Shell script eval, ' and $ (), [[and] [, $ (())

Eval

The role of Eval is to perform command-line processing again, that is, to perform two command-line processing on a single command line.

Syntax: eval cmdline

Eval will scan the back of the CmdLine two times, if CmdLine is a normal command after the first scan, execute this command;
If the cmdline contains an indirect reference to a variable, the semantics of the indirect reference is guaranteed.

Eval echo \$$# get last parameter

such as: Cat Last

Eval echo \$$#

./last one Teo three four

Four

After the first scan, the shell removes the backslash. When the shell scans the row again, it replaces the value of the $4 and executes the echo command

`` (anti-quote) and $()

In the bash shell, $ () and "(anti-quote) are used for command substitution.

$ echo the last Sunday was $ (date-d "last Sunday" +%y-%m-%d)

Get the date of the previous Sunday

The reason for using $ ()

1. "Easy to mix with" (single quote). Sometimes in some strange glyphs, the two symbols are identical (vertical two points).

2. In a multi-level compound substitution, ' extra hops (\ ') are required, while $ () is more intuitive. For example:

Command1 ' Command2 ' Command3 '

The original intention is to Command2 ' Command3 ' in the first to replace the Command3 to Command2 processing, and then pass the results to Command1 ' Command2 ... ' to deal with.

However, the real results are divided into ' command2 ' and ' two segments ' in the command line.

The correct input should be as follows:

Command1 ' command2 \ ' command3\ '

Change to $ () at a glance:

Command1 $ (Command2 $ (COMMAND3))

$ () Insufficient

"Basically, it can be used in all of the Unix shells, if it's written in shell script, the transplant is relatively high. and $ () not every shell can be used.

[[And [

1."[[" is the keyword , and many shells (such as ash BSH) do not support this approach. Ksh, Bash (reportedly introduced support for [[support] from 2.02 onwards.

"[" is a command that is equivalent to test and is supported by most shells. In most modern SH implementations, "[" and "Test" are internal (builtin) commands.

2. Both support arithmetic comparisons and string comparison expressions (the use may be a bit different)

(1) "-gt", "-lt" is an arithmetic comparison operator that is used to compare the size of integers.
(2) ">", "<" is a string comparison operator used to compare the size of a string, using the dictionary order, relating to the current locale.

(3) It is recommended to use the Let, (()) command when using numeric comparisons, otherwise error-prone.

[and [[] different comparisons:

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As can be seen from the above two graphs, [the character comparison requires the use of the escape character \, and [[No

3. Logical AND Logical OR

(1) "[": Logic and: "-a"; Logic or: "-o";

(2) "[[": Logic and: "&&"; Logic or: "| |"

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4.

1) [...] is a shell command, so the expression in it should be its command-line argument, so the string comparison operator ">" and "<" must be escaped, otherwise it becomes IO redirection;

(2) because "[[" is a keyword, does not do the command line extension, so in [["<" and ">" Do not need to escape, but the relative syntax is slightly stricter. For example in [...] You can enclose the operators in quotation marks, because they are removed when you do a command-line extension, and in [[...] is not allowed to do so;

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5.[[...] Perform arithmetic expansion, while [...] Don't do

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$(( ))

Used for integer arithmetic. In bash, the integer operation symbol for $ (()) roughly has these:

+-*/Add, subtract, multiply, divide

% remainder operation

& | ^ ! And, or, XOR, not arithmetic

Example:

$ a=5; b=7; c=2

$ echo $ ((a+b*c))

19

$ echo $ (((a+b)/C))

6

$ echo $ (((a*b)%c))

1

The variable name in $ (()) can also be preceded by a $ symbol: $ (($a + $b * $c)) can also get 19 results.

In addition, $ (()) can also be used for different binary (such as binary, octal, hexadecimal) operations, but the output is only decimal.

echo $ ((16#2A)) result is 42 (16 decimal)

To give a practical example:

The current umask is 022, and the permissions for the new file are:

$ umask 022

$ echo "OBASE=8; $ ((8#666 & (8#777 ^ 8#$ (umask))) "| Bc

644

In fact, it is possible to redefine the value of a variable by simply (()), or as a testing:

a=5; ((a++)) to redefine $a to 6

a=5; ((A –)) a=4

a=5; b=7; ((a < b) will get a return value of 0 (TRUE)

Common test symbols for (()) have the following:

< less than

> Greater than

<= less than or equal to

>= greater than or equal to

= = equals

! = does not equal

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Shell script eval, ' and $ (), [[and] [, $ (())