Shell Learning Note 2

First, Shell special variables

1. Position Variables

Positional variables	Meaning
$	Gets the file name of the currently executing shell script, including the script path
$n	Gets the nth parameter of the currently executing shell script, n=1..9, which represents the file name of the script when n is 0, if n is greater than 9, enclosed in curly braces ${10}
$*	Get all parameters of the current shell, treat all command-line arguments as a single string, equivalent to "$1$2$3", and note the #的区别: Parameter 1, parameter 2, Parameter 3
$#	Gets the total number of parameters in the current shell command line
[Email protected]	All parameters of this program "$" "$" "$" "...", this is the best way to pass parameters to other programs because it preserves any whitespace embedded in each parameter.

2. Process state variables

Process state variables	Meaning
$$	Gets the current shell's process number (PID): 1534
$!	Gets the PID that executes the previous instruction
$?	Gets the return value of the last instruction executed
$_	The last parameter of the command or script that was executed before this

Ii. Examples and explanations

1. Position variables

$ $ Gets the file name of the currently executing shell script, including the script path

[[email protected] ~]# cat 0.shecho $0[[email protected] ~]# sh 0.sh0.sh[[email protected] ~]# sh/root/0.sh/root/0.sh

$n

[[email protected] ~]# cat n.shecho $1[[email protected] ~]# sh  n.sh lelelele[[email protected] ~]# sh n.sh lele baobaolele[[email  protected] ~]# sh n.sh  "Lele baobao" lele baobao[[email protected] ~]#  seq 15 | sed  ' s#^#$ #g ' |tr  "\ n"   " "  > n1.sh[[email  Protected] ~] #vim  n1.shecho echo $1 $2 $3 $4 $5 $6 $7 $ 8 $9 $10 $11 $12 $13 $14 $15[[email protected] ~]# sh  N1.sh {a. Z}a b c d e f g h i a0 a1 a2 a3 a4 a5      #如果n大于9, enclosed in curly braces ${10}[[email protected] ~]# cat n1.shecho $1  $2 $3 $4 $5 $6 $7 $8 $9 ${10} ${11} ${12} ${13}  ${14} ${15}[[email protected] ~]# sh n1.sh {a. Z}a b c d e f g h i j k l m n o

$* and $#

[[Email protected] ~]# set--"I am a" good teacher #--end option section, new parameters from "I am" [[email protected] ~]# echo $*i am a good tea Cher[[email protected]ost ~]# echo $ #3

$* and [email protected]

[[Email protected] ~] #set--"I am a" good teacher[[email protected] ~]# for I in "$*"; Do echo $i;d one #在有双引号情况, the parameters quoted as a parameter output I am a good teacher[[email protected] ~]# for i in "[email protected]"; Do echo $i;d one #在有双引号的情况, each parameter independently outputs I am agoodteacher if $* and [email protected] without quotation marks [[email protected] ~]# for i in $*; Do echo $i;d one I ama Goodteacher[[email protected] ~]# for i in [email protected]; do echo $i; Donei AMA Goodteacher

$* treats all command-line arguments as a single string, equivalent to "$1$2$3"

[email protected] treats each parameter of the command line as a separate string, equivalent to "$" "$" "$", which is the best way to pass parameters to other programs because it preserves any whitespace embedded in each parameter.

Note: The above differences are only when double quotes are added, i.e. "$*" "[Email protected]"

2. Process variables

Explain the most commonly used process variables $?

$?  gets the return value of the last instruction executed

Value	Meaning
0	Command runs successfully
2	Permission denied
1-125	Command run failure, specific exit meaning, is defined by individual commands
126	The command was found, but the file could not be executed
127	Command not Found
>128	command is forced to end by system

[[email protected] ~]# ll/dev/null crw-rw-rw-1 root root 1, 3 June 9 17:37/dev/null[[email protected] ~]# echo $?0[[ema Il protected] ~]# ll fools:cannot access foo:no such file or Directory[[email protected] ~]# echo $?2

Shell Learning Note 2