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Dollar character before double quotes in Bash
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Closed 5 years ago.
I was just reading through the /etc/init.d/httpd on a Centos 6.5 box and noticed that all of the strings seem to be quoted like $"Hello World.". I've never seen this syntax before, and I can't seem to turn up anything via google.
Excerpt:
if ! LANG=$HTTPD_LANG $httpd $OPTIONS -t >&/dev/null; then
RETVAL=6
echo $"not reloading due to configuration syntax error"
failure $"not reloading $httpd due to configuration syntax error"
What's the deal?
From man bash
Words of the form $'string' are treated specially. The word expands to
string, with backslash-escaped characters replaced as specified by the
ANSI C standard. Backslash escape sequences, if present, are decoded
as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is the
hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character
The expanded result is single-quoted, as if the dollar sign had not
been present.
A double-quoted string preceded by a dollar sign ($"string") will cause
the string to be translated according to the current locale. If the
current locale is C or POSIX, the dollar sign is ignored. If the
string is translated and replaced, the replacement is double-quoted.
Related
I'd expect echo -E \n and echo -E "\n" to be equivalents. However echo -E \n prints n (backslash is not printed) whereas echo -E "\n" prints \n (backslash is printed). Apparently backslashs are interpreted differently not only in single and double quotes, but also in double quotes and outside of quotes. How are backslashs interpreted outside of quotes?
Backslashes are always removed, unless they're themselves escaped (with another backslash) or inside single- or double-quotes. From the POSIX standard for shell syntax, section 2.2.1 "Escape Character (Backslash)":
A <backslash> that is not quoted shall preserve the literal value of the following character, with the exception of a <newline>. If a <newline> follows the <backslash>, the shell shall interpret this as line continuation. The <backslash> and <newline> shall be removed before splitting the input into tokens. Since the escaped <newline> is removed entirely from the input and is not replaced by any white space, it cannot serve as a token separator.
...so outside of quotes, the shell interprets \n as a literal n.
Inside double-quotes, on the other hand (section 2.2.3 "Double-Quotes"):
\
The <backslash> shall retain its special meaning as an escape character (see Escape Character (Backslash)) only when followed by one of the following characters when considered special:
$ ` " \ <newline>
...since in "\n", the backslash is not followed by one of those characters, it doesn't retain its special meaning and is just passed through as a literal backslash.
BTW, just to add confusion, some versions of echo will do their own backslash-interpretation (on any that make it past the shell's parsing process), using yet another set of rules. In some versions, -E disables this... but some just print "-E" as part of their output. If you want predictability, use printf instead:
printf '%s\n' \n # Prints just 'n'
printf '%s\n' "\n" # Prints '\n'
Outside of quotes, unescaped backslashes are always deleted. They are only used to disable the special meaning of other symbols.
Inside double quotes, backslashes are kept, except when escaping one of $, `, ", \, or marking a line continuation.
From POSIX Shell Command Language, emphasis mine:
2.2.1 Escape Character (Backslash)
A <backslash> that is not quoted shall preserve the literal value of the following character, with the exception of a <newline>. If a <newline> follows the <backslash>, the shell shall interpret this as line continuation. The <backslash> and <newline> shall be removed before splitting the input into tokens. [....]
2.2.3 Double-Quotes
Enclosing characters in double-quotes ( "" ) shall preserve the literal value of all characters within the double-quotes, with the exception of the characters backquote, <dollar-sign>, and <backslash>, as follows:
[...]
\
The <backslash> shall retain its special meaning as an escape character (see Escape Character (Backslash)) only when followed by one of the following characters when considered special:
$ ` " \ <newline>
I'm having trouble determining what characters '$#' actually counts
Example:
bold=$'\e[1m'
red=$'\e[0;31m'
clr=$'\e[0m'
string="${red}[!]${clr} ${bold}Warning:${clr} foo bar"
printf "String count: %s\n" "${#string}"
Output:
String count: 39
The length of string with its variables is 45 characters. The length of string with its variables substituted is 43 characters (i.e. \e[0;31m in place of ${red}, etc.).
So, what characters is the shell not counting for it to output 39 as the total length of the string?
To simplify the answer, consider the strings '\n', and $'\n'
This first is 2 characters. The second is single character (newline).
As per bash man:
Words of the form $'string' are treated specially. The word expands
to string, with backslash-escaped characters replaced as specified by
the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
...
\n new line
\r carriage return
\t horizontal tab
...
Replacement of backslash-escaped characters also occur with echo -e, evaluation of various dynamic parameters (TIMEFORMAT, PS2, ..). Many utilities will parse escape sequences (e.g., sed, tr), sometimes with different rules. Unfortunately, those situation, in many cases dictated by backward compatibility can be confusing and may conflict with each other, sometimes resulting in having to double, triple and sometimes quadruple escapes, when strings have to be passed to external programs.
Got kinda surprised with:
$ node -p 'process.argv' $SHELL '$SHELL' \t '\t' '\\t'
[ 'node', '/bin/bash', '$SHELL', 't', '\\t', '\\\\t' ]
$ python -c 'import sys; print sys.argv' $SHELL '$SHELL' \t '\t' '\\t'
['-c', '/bin/bash', '$SHELL', 't', '\\t', '\\\\t']
Expected the same behavior as with:
$ echo $SHELL '$SHELL' \t '\t' '\\t'
/bin/bash $SHELL t \t \\t
Which is how I need the stuff to be passed in.
Why the extra escape with '\t', '\\t' in process argv? Why handled differently than '$SHELL'? Where's this actually coming from? Why different from the echo behavior?
First I thought this to be some extras on the minimist part, but then got the same with both bare Node.js and Python. Might be missing something obvious here.
Use $'...' form to pass escape sequences like \t, \n, \r, \0 etc in BASH:
python -c 'import sys; print sys.argv' $SHELL '$SHELL' \t $'\t' $'\\t'
['-c', '/bin/bash', '$SHELL', 't', '\t', '\\t']
As per man bash:
Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal value nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character
In both python and node.js, there is a difference between the way print works with scalar strings and the way it works with collections.
Strings are printed simply as a sequence of characters. The resulting output is generally what the user expects to see, but it cannot be used as the representation of the string in the language. But when a list/array is printed out, what you get is a valid list/array literal, which can be used in a program.
For example, in python:
>>> print("x")
x
>>> print(["x"])
['x']
When printing the string, you just see the characters. But when printing the list containing the string, python adds quote characters, so that the output is a valid list literal. Similarly, it would add backslashes, if necessary:
>>> print("\\")
\
>>> print(["\\"])
['\\']
node.js works in exactly the same way:
$ node -p '"\\"'
\
$ node -p '["\\"]'
[ '\\' ]
When you print the string containing a single backslash, you just get a single backslash. But when you print a list/array containing a string consisting of a single backslash, you get a quoted string in which the backslash is escaped with a backslash, allowing it to be used as a literal in a program.
As with the printing of strings in node and python, the standard echo shell utility just prints the actual characters in the string. In a standard shell, there is no mechanism similar to node and python printing of arrays. Bash, however, does provide a mechanism for printing out the value of a variable in a format which could be used as part of a bash program:
$ quote=\"
# $quote is a single character:
$ echo "${#quote}"
1
# $quote prints out as a single quote, as you would expect
$ echo "$quote"
"
# If you needed a representation, use the 'declare' builtin:
$ declare -p quote
declare -- quote="\""
# You can also use the "%q" printf format (a bash extension)
$ printf "%q\n" "$quote"
\"
(References: bash manual on declare and printf. Or type help declare and help printf in a bash session.)
That's not the full story, though. It is also important to understand how the shell interprets what you type. In other words, when you write
some_utility \" "\"" '\"'
What does some_utility actually see in the argv array?
In most contexts in a standard shell (including bash), C-style escapes sequences like \t are not interpreted as such. (The standard shell utility printf does interpret these sequences when they appear in a format string, and some other standard utilities also interpret the sequences, but the shell itself does not.) The handling of backslash by a standard shell depends on the context:
Unquoted strings: the backslash quotes the following character, whatever it is (unless it is a newline, in which case both the backslash and the newline are removed from the input).
Double-quoted strings: backslash can be used to escape the characters $, \, ", `; also, a backslash followed by a newline is removed from the input, as in an unquoted string. In bash, if history expansion is enabled (as it is by default in interactive shells), backslash can also be used to avoid history expansion of !, but the backslash is retained in the final string.
Single-quoted strings: backslash is treated as a normal character. (As a result, there is no way to include a single quote in a single-quoted string.)
Bash adds two more quoting mechanisms:
C-style quoting, $'...'. If a single-quoted string is preceded by a dollar sign, then C-style escape sequences inside the string are interpreted in roughly the same way a C compiler would. This includes the standard whitespace characters such as newline (\n), octal, hexadecimal and unicode escapes (\010, \x0a, \u000A, \U0000000A), plus a few non-C sequences including "control" characters (\cJ) and the ESC character \e or \E (the same as \x1b). Backslashes can also be used to escape \, ' and ". (Note that this is a different list from the list of backslashable characters in double-quoted strings; here, a backslash before a dollar sign or a backtic is not special, while a backslash before a single quote is special; moreover, the backslash-newline sequence is not interpreted.)
Locale-specific Translation: $"...". If a double-quoted string is preceded by a dollar sign, backslashes (and variable expansions and command substitutions) are interpreted as with a normal double-quoted strings, and then the string is looked up in a message catalog determined by the current locale.
(References: Posix standard, Bash manual.)
I'm taking a course on lexical analysis, and \t\v\r is used in the lexer token definitions to represent white spaces. What are \v and \r exactly??
\t is a horizontal tab, \v is a vertical tab and \r is a carriage return.
They are certainly a sub-set of white space characters.
\' Single quote (')
\" Double quote (")
\a ASCII Bell (BEL)
\b ASCII Backspace (BS)
\f ASCII Formfeed (FF)
\n ASCII Linefeed (LF)
\N{name} Character named name in the Unicode database (Unicode only)
\r ASCII Carriage Return (CR)
\t ASCII Horizontal Tab (TAB)
\uxxxx Character with 16-bit hex value xxxx (Unicode only) (1)
\Uxxxxxxxx Character with 32-bit hex value xxxxxxxx (Unicode only) (2)
\v ASCII Vertical Tab (VT)
does not work
echo ${REV%%\n*}
does work
echo ${REV%%
*}
After reading trough http://tldp.org/LDP/abs/html/parameter-substitution.html I still can not figure out how to make \n work.
${REV%%$'\n*'} seems to work. See the quoting section of the bash documentation.
If your intent is to try and get it on one line and you're willing to go "outside" of bash, you can use:
echo "$(echo "${REV}" | head -1l)"
But, assuming your version of bash is recent enough, you can try:
pax> export REV="abc
...> def"
pax> echo "${REV}"
abc
def
pax> echo "${REV%%$'\n'*}"
abc
The reason you need $'\n' is because the bash definition of word is somewhat restrictive, compared to what you expect. The bash manpage has this to say:
Words of the form $'string' are treated specially. The word expands
to string, with backslash-escaped characters replaced as specified by
the ANSI C standard. Backslash escape sequences, if present, are decoded
as follows:
\a alert (bell)
\b backspace
\e
\E an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\' single quote
\" double quote
\nnn the eight-bit character whose value is the octal value
nnn (one to three digits)
\xHH the eight-bit character whose value is the hexadecimal
value HH (one or two hex digits)
\uHHHH the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHH (one to four hex digits)
\UHHHHHHHH
the Unicode (ISO/IEC 10646) character whose value is
the hexadecimal value HHHHHHHH (one to eight hex digits)
\cx a control-x character
More portably (ie, POSIX):
var="yo
yo"
newline="
"
echo "${var#*"$newline"}"