Shell quotation marks and escape are rarely noticed, but they are often used in writing scripts

Shell Quotes and escapes

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Bash There is only one data type , It's just a string . No matter what data the user enters ,Bash Are treated as strings . therefore , String related quotes and escapes , Yes Bash It's very important for .

1. escape

Some characters are in Bash It has a special meaning （ such as $、&、*）.

$ echo $date

$

In the example above , Output $date There will be no result , because $ It's a special character .

If you want to output these special characters as they are , You have to precede them with a backslash , Make it a normal character . This is called “ escape ”（escape）.

$ echo \$date
$date

In the above command , Only in special characters $ Preceded by a backslash , Can be output as is .

The backslash itself is also a special character , If you want to output the backslash as it is , It needs to escape itself , Use two consecutive backslashes （\）.

$ echo \\
\

The example above outputs the backslash itself .

Backslashes are used except to escape , It can also represent some non printable characters .

• \a： Ring the bell
• \b： Backspace
• \n： Line break
• \r： enter
• \t： tabs

If you want to use these non printable characters on the command line , You can put them in quotation marks , And then use echo Ordered -e Parameters .

$ echo a\tb
atb

$ echo -e "a\tb"
a        b

In the example above , The command line directly outputs non printable characters \t,Bash Cannot explain correctly . You must put them in quotation marks , And then use echo Ordered -e Parameters .

The newline character is a special character , Indicates the end of the command ,Bash After receiving this character , The input command will be interpreted and executed . Escape with a backslash in front of a newline character , This makes the newline character a normal character ,Bash It will be taken as having a length of 0 Null character processing , Thus one line of command can be written as multiple lines .

$ mv \
/path/to/foo \
/path/to/bar


#  Equate to 
$ mv /path/to/foo /path/to/bar

In the example above , If a command is too long , You can use a backslash at the end of a line , Rewrite it into multiple lines . This is a common way to write multiline commands .

2. Single quotation marks

Bash Allow strings to be in single or double quotation marks , To quote .

Single quotation marks are used to preserve the literal meaning of characters , All kinds of special characters are in single quotation marks , Will become normal characters , Like the asterisk （*）、 Dollar symbol （$）、 The backslash （\） etc. .

$ echo '*'
*

$ echo '$USER'
$USER

$ echo '$((2+2))'
$((2+2))

$ echo '$(echo foo)'
$(echo foo)

In the above command , Single quotation marks make Bash Expand 、 Variable references 、 Arithmetic operations and subcommands , All failed . If you do not use single quotation marks , They will all be Bash Automatic extension .

Because the backslash becomes a normal character in single quotation marks , So if it's in single quotation marks , And use single quotes , You can't use an escape , You need to put a dollar sign in front of the outer single quotation mark （$）, And then the single quotation mark in the inner layer is escaped .

#  Incorrect 
$ echo it's #  Incorrect  $ echo 'it\'s' #  correct  $ echo $'it\'s'

however , A more reasonable way is to use single quotation marks instead of double quotation marks .

$ echo "it's"
it's

3. Double quotes

Double quotation marks are looser than single quotation marks , Most special characters are in double quotes , Will lose its special meaning , Become normal characters .

$ echo "*"
*

In the example above , wildcard * It's a special character , Put it in double quotation marks , It becomes ordinary characters , It will output as is . This needs special attention , It means , There will be no file name extension in double quotation marks .

however , Except for three special characters ： Dollar symbol （$）、 The quotation marks （） And the backslash （\）. These three characters are in double quotation marks , Still has a special meaning , Will be Bash Automatic extension .

$ echo "$SHELL"
/bin/bash

$ echo "`date`"
Mon Jan 27 13:33:18 CST 2020

In the example above , Dollar symbol （$） And the quotation （） In double quotes , Keep special meaning . The dollar sign is used to refer to variables , Backquotes are used to execute subcommands .

$ echo "I'd say: \"hello!\""
I'd say: "hello!"

$ echo "\\"
\

In the example above , The backslash holds a special meaning in the double quotation marks , Used to escape . therefore , You can use backslashes , Insert double quotation marks between double quotation marks , Or insert the backslash itself .

The newline character is in double quotation marks , It will lose its special meaning ,Bash It is no longer interpreted as the end of the command , Just as a normal newline character . So you can use double quotation marks , Enter multiline text on the command line .

$ echo "hello world"
hello
world

In the above command ,Bash Normally, the newline character will be interpreted as the end of the command , But the newline character loses its special function in double quotation marks , Just for line breaks , So you can enter multiple lines .echo The command will output the newline character as it is , When displayed, it is normally interpreted as line feed .

Another common use of double quotes is , The filename contains spaces . You have to use double quotation marks （ Or single quotation mark ）, Put the file name in it .

$ ls "two words.txt"

In the above command ,two words.txt Is a file name with spaces , If you don't put it in double quotation marks , Will be Bash As two files .

Double quotation marks will keep extra spaces as they are .

$ echo "this is a test"
this is a     test

Double quotes have another function , Is to save the output format of the original command .

#  Single line output 
$ echo $(cal)
 January  2020  Japan   One   Two   3、 ... and   Four   5、 ... and   6、 ... and  1 2 3 ... 31

#  Original format output 
$ echo "$(cal)"
       January  2020
 Japan   One   Two   3、 ... and   Four   5、 ... and   6、 ... and 
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31

In the example above , If $(cal) Not in double quotation marks ,echo All the results are output in one line , Discarding all the original formats .

4. Here file

Here file （here document） Is a method of entering multiple lines of string , The format is as follows .

<< token text token

Its format is divided into start marks （<< token） And closing marks （token）. The start mark is two less than signs + Here The name of the document , The name can be taken at will , Must be followed by a newline character ; The closing tag is written in a separate line at the top Here Document name , If it's not the top grid , The end tag does not work . Between the two is the content of the multiline string .

Here is a pass through Here Document output HTML Code example .

$ cat << _EOF_ <html> <head> <title> The title of your page </title> </head> <body> Your page content goes here. </body> </html> _EOF_

Here Variable substitution will occur inside the document , It also supports backslash escape , However, wildcard extensions are not supported , Double quotation marks and single quotation marks also lose their grammatical function , Into ordinary characters .

$ foo='hello world'
$ cat << _example_ $foo "$foo" '$foo' _example_

hello world
"hello world"
'hello world'

In the example above , Variable $foo A replacement has occurred , But double quotation marks and single quotation marks are output as they are , It indicates that they have lost the function of reference .

If you don't want variable substitution , You can put Here The opening tag of the document is placed in single quotation marks .

$ foo='hello world'
$ cat << '_example_' $foo "$foo" '$foo' _example_

$foo
"$foo"
'$foo'

In the example above ,Here Start tag of the document （_example_） In single quotes , Cause variable replacement to fail .

Here The essence of a document is redirection , It outputs the string redirection to a command , It is equivalent to including echo command .

$ command << token string token


#  Equate to 

$ echo string | command

In the above code ,Here The document is equivalent to echo Redirection of commands .

therefore ,Here Strings are only suitable for commands that can accept standard input as arguments , Not valid for other commands , such as echo Orders can't be used Here Document as a parameter .

$ echo << _example_ hello _example_

The above example will not have any output , because Here Document for echo Invalid command .

Besides ,Here The document cannot be used as the value of a variable , Parameters that can only be used for commands .

5. Here character string

Here There is also a variation of the document , be called Here character string （Here string）, Use three less than signs （<<<） Express .

<<< string

Its function is to input a string through standard input , Pass to command .

Some commands take a given argument directly , And accept parameters through standard input , The result is different . That's why we have this grammar , Makes it easier to pass strings to commands through standard input , such as cat The command only accepts strings passed in by standard input .

$ cat <<< 'hi there'
#  Equate to 
$ echo 'hi there' | cat

The first syntax above uses Here character string , It looks semantic better than the second grammar , It's also simpler .

$ md5sum <<< 'ddd'
#  Equate to 
$ echo 'ddd' | md5sum

In the example above ,md5sum The command can only accept standard input as an argument , You cannot put a string directly after a command , Will be treated as a file name , namely md5sum ddd Inside ddd Will be interpreted as a file name . You can use it at this point Here character string , Pass a string to md5sum command .

Reference resources ：

• bash Quotes and escapes