The concept of the Shell is derived from the Unix command interpreter. The Shell not only interprets the commands entered by the user, but also interprets the scripting language used to execute commands. Shell scripts can improve the efficiency of user operations and system management.
Write a script
Basic elements of the shell
- #! /bin/bash Mandatory, specifying the type of shell
- # comments. In the shell, comments are written after #, and nothing after # is executed
- variable
- Control loop branch
Special symbols in the shell
| symbol | role |
|---|---|
| #! | Specify the shell used to execute the script |
| $ | Instead of a backslash escape, use the plain character that follows as the variable name, such as $a, to indicate the value of variable A. If the variable contains more than one character, use {} to enclose the variable |
| ‘ | Echo ‘my $SHELL’ echo ‘my $SHELL’ |
| “ | For the contents in quotation marks, except for the special functions of $, escape character \, and inverted quotation mark ‘, all the other characters are ordinary characters. |
| ` | The string in quotes is interpreted as a shell command line (the same function can also be used with $()), and the result replaces the entire backquote section. |
| \ | Backslashes are escape characters that turn special characters into ordinary characters. Using a backslash (\) before a character prevents the shell from interpreting subsequent characters as special characters. |
| * | The value contains 0 or more special characters. Example yum.* can also be yum. A, yum. Ab, yum |
| ? | Represents any character. Example yum.? It can be yum. A yum. B yum. C, but be careful that there must be any character after the decimal point |
| [] | Represents any one of the brackets. [abcdef] can be any of the letters a, B, C, D, e, or F. [-] represents a range, and [a-z] represents any letter from a to Z. [^]^ [^ ABC] [^ ABC] [^ ABC] [^ ABC] [^ ABC] [^ ABC] [^ ABC |
| The $() | You can assign a command replacement output to a variable |
| {} | The required strings can be generated by parenthesis extension, which can contain contiguous sequences containing a starting point and an ending point, or multiple items separated by commas |
user@computer: ~$echo {a,b,c} a b c user@computer: ~$echo user{1,5,8} user1 user5 user8 user@computer: ~$ echo {0.. 10} 0 1 2 3 4 5 6 7 8 9 10 user@computer: ~$mkdir {dir1,dir2,dir3} user@computer: ~$ls -- ld dir{1,2,3}Copy the codevariable
Shell variables can hold pathnames, file names, a number, and so on. Divided into three categories:
- Local variables: (Local variables) are used only in the Shell in which they were created, and can be used and modified arbitrarily within the Shell program.
- Environment variables: They can be used in the Shell that created them and in any subroutine derived from them. Some variables are user-created, others are proprietary (PATH, HOME). They are part of the system environment and can be used in shell programs without having to define them. You can also modify it in the shell.
- Internal variables: provided by the system. Unlike environment variables, users cannot modify them.
The local variable
Local variables are used in scripts for the user’s current shell lifetime
Considerations for using local variables
- There can be no Spaces on either side of the equal sign
- The value contains Spaces and must be enclosed in double quotation marks
- Shell variables can be case sensitive and can be written in upper and lower case letters
- Variables are weakly typed and do not declare a type
Variable declaration and assignment formats
Variable = value (no Spaces on equal sign)Copy the codeVariable reference
$variable name ${variable name} Recommended use method 1 if the variable name contains one character. Recommended use Method 2 If the variable name contains more than one character. Example: $a ${ABC}Copy the codeRemove the variable
user@computer: ~$name=Jack user@computer: ~$echo ${name} user@computer: ~$unset name #Copy the codeSetting read-only variables
If you do not want to change the value of a variable, you can set it as a read-only variable. Variable name = value ReadOnly Variable nameCopy the codeThe environment variable
Bash presets a number of environment variables that, in practice, you can call directly. Environment variables can be used for all subroutines, including editors, scripts, and applications
Built-in environment variables
| variable | meaning |
|---|---|
| HOME | Represents the user directory. CD ~ to the user’s home directory or use the CD can be directly back to the user’s directory. |
| SHELL | Which SHELL program is currently used in this environment? For bash, the default is /bin/bash |
| PWD | The path to the user’s current working directory, indicating where the user is currently in the Linux file system |
| HISTSIZE | This withhistoryCommands can be recorded by the system, and the “number” of records is set by this value. |
| PATH | Directories are separated by colons (:). Files are searched in order by directories in the variable PATH, so the order of directories is also important. |
Environment variables precautions
- Environment variables can be set on the command line, but these values are lost when the user logs out
- Environment variables are capitalized
- It must be exported using the export command
Setting environment Variables
Variable-name =value export variable-nameCopy the codeDisplay environment variables
Env can see all environment variables echo $environment variable name (display a variable)Copy the codeClearing environment variables
Unset Specifies the name of the environment variableCopy the codeModify the PATH environment variable
Modify the PATH environment variable so that the script can be executed by directly entering the name of the file without adding the PATH. Mysql > alter environment variables; Mkdir shdir && CD shdir vi hello chmod 755 hello CD ~ export PATH=$PATH:$HOME/shdir In any directory, enter Hello to execute the file. If the environment variables are incorrectly modified, you can restore the default values after you log in to the system again after exit.Copy the codeModify environment variables in the configuration file
You need to know which profiles environment variables are associated with: Different distributions have different names, but the names are common: find / -name "*profile" find / -name "*bashrc" Global profile /etc/profile Local profile ~/.bashrcCopy the codeInternal variables
Internal variables are a special type of variable provided by Linux that is used in programs to make judgments. The values of these variables cannot be changed in shell programs.
Partial internal variables
| variable | meaning |
|---|---|
| $# | The number of positional arguments passed to the shell program |
| $? | The completion code of the last command or shell program executed inside the shell program (return value) |
| $0 | The name of the shell program |
| $* | A single string of all arguments passed when the shell program is called, “Parameter 1″,” parameter 2″… Form saves parameters |
| $@ | “Parameter 1″,” parameter 2″… Form saves parameters |
| $n | The NTH argument |
| ? | PID of this program |
| $! | PID of the previous command |
Input and output
The input
-n NCHARS read n characters -p PROMPT display a PROMPT -r cancel transfer -s quiet mode, -t TIMEOUT will not prompt you if the specified time is exceeded. Read will stop automaticallyCopy the codeThe output
Echo [options] String common options -n does not wrap at the end -e enables the conversion of backslash control characters -e does not handle escape characters. This is the default option;Copy the code#! /bin/bash
# Enter a sentence, print the input words
read -p 'please type some words, I will print them: ' words
echo $words
Copy the codeConditions for testing
Test conditional expression
# test The test command returns 0 if the test condition is true, otherwise a non-0 numeric test statement is returned along with the if/ THEN and case statements to form the control transfer structure of shell programmingCopy the code[] Conditional expression
If the value of the conditional expression is true, it returns zero. If it is false, it returns a non-zero valueCopy the codeFile status judgment
| Conditional statements | True or false |
|---|---|
| -d filename | Return true if filename is a directory file |
| -f filename | If the file exists and is a normal file, true is returned |
| -r filename | Return true if filename is readable |
| -s filename | Return true if the length of filename is greater than 0 |
| -w filename | Return true if filename is writable |
| -x filename | Return true if filename is executable |
| -e filename | Whether the file exists |
#! Read -p 'input file path: 'file if [-e $file] then echo' exists' else echo 'does not exist' fiCopy the codeConditional statements
if
if[conditional expression];thenCommand sequencefi
当"Conditional expression"Is executed when the test value of"Sequence of commands"Otherwise, the command following the conditional statement is executed. Conditional expressions andthenThe semicolon between";"Acts as a command separator.Copy the codeif-else
if[Conditional expression]thenCommand Sequence 1elseCommand Sequence 2fi
当"Conditional expression"Is executed when the test value of"Command Sequence 1"Otherwise, execute the command"Command Sequence 2". There can be one or more commands in a command sequence.Copy the codeif-elif-else
if testConditional expression 1thenCommand Sequence 1elif[Conditional expression 2]thenCommand Sequence 2elseCommand Sequence 3fiThis is a conditional statement containing two levels of nesting, when"Conditional expression 1"If true, the command is executed"Command Sequence 1"Otherwise, in"Condition table reached formula 2"If true, execute"Command Sequence 2"Otherwise, execute the command"Command Sequence 3"."Command Sequence 3"Is part of the second clause statement.Copy the codeApplication, for example,
#! /bin/bash # Read -p 'Please input the file path: ' file if [ -d $file ] then echo 'this is a directory' elif [ -f $file ] then echo 'this is a file' else echo 'wrong file type, or the file do not exist' fiCopy the codeNumeric operator
| The operator | meaning |
|---|---|
| N1 – eq n2 | Determine whether n1 and n2 are equal. If they are, return 0; otherwise, return 1 |
| N1 n2 – can be | Check whether numbers n1 and n2 are not equal. If they are not, return 0; otherwise, return 1 |
| N1 – lt n2 | Determine whether the number n1 is less than n2, if, return 0, otherwise return 1 |
| N1 – gt n2 | Determine whether the number n1 is greater than n2, if, return 0, otherwise return 1 |
| N1 – le n2 | Determine whether the number n1 is less than or equal to n2, if, return 0, otherwise, return 1 |
| N1 – ge n2 | Determine whether the number n1 is greater than or equal to n2, if, return 0, otherwise, return 1 |
String operator
| The operator | meaning |
|---|---|
| string | Return true if the string string is not empty |
| -n string | Return true if the string length is greater than 0 |
| -z string | If the string length is 0, true is returned |
| string1 = string2 | Return true if the strings string1 and string2 are equal |
| string1 ! = string2 | Return true if the strings string1 and string2 are unequal |
Logical operator
| The operator | meaning |
|---|---|
| E1 – a e2 | Returns 0 if both the logical expressions e1 and e2 are true, and 1 otherwise |
| E1 – o e2 | Returns 0 if either of the logical expressions E1 and e2 is true, and 1 otherwise |
| ! e1 | Returns 0 if the logical expression E1 is not true, and 1 otherwise |
Looping statements
Python-style for loops
For variable name in parameter list do command list done Assigns each element in the parameter list to the variable name. After each assignment, execute the command list. Parameter List indicates the range of variable namesCopy the codeC style for loop
For ((initializes a variable value; End loop condition; Do command sequence doneCopy the codewhile
While [conditional expression] do command list done The loop executes the commands in the command list until the value of the conditional expression is false.Copy the codeuntil
Until [condition] do sequence Done Until the condition is met the loop endsCopy the codeApplication, for example,
#! /bin/bash # rename all.txt files in the specified directory to *.doc directory=$1 if [! $directory ] then echo "please input the argument directory" exit fi files=`ls ${directory}` for file in $files; do if [ -f ${file} ] echo $file then suffix=${file#*\.} echo $suffix if [[ $suffix == "txt" ]] then prefix=${file%\.*} mv $directory/$file $directory/$prefix.doc fi fi doneCopy the codefunction
Functionname () {command list return} functions can be called as: functionname argumentsCopy the code