Shell script Programming-structured commands 1

1, Branch statement (1) IF-THEN-FI statement: Format:

    if  Then         commands    fi    

When the command command exit status code is $?=0, enter the branch, or continue with the following command.

(2) IF-THEN-ELSE-FI statement: Format:

    if  Then         commands1    else        commands2    fi  

When the command command exit status code is $?=0, go to then branch, otherwise execute the Else Branch command. (3) IF-THEN-ELIF-THEN-FI statement: Format:

    if  Then         commands1    elifthen        commands2    fi

Nested processing of If branches.        2, the test command can only be tested by the test command with the command exit code independent conditions.        The test command provides a way of testing different conditions in a if-then statement. The test command can determine three types of conditions, numeric comparisons, string comparisons, and file comparison formats:

    if  Then         commands    fi

Another way to declare the test command in a IF-THEN-FI statement is provided in the bash shell:

 if  [condition]; then   commands  fi  

        square brackets define the conditions used in the test command, note: there must be a space between the left and right brackets (1) numeric comparison         N1-EQ n2 &NB condition Sp   Check if N1 equals n2         n1-ge n2     Check N1 is greater than or equal to n2         N1 -GT n2      Check if N1 is greater than n2        n1-le n2     Check N1 is less than or equal to n2        N1-LT n2       Check N1 is less than n2        n1-ne n2     Check N1 is not equal to n2  &nbsp ; Note: Numeric comparisons can only be used to process integers, cannot handle floating-point numbers (2) string comparisons         STR1 = str2         Check if STR1 is the same as str2   & nbsp     STR1! = str2       Check str1 is different from str2         STR1 < str2     &N Bsp   Check if STR1 is smaller than str2         STR1 > str2         Check str1 is greater than str2     &NBS P  -N str                 Check str length is not 0       -Z str                 Check str length is 0    Note:            when using greater than and less than You need to use the escape character \< or \>, otherwise the uppercase letters will be processed as lowercase when you compare the redirect symbol conflict            , but when you compare the same string, Small Letter larger (3) file comparison        -D file         Check if file exists and is directory      -E File         Check if file exists        -F file         Check if file exists and is nbsp      -R file         Check if file exists and is readable        -s file     & nbsp   Checks if file exists and is not empty        -W file         Check if file exists and can be written       &NB Sp -X file         Check file exists and executable        -o file         Check if file exists and is current user all        -G file         Check if file exists and default group is the same as current user         File1-nt file2 &NBSp   Check if File1 is newer than file2         File1-ot file2     Check file1 is better than file2 old (4) qualifying test     & nbsp The   IF-THEN-FI statement allows the use of Boolean logic to combine tests, two Boolean logic symbols available:            [condition1] && [conditio N2]            [Condition1] | | [Condition2] 3, If-then advanced Features (1) using the double parenthesis         format: (expression))         Expressio n can be any mathematical assignment or comparison expression, except for the math operator in the test command, the following symbols:            val++/val--    post-increment/post-subtract & nbsp           ++val/--val     Pre-add/forward minus            !                   logic take-back             ~   &nbs P               Click-to-reverse             *       &NBS P         power operations             << / >>         Bitwise LEFT SHIFT/Right shift             &/|               by position and/or             &&/| |           logic and/or         NOTE: Mathematical operators for double parentheses do not need to be escaped (2) use both brackets         Format: [[Expression]]        expression can use the standard string comparison in the test command and provide a pattern match that can be matched using regular expressions     & nbsp   Example:

    if  Then         commands    fi

Matches whether the $USER starts with R and, if passed, executes the commands content. 4. Case Statement Format:

     Case inch     | val2)    Command1;;    Val3    Command2;     *)    default command3;;     Esac

To solve the problem of multi-layered nesting of if-then

Shell script Programming-structured commands 1