $, `, or \. The
first backquote not preceded by a backslash terminates the command sub-
stitution. When using the $(command) form, all characters between the
parentheses make up the command; none are treated specially.
In addition to the other answers,
$(...)
stands out visually better than
`...`
Backticks look too much like apostrophes; this varies depending on the font you're using.
(And, as I just noticed, backticks are a lot harder to enter in inline code samples.)
$() allows nesting.
out=$(echo today is $(date))
I think backticks does not allow it.
It is the POSIX standard that defines the $(command) form of command substitution. Most shells in use today are POSIX compliant and support this preferred form over the archaic backtick notation. The command substitution section (2.6.3) of the Shell Language document describes this:
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 $(command) form, all characters following the open
parenthesis to the matching closing parenthesis constitute the
command. Any valid shell script can be used for command, except a
script consisting solely of redirections which produces unspecified
results.
The results of command substitution shall not be processed for further
tilde expansion, parameter expansion, command substitution, or
arithmetic expansion. If a command substitution occurs inside
double-quotes, field splitting and pathname expansion shall not be
performed on the results of the substitution.
Command substitution can be nested. To specify nesting within the
backquoted version, the application shall precede the inner backquotes
with <backslash> characters; for example:
\`command\`
The syntax of the shell command language has an ambiguity for expansions beginning with "$((",
which can introduce an arithmetic expansion or a command substitution that starts with a subshell.
Arithmetic expansion has precedence; that is, the shell shall first determine
whether it can parse the expansion as an arithmetic expansion
and shall only parse the expansion as a command substitution
if it determines that it cannot parse the expansion as an arithmetic expansion.
The shell need not evaluate nested expansions when performing this determination.
If it encounters the end of input without already having determined
that it cannot parse the expansion as an arithmetic expansion,
the shell shall treat the expansion as an incomplete arithmetic expansion and report a syntax error.
A conforming application shall ensure that it separates the "$(" and '(' into two tokens
(that is, separate them with white space) in a command substitution that starts with a subshell.
For example, a command substitution containing a single subshell could be written as:
$( (command) )
I came up with a perfectly valid example of $(...) over `...`.
I was using a remote desktop to Windows running Cygwin and wanted to iterate over a result of a command. Sadly, the backtick character was impossible to enter, either due to the remote desktop thing or Cygwin itself.
It's sane to assume that a dollar sign and parentheses will be easier to type in such strange setups.
Here in 2021 it is worth mentioning a curious fact as a supplement to the other answers.
The Microsoft DevOps YAML "scripting" for pipelines may include Bash tasks. However, the notation $() is used for referring to variables defined in the YAML context, so in this case backticks should be used for capturing the output of commands.
This is mostly a problem when copying scripting code into a YAML script since the DevOps preprocessor is very forgiving about nonexisting variables, so there will not be any error message.