When I type Grave accent(`) symbol in terminal, It is like down below.
>
>
>
What does Grave accent(`) symbol do?
The '>' is displayed because the shell 'waits' for the closing quote.
From the man page:
Command Substitution
Command substitution allows the output of a command to replace the com-
mand name. There are two forms:
$(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 $(< file).
Related
I'm trying to understand how the bash works from the inside, and I have a problem understanding why the heredoc's output gets expanded when we set a delimiter without quotes and get expanded with them.
For example :
Delimiter without quotes
bash-3.2$ cat << a
> test
> $SHELL
> a
test
/bin/zsh
Delimiter with quotes :
bash-3.2$ cat << 'a'
> test
> $SHELL
> a
test
$SHELL
Quoting the heredoc delimiter as 'a' or "a" or \a or any other way causes the heredoc to be treated literally with expansion disabled. This is a useful feature for cases where you don't want dollar signs and other special characters to be expanded: for example, when the heredoc contains an embedded shell script.
This behavior is described in the Bash manual (emphasis added):
3.6.6 Here Documents
This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input (or file descriptor n if n is specified) for a command.
The format of here-documents is:
[n]<<[-]word
here-document
delimiter
No parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion is performed on word. If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \newline is ignored, and \ must be used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
In bash it's possible to put a file as an argument using the form "$(< file)". Where is the documentation for the presumably special casing of $(<?
It's in the man page, under "Command substitution":
Command Substitution
Command substitution allows the output of a command to replace the com-
mand name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command in a subshell environ-
ment and replacing the command 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 com-
mand substitution $(cat file) can be replaced by the equivalent but
faster $(< file).
I'm currently learning Unix bash, through the terminal in Ubuntu 16. I was writing a simple script, so I can practice writing code for Unix. Here is a sample of the code:
report_uptime(){
cat << _EOF_
<H2>System Uptime</H2>
<PRE>$(uptime)</PRE>
_EOF_
return
}
This code doesn't work and the reason is because after cat I'm supposed to use <<- instead of <<. Sometime << works. So when am I supposed to use << and when am I supposed to use<<-?
This works:
report_uptime(){
cat <<- _EOF_
<H2>System Uptime</H2>
<PRE>$(uptime)</PRE>
_EOF_
return
}
This isn't cat syntax per se; it's one of the redirection operators supported by your shell:
https://www.gnu.org/software/bash/manual/bashref.html#Here-Documents
3.6.6 Here Documents
This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion is performed on word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \newline is ignored, and ‘\’ must be used to quote the characters ‘\’, ‘$’, and ‘`’.
If the redirection operator is ‘<<-’, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
This is not strictly speaking about the syntax of the cat utility, but of the syntax of the shell itself, i.e. of bash.
The construct that you're using is called a "here-document". A here-document feeds its contents into the standard input of whatever command you put before <<.
The syntax is this:
command <<word
...
contents
...
END_TAG
The word here is either exactly the same as the END_TAG, or it may be 'END_TAG', -END_TAG, or -'END_TAG'.
END_TAG: Without single quotes, the contents of the here-document will undergo substitution. This means that any variable, or simply put, "anything that contains a $" will be replaced with its value.
$ tr 'a-z' 'A-Z' <<TR_END
> This is my $HOME
> TR_END
THIS IS MY /USERS/KK
(The >, a greater-than sign and a space, is what's called the secondary prompt. I get that because I'm typing this directly into the shell, and it needs more lines of input before it can execute the whole command. It is not typed by me.)
'END_TAG': With single quotes, the contents of the here-document will not undergo substitution. This means, for example, that if you write $HOME in the here-document, it will be fed into the command just like that, not like /home/myname (or whatever your home directory may be).
$ tr 'a-z' 'A-Z' <<'TR_END'
> This is my $HOME
> TR_END
THIS IS MY $HOME
With a leading dash (-), the shell will strip off all tabs (but not spaces) at the start of each line of the here-document, including the line with the END_TAG at the end.
$ tr 'a-z' 'A-Z' <<-TR_END
> This line has a tab.
> This one does not.
> TR_END
THIS LINE HAS A TAB.
THIS ONE DOES NOT.
Without a leading dash, the shell will not strip off tabs. The END_TAG needs to be the first (and only) thing on the line ending the here-document.
$ tr 'a-z' 'A-Z' <<TR_END
> This line has a tab.
> This one does not.
> TR_END
THIS LINE HAS A TAB.
THIS ONE DOES NOT.
The bash shell also has something called "here-strings". It works in a similar fashion, but you're only feeding a single line into the command:
command <<<word
For example:
$ tr 'a-z' 'A-Z' <<<"hello world!"
HELLO WORLD!
Generally, the end marker should not be indented. Move it to column 1. Also remove the return statement, and it works for me in bash.
I have a little problem with my bash script.
#!/bin/bash
ex xxx.html << "HERE"
1,$s/\(foo\)/$1\1/
wq
HERE
This is just a little piece of my script. when I run it this is the output.
$1foo
Any way to fix this so the $1 will be the argument given to the script?
Thanks!
Try replacing "HERE" with HERE (unquoted). Also 1,$s becomes 1,\$s.
Here Documents
This type of redirection instructs the shell to read input from the
current source until a line containing only delimiter (with no trailing
blanks) is seen. All of the lines read up to that point are then used
as the standard input for a command.
The format of here-documents is:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion, or
pathname expansion is performed on word. If any characters in word are
quoted, the delimiter is the result of quote removal on word, and the
lines in the here-document are not expanded. If word is unquoted, all
lines of the here-document are subjected to parameter expansion, com-
mand substitution, and arithmetic expansion. In the latter case, the
character sequence \<newline> is ignored, and \ must be used to quote
the characters \, $, and `.
If the redirection operator is <<-, then all leading tab characters are
stripped from input lines and the line containing delimiter. This
allows here-documents within shell scripts to be indented in a natural
fashion.
Bash manual.
Remplace "HERE" with HERE (without quotes) and remplace 1,$ with 1,\$ or %
You could write your script as follows:
#!/bin/bash
ex xxx.html <<-HERE
%s/foo/$1&/
x
HERE
Although you could also build a smaller script:
#!/bin/bash
sed -i "s/foo/$1&/g" xxx.html
Try this.
#!/bin/bash
(echo '1,$s/\(foo\)/'"$1"'\1/'; echo 'wq') | ex xxx.html
That's 1,$s/\(foo\)/ in single quotes, adjacent to $1 in double quotes (so the shell substitutes the parameter), adjacent to \1/ in single quotes.
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.