Bash Shell Script Programming Learning Summary

shell scripting
Compiler ------- interpreter
Static language: compiled language, strongly typed (variable); converted to executable format in advance. C / C ++ / JAVA / C #
Dynamic language: interpreted language, weakly typed. Explain while changing execution. PHP, SHELL, python, perl
Process-oriented: shell, c; Object-oriented: java, python, perl, c ++
Variable assignment: VAR_NAME = VALUE;

bash variable types:
    Environment variable: The scope is the current shell process and its child processes.
        export VARNAME = VALUE or VARNAME = VALUE export VARNAME "Export" is defined as an environment variable.
        The script starts a subshell process when executed:
        [[email protected] ~] # NAME = LINJUNBIN
        [[email protected] ~] # export NAME
        [[email protected] ~] # echo $ NAME
        LINJUNBIN
        [[email protected] ~] # bash // Switch to a subshell;
        [[email protected] ~] # echo $ NAME // The environment variable is still valid.
        LINJUNBIN

    Local variables (local variables):
        The entire bash process: local variable: VARNAME = VALUE
        Local variables: The scope is valid for the current code segment. local VARNAME = VALUE
    Position variable:
        $ 1, $ 2, ....
    Special variables: (built-in bash): $ ?: the execution status of the previous command, the status value;
        There are generally two return values for program execution; execution status $? = 0-255
        0: Correct execution; 1-255: Incorrect execution;
        1,2,127: Reserved by the system.
        $ ?: the status code of the last command execution;
        $ #: Number of parameters
        $ *: Parameter list
        [email protected]: Parameter list

Reference variable: $ {VAR_NAME}, reference the variable.
    [[email protected] ~] # echo "there are some $ {ANIMAL} s."
    there are some PIGs.
    [[email protected] ~] # echo ‘there are some $ {ANIMAL} s.’
    there are some $ {ANIMAL} s.
    Note: ‘‘ The difference between single quotes and “” double quotes.

Output redirection:
    >: >>: 2>: 2 >>, &>
Enter:
    / dev / null: software device; / dev / null: data black hole;
    eg: Determine whether a user is included.
        [[email protected] ~] # id student &> / dev / null
        [[email protected] ~] # echo $?
        1
Undo variable:
    unset VARNAME without adding $;
View variables:
    Use the set command; set: contains environment variables and local variables;
    View environment variables: printenv, env, export; all three are fine.
eg:
    [[email protected] ~] # ANIMALS = pig
    [[email protected] ~] # ANIMALS = $ ANIMALS: goat
    [[email protected] ~] # echo $ ANIMALS
    pig: goat
Scripting: stacking of commands,
Determine if the user exists: id user1 &> / dev / null && echo "hello user1";
    [[email protected] test] # id root &> / dev / null && echo ‘root’
    root
shell condition judgment
Determine the execution result $? To determine the execution status code to determine whether the execution was successful or failed.
Integer test:
    -eq: test whether two integers are equal;
    -ne: test whether two integers are not equal, not equal, true; equal, false;
    -gt: test whether one number is greater than another number;
    -lt: test if one number is less than another number;
    -ge: greater than or equal to;
    -le: less than or equal to;
    eg:
            [[email protected] test] # A = 3
            [[email protected] test] # B = 4
            [[email protected] test] # [$ A -eq $ B]
            [[email protected] test] # echo $?
            1
            [[email protected] test] # B = 3
            [[email protected] test] # [$ A -eq $ B]
            [[email protected] test] # echo $?
        0
Character test:
File test:

Conditional test expression: Note: Spaces are required on both sides.
    [expression expression], [[expression]], test expression
expression:

Logical relationship between commands;
    Logical AND: &&, the first condition is false and the second condition does not need to be executed, both must be true.
    Logical OR: || The first condition is false, and the second condition must be judged. One of the two can be true.
    eg: if user user6 does not exist, add user user6
        ! id user6 &> / dev / null && useradd user6
        id user6 &> / dev / null || useradd user6
LINE = `wc -l / etc / inittab`; Put the result of this command in a variable.
eg: If the number of lines in the file / etc / inittab is greater than 100, a large file is displayed, otherwise a small file is displayed.
        [[email protected] test] # cat souce.sh
        #! / bin / bash
        LINES = `wc -l / etc / inittab`
        echo $ LINES
        FINLINES = `echo $ LINES | cut -d‘ ‘-f1`
        echo $ FINLINES;

        [$ FINLINES -gt 100] && echo "/ etc / inittab is a big file." || echo "/ etc / inittab is a small file"

        [[email protected] test] # bash souce.sh
        17 / etc / inittab
        17
        / etc / inittab is a small fi

eg: Add users to the system:
`! id user1 &> / dev / null && useradd user1 && echo" user1 "| passwd --stdin user1 &> / dev / null || echo‘ user1 exists`
    After adding users, pipe to passwd. Set password; output the total number of lines at the same time.
    [[email protected] test] # bash addUser.sh
    user1 exists
    39
    [[email protected] test] # cat addUser.sh
    #! / bin / bash
    ! id user1 &> / dev / null && useradd user1 && echo "user1" | passwd --stdin user1 &> / dev / null || echo ‘user1 exists’
    ! id user2 &> / dev / null && useradd user2 && echo "user2" | passwd --stdin user2 &> / dev / null || echo ‘user2 exists’
    ! id user3 &> / dev / null && useradd user3 && echo "user3" | passwd --stdin user3 &> / dev / null || echo ‘user3 exists’
    #Get the total number of rows
    LINES = `wc -l / etc / passwd | cut -d‘ ‘-f1`
    echo $ LINES;

    eg:
    # Determine whether the current user is a root user;
            [[email protected] test] # bash is_root.sh
            is root
            [[email protected] test] # cat is_root.sh
            #! / bin / bash
            USER_UID = `id -u`;
            [$ USER_UID -eq 0] && echo ‘is root’ || echo "is not root and this uid is $ {UID}"
        Note: The error is_root.sh: line 2: UID: readonly variable. The original is the same as the UID variable setting and environment variable. The main variable is named.
if condition judgment
Single branch if statement:
    if condition
    then
        statement1; statement2;
    fi
Double branch if statement:
    if judgment condition; then
        statement1; statement2; ...
    else
        statement3; statement4; ...
    fi
Multi-branch if statement
    if condition judgment 1; then
        statement1; ...
    elif judgment condition 2; then
        statement2; ...
    elif judgment condition 3; then
        statement3; ...
    ...
    else
        statement4; ...
    fi

eg:
    #Determine if a user exists:
    [[email protected] test] # bash user.sh
    user12 IS NOT exists
    [[email protected] test] # cat user.sh
    #! / bin / bash
    NAME = user12;
    if id $ NAME &> / dev / null; then
    echo "$ NAME is exists"
    else
    echo "$ NAME IS NOT exists"
    fi
    #If the user exists, display the existence, if not, add it;

eg:
#Determine if the current system has a user's default shell as bash: if there is, display it How many, otherwise it shows that there are no such users
        [[email protected] test] # ./is_bash.sh
        the shells of 10 users is bash shell
        [[email protected] test] # cat is_bash.sh
        #! / bin / bash
        #Get the default shell for bash
        grep "bash $" / etc / passwd &> / dev / null;
        RETVAL = $ ?;
        if [$ RETVAL -eq 0]; then
          USERS = `grep" \ <bash $ "/ etc / passwd | wc -l`
          echo "the shells of $ USERS users is bash shell";
        else
         echo "no bash user"
        fi
        [[email protected] test] #

#Verify that there are no blank lines in / etc / inittab, how many blank lines
        [[email protected] test] # cat has_space.sh
        #! / bin / bash
        grep "^ $" / etc / inittab &> / dev / null;
        HAS_SPACE = $?
        if [$ HAS_SPACE -eq 0]; then
        SPACES = `grep" ^ $ "/ etc / inittab | wc -l`;
        echo "total SPACES is $ SPACES LINES";
        else
        echo "no any spaces in / etc / inittab";
        fi
        [[email protected] test] #
    Note: there must be no spaces on either side of the equal sign;

#Verify that the user's uid is equal to the GID, two implementation methods
        [[email protected] test] # cat uid_eq.sh
        #! / bin / bash
        USERNAME = user1
        # USERID = `id -u $ USERNAME`;
        USERID = `grep" ^ user1 "/ etc / passwd | cut -d: -f3`
        GROUPID = `grep" ^ user1 "/ etc / passwd | cut -d: -f4`
        # GROUPID = `id -g $ USERNAME`;

        if [$ USERID -eq $ GROUPID]; then
        echo ‘Good guy’;
        else
        echo ‘Bad guy’;
        fi
#Note: date +% s represents the number of seconds since 1970 in the timestamp; man date
echo $ HISTSIZE;

#Give a file to test its existence
#Single step execution; bash -x
    eg: Test script execution. bash -x
        [[email protected] test] # bash -x file_test.sh
        + FILE = / etc / inittab
        + ‘[’ -E / etc / inittab ‘]’
        + echo OK
        OK
        [[email protected] test] #

# Given a file, if it is a normal file, it will be displayed, if it is a directory, it will be displayed, otherwise the display cannot identify the file.
    [[email protected] test] # ./file_test.sh
    common file
    [[email protected] test] # cat file_test.sh
    #! / bin / bash

    FILE = / etc / inittab

    if [! -e $ FILE]; then
    echo ‘no such file’;
    exit 6;
    fi
    if [-f $ FILE]; then
    echo ‘common file’;
    elif [-d $ FILE]; then
    echo ‘directory’;
    else
    echo ‘unknown’;
    fi
    Note: the predicate! There must be a space after it;
arithmetic operations in the shell
All variables in the shell are strings by default;
    [[email protected] test] # A = 3
    [[email protected] test] # B = 4
    [[email protected] test] # echo $ A + $ B
    3 + 4
Generally use these three methods: 1, 2, 3:
1. Arithmetic operation let:
    let C = $ A + $ B
2. [[Arithmetic operation expression]
    C = $ [$ A + $ B]
3. $ ((Arithmetic operation expression))
    C = $ (($ A + $ B))
4, expr arithmetic operation expressions, there must be spaces between the operands and operators in the expression, and use command references
    F = `expr $ A + $ B`
5, using the arithmetic unit bc
Exit script
exit: exit the script;
exit n; n is the exit status code;
If the script does not explicitly define the exit status code, then the exit code of the last command is the exit status code of the script;
Test Methods
Use square bracket test method:
[expression]: Remember to use spaces on both sides and order the test method;
[[expression]]: keyword test method;
test expression:
Only use -gt, -le, -ne, -eq, -ge, -lt: square brackets are required, others are not necessarily square brackets.
if `grep" ^ $ USERNAME \> "/ etc / passwd`; then

File test:
    -e FILE: Whether the file exists.
    -f FILE: test whether the file is a normal file
    -d FILE: test whether the specified path is a directory;
    -r FILE: test whether the current user has read permissions on the specified file
    -w FILE: test whether the current user has write permission on the specified file
    -x FILE: test if the current user has execute permission on the specified file
    eg:
    [-e / etc / inittab], [-x /etc/rc.d/rc.sysinit]
        [[email protected] test] # ./exitst_file.sh
        NO / etc / iniittab
        [[email protected] test] # cat exitst_file.sh
        #! / bin / bash

        FILE = / etc / iniittab
        if [! -e $ FILE]; then
          echo "NO $ FILE"
          exit 8;
        fi

Round: discard the content after the decimal point; /
Show history commands:
    [[email protected] test] # history | tail -1 | cut -d ‘‘ -f2
    1012
Position variable (parameter)
Position variables $ 1, $ 2, ...
./file_test.sh / etc / fstab / etc / inittab;
$ 1: / etc / fstab; $ 2: / etc / inittab;
shift: Pop the first parameter in the parameter list; equivalent to shift in the array to pop the first element;
shift n: indicates that n parameters are popped at a time;

#Judgment: Receive a parameter, display OK if an existing file, no such file if it does not exist
    eg: $ #: display the number of all parameters; $ *: parameter list [email protected]: parameter list
            [[email protected] test] # ./exitst_file.sh / etc / inittab / ffff / ttddd
            3
            / etc / inittab / ffff / ttddd
            / etc / inittab / ffff / ttddd
            OK
            [[email protected] test] # cat exitst_file.sh
            #! / bin / bash
            echo $ #
            echo $ *
            echo [email protected]

            if [! $ 1]; then
             echo ‘please add parame file’;
            exit;
            fi

            if [-e $ 1]; then
              echo "OK"
            else
              echo "no such file";
            fi
            [[email protected] test] #

#Test shift
        [[email protected] test] # ./shift.sh 1 2 3 4 5
        5
        1
        2
        4
        [[email protected] test] # cat shift.sh
        #! / bin / bash

        echo $ #;
        echo $ 1;
        shift
        echo $ 1;
        shift 2
        echo $ 1
Bash shell scripting learning summary