Shell functions and built-in variables

1, Special shell variable

$# the number of arguments passed to the script
$* displays all parameters passed to the script in a single string
$$ the current process ID number for the script to run
$! the ID number of the last process running in the background
[Email protected] is the same as $*, but is used with quotation marks and returns each parameter in quotation marks.
$-shows the current options that the shell uses, as is the SET command function.
$? Displays the exit status of the last command. 0 means there is no error, and any other value indicates an error.

Command > file redirects the output to file.
Command < file redirects the input to file.
Command >> file redirects the output to file in an append manner.
n > file redirects the file descriptor n files to filename.
n >> files redirect files with file descriptor N as an append to file.
N >& m merges the output file m and N.
N <& m merges the input file m and N.
<< tag will start to tag the contents of tags between tag and end tag as input.

$ command >/dev/null
/dev/null is a special file, and the content written to it is discarded, and if you try to read from the file, nothing is read. However, the/dev/null file is very useful and redirects the output of the command to it, which results in a "no output" effect.
If you want to mask stdout and command >/dev/null 2>&1

$ A script name
Location parameter #1
$-$9 positional parameter #2-#9
${10} positional Parameters #10
$# Number of positional parameters
"$*" all positional parameters (as a single string) *
"[Email protected]" All positional parameters (each as a separate string)
${#*} The number of command-line arguments passed to the script
${#@} The number of command-line arguments passed to the script
$? return value
Process ID (PID) of the $$ script
$-the flags passed to the script (using set)
$_ the last parameter of the previous command
$! the Process ID (PID) of the last job running in the background

2, two Yuan comparison

-eq equals
= equals
= = equals
-ne Not equal to
! = does not equal
-lt less than
\< less than (ASCII) *
-le less than or equal to
-GT Greater than
\> greater than (ASCII) *
-ge greater than or equal to
-Z string is empty
-N string is not empty
> Greater than
>= greater than or equal to
< less than
<= less than or equal to

3, File test type

-e file is present
-S file size is not 0
-F is a standard file
-D is a directory
-r file with Read permission
-W file has write permissions
-X file has execute permissions
-h file is a symbolic link
-l file is a symbolic link
-B file is a block device
-c file is a character device
-G Sets the Sgid tag
-P file is a pipe
-U set the suid tag
-S file is a socket
-K Sets the "Paste bit"
-T file associated with a terminal
-N has been modified since the last time this file was read
-O The host of this file is your
-G file has the same group ID as the group you belong to
F1-nt F2 file F1 F2 new than file *
F1-ot F2 file F1 F2 older than file *
F1-ef F2 file F1 and file F2 are hard links to the same file *

!" Non "(reverses the test result above)

4, parameter substitution and extension

Meaning of an expression
${var} variable var value, same as $var

${var-default} If Var is not declared, then use $default as its value *
${var:-default} If Var is not declared, or its value is empty, then use $default as its value *

${var=default} If Var is not declared, then use $default as its value *
${var:=default} If Var is not declared, or its value is empty, then use $default as its value *

${var+other} If Var is declared, then its value is $other, otherwise it will be a null string
${var:+other} If Var is set, then its value is $other, otherwise it will be a null string

${var? Err_msg} If Var is not declared, then print $err_msg *
${var:? Err_msg} If Var is not set, then print $err_msg *

${!varprefix*} matches all variables declared at the beginning of Varprefix
${[email protected]} matches all previous variables declared at the beginning of the Varprefix



${#string} Length of $string

${string:position} in $string, extract the substring from position $position
${string:position:length} in $string, start extracting substrings of length $length from position $position

${string#substring} removes the substring of the shortest match $substring from the beginning of the variable $string
${string# #substring} Remove the substring of the longest matching $substring from the beginning of the variable $string
${string%substring} removes the substring of the shortest match $substring from the end of the variable $string
${string%%substring} Remove the substring of the longest matching $substring from the end of the variable $string

${string/substring/replacement} uses $replacement instead of the first matching $substring
${string//substring/replacement} uses $replacement instead of all matching $substring
${string/#substring/replacement} if the $string prefix matches $substring, then $replacement is used instead of the matching $substring
${string/%substring/replacement} If the $string suffix matches $substring, then $replacement is used instead of the matching $substring


Expr match "$string ' $substring ' matches the length of the $substring* at the beginning of the $string
Expr "$string": ' $substring ' matches the length of the $substring* at the beginning of the $string
Expr index "$string" $substring where the first character of the $substring that matches in $string appears
Expr substr $string $position $length to extract a substring of length $length from position $position in $string
Expr match "$string" \ ($substring \) ' Extracts $substring* from the beginning of $string
Expr "$string": ' \ ($substring \) ' Extract $substring* from the beginning of $string
Expr match "$string" '. *\ ($substring \) ' Extract $substring* from the end of $string
Expr "$string": '. *\ ($substring \) ' Extract $substring* from the end of $string

, 5, Brackets
If [CONDITION] test structure
if [[CONDITION]] Extended test structure
Array[1]=element1 Array Initialization
[A-z] the character range of a regular expression

, 6, curly braces
${variable} parameter substitution
${!variable} indirect variable reference
{Command1; command2;.. commandn;} code block
{String1,string2,string3,...} Curly brace Extension


, 7, round brackets
(Command1; command2) a command group executed in a child shell
array= (element1 element2 element3) array initialization
result=$ executes the command in the child shell and assigns the result to the variable
> (COMMAND) process substitution
< (COMMAND) process substitution

, 8, double parenthesis
Integer operation ((var = 78))
var=$ ((20 + 5)) integer operation with the result assigned to the variable

, 9, quotation marks
"$variable" "weak" reference
' String ' "strong" reference

10, post reference
result= ' command ' runs the command in the child shell and assigns the result to the variable

Shell functions and built-in variables