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
Related
$ a=33
$ echo "$(echo '$a')" # (1)
$a
$ echo "$(echo "$a")" # (2)
33
I can't understand the shell command-line parsing result above.
For command line (1), according to man bash, single quote inside the double-quotes will be parsed as literal, and $a inside the double quotes will be interpolated, so I think the result of command line (1) should be '33'.
You don't have single-quotes in double-quotes. $(....) is a command substitution. Essentially what occurs within (...) takes place within its own subshell. You must employ quoting rules within that subshell. The subshell represents its own environment.
For example:
$ a=33
$ echo $(echo "'$a'") # single-quotes within double-quotes
The quoting rules within the command substitution are applied correctly resulting in the output:
$ '33'
Let me know if you have further questions.
Why is the $bar being printed here as a literal, even thought the outer subshell should expand it's parameters according to bash command line processing rules?
$ foo='$bar' bar=expanded
$ echo $(echo $(echo $foo))
$bar
The inner subshell prints $bar, but why doesn't the outer subshell expand it? Does the bash implicitly pass it as a literal and if so, why and how? According to my knowledge, the parameter expansions happens after each fork of the subshell, inside the new process. In the case of nested subshells, the command substitution is done from inside out, inner subshell printing out the literal, raw text representation of the outer shell command line before the fork happens and the command line (string of characters) is being split, expanded and processed by the new shell. Now the question is, why the text $bar is not expanded in the outer subshell, even thought it actually doesn't contain quotes? What causes it to be implicitly quoted here?
Here is example of the same logic and expected output without nested shells
$ foo='$bar' bar=expanded
$ echo $foo
$bar
$ echo $bar
expanded
Also, by adding eval I get the result which I would expect in the first example, but I don't undertand why it's necessary and how it wokrs.
$ echo $(eval echo $(echo $foo))
expanded
The Bash manual explains the ordering shell expansions: (reformatted for clarity)
The order of expansions is:
brace expansion;
tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion);
word splitting;
and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution.
After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (quote removal).
This essentially echoes the Posix shell specification with the addition of some bash-specific expansions.
Note that the second group of expansions, which includes command substitution ($(...)) is only performed once, left-to-right. They are not performed repetitively, so the result of a command substitution is not subject to parameter expansion. Unless quoted, it is subject to word-splitting, filename expansion, and quote removal.
The commands evaluated in subshells are, indeed, evaluated inside out, but at each level the inner command substitution is only subject to word-splitting, filename expansion and quote removal (none of which apply in thus example).
So the only parameter expansion done is the replacement of $foo with its value.
I'm writing a shell, and am getting unexpected parsing from both bash, dash, and busybox's ash:
echo "`echo a #`"
prints a, however
echo "$(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 $( command substitution marker is terminated by the "first matching )"; the implication (made explicit by examples in the Rationale, Note 1) is that the matching ) cannot be inside of a shell comment, here-doc or quoted string.
The quotes surrounding the command substitution are not relevant in either case (although, of course, "undefined results" could be different in the quoted case, since they are undefined.)
In bash and certain other shells, comments could also be present inside process substitution (eg., <(…)); however, process substitution cannot be quoted.
Notes:
Thanks to #mklement0, who included this link in a comment.
Sample bash script
QRY="select * from mysql"
CMD="mysql -e \"$QRY\""
`$CMD`
I get errors because the * is getting evaluated as a glob (enumerating) files in my CWD.
I Have seen other posts that talk about quoting the "$CMD" reference for purposes of echo output, but in this case
"$CMD"
complains the whole literal string as a command.
If I
echo "$CMD"
And then copy/paste it to the command line, things seems to work.
You can just use:
qry='select * from db'
mysql -e "$qry"
This will not subject to * expansion by shell.
If you want to store mysql command line also then use BASH arrays:
cmd=(mysql -e "$qry")
"${cmd[#]}"
Note: anubhava's answer has the right solution.
This answer provides background information.
As for why your approach didn't work:
"$CMD" doesn't work, because bash sees the entire value as a single token that it interprets as a command name, which obviously fails.
`$CMD`
i.e., enclosing $CMD in backticks, is pointless in this case (and will have unintended side effects if the command produces stdout output[1]); using just:
$CMD
yields the same - broken - result (only more efficiently - by enclosing in backticks, you needlessly create a subshell; use backticks - or, better, $(...) only when embedding one command in another - see command substitution).
$CMD doesn't work,
because unquoted use of * subjects it to pathname expansion (globbing) - among other shell expansions.
\-escaping glob chars. in the string causes the \ to be preserved when the string is executed.
While it may seem that you've enclosed the * in double quotes by placing it (indirectly) between escaped double quotes (\"$QRY\") inside a double-quoted string, the shell does not see what's between these escaped double quotes as a single, double-quoted string.
Instead, these double quotes become literal parts of the tokens they abut, and the shell still performs word splitting (parsing into separate arguments by whitespace) on the string, and expansions such as globbing on the resulting tokens.
If we assume for a moment that globbing is turned off (via set -f), here is the breakdown of the arguments passed to mysql when the shell evaluates (unquoted) $CMD:
-e # $1 - all remaining arguments are the unintentionally split SQL command.
"select # $2 - note that " has become a literal part of the argument
* # $3
from # $4
mysql" # $5 - note that " has become a literal part of the argument
The only way to get your solution to work with the existing, single string variable is to use eval as follows:
eval "$CMD"
That way, the embedded escaped double-quoted string is properly parsed as a single, double-quoted string (to which no globbing is applied), which (after quote removal) is passed as a single argument to mysql.
However, eval is generally to be avoided due to its security implications (if you don't (fully) control the string's content, arbitrary commands could be executed).
Again, refer to anubhava's answer for the proper solution.
[1] A note re using `$CMD` as a command by itself:
It causes bash to execute stdout output from $CMD as another command, which is rarely the intent, and will typically result in a broken command or, worse, a command with unintended effects.
Try running `echo ha` (with the backticks - same as: $(echo ha)); you'll get -bash: ha: command not found, because bash tries to execute the command's output - ha - as a command, which fails.
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.