Symbolic descriptions of shell scripts in Linux __linux

#。 The first sentence in the footsteps, indicating the program used to execute the script

    Example: #!/bin/bash

# comments, for help information or for ignoring statements that are temporarily not executed

# This is a comment

$ variable substitution symbol

Example: A=123;echo $a

${} variable regular expression to avoid early truncation of variable names

Example: ${13}

$num Position Parameters

Example: $0,$1,..., ${10}

$? The state of the last command, General 0 is correct, others indicate error

Example: If [$?-ne 0]; Then

echo "Encounter Error"

Fi

$! ID of the background command executed by the last command

Example: pid=$!

$$ the ID of the script process to run

Example: pid=$$

$* passed to the steps of the parameters, add double quotes when the equivalent of "$1$2 ..."

Example: Echo "Theparameters are $*"

$@ is the same as $*, with double quotes equal to "$" ...

Example: a=$@

$# the number of parameters passed to the script without the $ parameter

Example: Echo "theparameter number is $#"

$-saves the current shell's settings information and, if there is I in the result, is an interactive shell

Example: Echo $-

; A separator that writes multiple instructions on the same line

Example: A=123;echo $a

;; Special language case option, which acts like a break in a C-language switch statement

Example: Case${tao} in

            A) echo "a";;

             b) echo "B";;

Esac

. One represents the current directory and two represents the previous level directory

Example: CD..

, similar to the comma expression in the C language

Example: Let "t1= ((A=5+3,B=7-1,C=15/3))"; Echo $t 1

! Representation of logical operation "non"

Example: If ["$?"! = "0"]; Then

echo "Execute Error"

Exit 1

Fi

: In bash, this is a built-in command, "do nothing", but returns a status value of 0

Example::; Echo$?; :> f.out equivalent to Cat/dev/null > f.out

? Wildcard characters that match any one character, but do not include null

Example: LS f?n

"Treats its contents as strings, and the $ and wildcard characters do not extend or escape

Example: a=1; echo ' $a ' result is $a

"" To use its contents as a string, but $ can be extended, wildcard characters cannot be

Example: b= "string"; echo "$b" result is string

' Inverted quotation mark (on the left of keyboard number 1), where the string is executed as a command

Example: a= ' Date '; echo "$a"

() instruction group that encloses a sequence of instructions, and the shell generates Subshell to execute them

Example: (a=1;echo$a)

(()) bash's built-in functionality for arithmetic operations

Example: a=10;echo$a ((a++)); Echo $a

[] The same as test, used to judge the true and false expression

Example: if [1-eq 2];then

echo ' Kill me '

Fi

[[]] The enhanced version of Bash to [], which allows the use of | | And &&, and you can use regular expressions

Example: Read ver

if [[$ver > 5 && $ $ver < 9]]; Then

echo "That ' s it"

Fi

{} instruction group, similar to (), but executed in the current shell and can also be used for a combination of strings

Example: Mkdir{usera, UserB, UserC}-{home, bin, data}

\<....\> regular expression, matching word first and suffix

Example: Find ' \<the\> ' file

+ addition, regular expression to denote character repeat 1~n times

Example: grep ' 10\+9 ' file

-Subtraction, standard input, last working directory represented in CD command

Example: expr 1-2 + 3; TAR-ZXFV-; cd–

* multiplication, wildcard, representing any character; repetition of one character 0~n times

Example: let "var=2*3"; LS zha*t; grep ' a1[0-9]* '

* * Sub-square operation

Example: Let "var=2**8"

/Division; Directory Separator

Example: Expr 10/2;cd/home/zc

% remainder

Example: expr 10%2

= Assignment; string comparisons, different in [] and [[]] (see = =)

Example: a= "we"; if [$a = = ' wee ']; Then Exit0 fi

= = string comparison, with a single equal sign

Example: a= ' we '

if[[$a = = w*]]; Then

echo "OK"

Fi

!= string comparison, not equal to

Example: if ["$a"!= "$b"]; Then ...

< string comparison, less than; input redirection

Example: Cat>test < Suck.txt

<< read from standard input until separator

Example: 110<< EOF

> Output redirection, file overwrite, no presence, new

Example: Ls>out 2>&1

>> output redirection, the file exists append, does not exist is created

Example: Ls>>out

& Background Execution Work characters

Example: Tar zcvfdata.tar.gz data >/dev/null &

&& logic and short circuit operators

Example: CP 1.txt2.txt && RM 1.txt && Echo ' success '

| Pipe operator

Example: Cat file| grep ' Fan '

|| Logic or, short-circuit operator

Example: RM 1.txt| | echo ' fail '

~ root directory

Example: CD ~

^ The beginning of a matching line in a regular expression

Example: grep ' ^home ' file

\ escape characters, continuation line symbol

Example: echo \ $A = $A \

This is the "the";