Shulou(Shulou.com)06/03 Report--

1. Special and important variables in shell 1. Special position variables

There are some special and important variables in shell, such as $0, $1, $#, which can be called special position variables. To pass parameters from the command line, function, or script execution, you need to use the positional argument variable in the Shell script.

Commonly used special position parameter variables, such as figure:

Examples are as follows:

, 0

[root@localhost ~] # cat a.shecho $0 [root@localhost ~] # sh a.sha.sh [root@localhost ~] # sh/root/a.sh / root/a.sh [root@localhost ~] # dirname / root/a.sh/root// only output path name [root@localhost ~] # basename / root/a.sha.sh// only output script name

$n

[root@localhost ~] # cat a.shecho $1 $2 $3 $4 $5 [root@localhost ~] # sh a.sh a b c d e fa b c d e [root@localhost ~] # cat a.shecho $1 $2 $3 $4 $5 $6 $7 $8 $9 $10 $11 $12 $13 $14 $15 [root@localhost ~] # sh a.sh {a.z} a b c d e f g h i a0a1a2a4a5amp / output the 10th position parameter is not normal [root@localhost ~] # cat A.shecho $1 $2 $3 $4 $5 $6 $7 $8 $9 ${10} ${11} ${12} ${13} ${14} ${15} [root@localhost ~] # sh a.sh {a.. z} a b c d e f g h i j k l m n obank / so it's normal. The position variable is greater than 9, and the number must be enclosed in "{}"

$#

[root@localhost ~] # cat a.shecho $# [root@localhost ~] # sh a.sh a b c d e f6

* *

[root@localhost ~] # cat a.shecho $* [root@localhost ~] # sh a.sh "a b" c d ea b c d e

$@

[root@localhost ~] # cat a.shecho $@ [root@localhost ~] # sh a.sh "a b" c d ea b c d e f

The output of "$*" and "$@" without quotation marks is the same!

* "$" *

[root@localhost] # cat a.shfor I in "$*"; do echo $iTipdone [root@localhost ~] # sh a.sh "a b" c d ea b c d e

"$@"

[root@localhost ~] # cat a.shfor I in "$@"; do echo $iTipdone [root@localhost ~] # sh a.sh "a b" cd ea bcde2, special status variable

As shown in the figure:

Examples are as follows:

$?

[root@localhost ~] # pwd/root [root@localhost ~] # echo $? 0 [root@localhost ~] # fsdafasdfbash: fsdafasdf: command not found. [root@localhost ~] # echo $? 127 echo / the correct command return value is 0, incorrect is not 0. [root@localhost ~] # cat a.shif [$#-ne 2] then echo "cuowu!" Exit 123 / / when there is an error in executing the script, $? The value of is 123fi [root@localhost ~] # sh a.shcuowu! [root@localhost ~] # echo $? 123

This command is very useful for beginners!

"$?" The usage of the return value:

Determines whether a program such as a command, script, or function executes successfully; if "exit number" is called in the script, it will be returned to "$?" Variable; if it is in a function, pass this number to "$?" in the form of a function return value through the "return number" Variabl

The rest about "$$", "$!" and "$_" are not very common, and there are no examples here!

II. Shell built-in variable command

Shell contains some built-in commands that are not visible in the directory list and are provided by shell itself. Common internal commands are: echo, eval, exec, export, read, shift. Let's take a look at the functions of these commands.

Echo command

Displays the strings and variables specified after the echo command to standard output.

Common parameters of echo, as shown in the figure:

Examples are as follows:

[root@localhost ~] # echo one;echo twoonetwo [root@localhost ~] # echo-n one;echo twoonetwo// does not wrap output [root@localhost ~] # echo-e "one\ ttwo\ nthree\ tfour" one twothree four// is the function of "\ t", "\ n" wrap [root@localhost ~] # echo-e "1\ b23" 23jump / "\ b" is backspace, so block 1

The echo command is very common, and it's not difficult, so that's all!

The eval command is not very common, and I haven't used it much, so I won't play tricks!

Exec command:

The exec command can be transferred to execute the specified command without creating a new child process. When the specified command is executed, the process (that is, the original Shell) is terminated, as shown in the following example:

[root@localhost ~] # exec dateSun Sep 15 12:05:22 CST 2019 [lisi@localhost ~] $/ / back to privileged mode

Read command:

Information such as standard input strings is passed to variables defined within the Shell program.

Shift command:

Move the location parameters $1, $2, etc., to the left. If the location variables are $1, $2, $3, then after performing a shift, $3 becomes $2, 2 becomes $1, and 1 disappears.

3. Shell variable string

Examples of common options:

(1) query character length value

Examples are as follows:

Define variables:

[root@localhost ~] # a=abcdef [root@localhost ~] # echo ${a} abcdef [root@localhost ~] # echo $aabcdef

Returns the length of the variable value

[root@localhost ~] # echo ${# a} 6

Extend:

[root@localhost ~] # echo $a | wc-L6 echo / use the wc command to count the length [root@localhost ~] # expr length "$a" 6max / use expr's length function to calculate [root@localhost ~] # echo "$a" | awk'{print length ($0)} '6max / use awk's length function to calculate the variable length

Of these, the fastest way is ${# variable name}.

(2) intercept character length

Define variables:

[root@localhost ~] # a = "a b c d e f" [root@localhost ~] # echo ${a} a b c d e f

Intercept the contents of the variable, starting with the second character (by default to the end)

[root@localhost ~] # echo ${aura 2} b c d e f

Intercept the contents of the variable and intercept 2 characters starting with the second character

The first method:

[root@localhost ~] # echo ${aVl2VR 4} b cplink / because spaces are also characters

The second method:

[root@localhost ~] # echo ${a} | cut-c 3-5b c (3) Delete the specified character

Define variables:

[root@localhost ~] # a=abcABC123ABCabc [root@localhost ~] # echo $aabcABC123ABCabc [root@localhost ~] # echo ${a#a*c} ABC123ABCabc [root@localhost ~] # echo ${a#a*C} 123ABCabc// delete the strings [root@localhost ~] # echo ${a##a*c} / / because they all meet the requirements from the beginning of the variable content. So it does not show that [root@localhost ~] # echo ${a##a*C} abc// deletes the string [root@localhost ~] # echo ${a%a*c} abcABC123ABC [root@localhost ~] # echo ${a%a*C} abcABC123ABCabc// from the beginning of the variable content to delete the string [root@] that matches the longest APCC and APCC from the end of the variable content. Localhost ~] # echo ${a%%a*c} [root@localhost ~] # echo ${a%%a*C} abcABC123ABCabc// deletes the string with the longest matching axic and astatc from the end of the variable content

A summary of deleting characters:

#: the shortest match is deleted from the beginning; # #: the longest match is deleted from the beginning;%: the shortest match is deleted from the end;%: the longest match is deleted from the end

Characters that ac matches, indicating all, starting with an and ending with c

Characters that aC matches, indicating all, starting with an and ending with C.

(4) replace the specified character

Define variables:

[root@localhost ~] # a = "abc abc" [root@localhost ~] # echo $aabc abc [root@localhost ~] # echo ${a/abc/ABC} ABC abc// replace the first character that meets the requirements [root@localhost ~] # echo ${a//abc/ABC} ABC ABC// replaces all characters that meet the requirements

A summary of the replacement:

One "/" represents the first string that replaces the match; two "/ /" represents all the strings that replace the match; 4. Special extension variables

Common options, as shown in the figure:

It's not very common, so I won't give an example here!

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