Note points that require special attention in writing shell scripts

1. Conditional Judgments in shell scripts

1. if [Condition];then Statement Fi 2. If [Condition];then Statement elif [Condition];then Statement Else Statement Fi

Note the point:

1-1. If and fi are paired, a syntax error occurs when the script executes when the fi is missing.

1-2. When the condition is judged, the use of square brackets requires special care, where there must be a space between if and [, there must be a space between the front bracket and the conditional expression,] there must be a space between the back bracket and the conditional expression, which requires attention to these three places.

Example: The script shown in 1, when there is no space between if and [

Figure 1

Its running result 2

Figure 2

Thus, the treatment of square brackets "[]" must be taken seriously.

Loops in a 2.shell script

2-1.for Loop Statements

#!/bin/bash For x in one, three four Do Echo Number $x Done

Attention:

In this loop, the value of each variable x is a type of value in the list after "in", which can be a file name

2-2.while Loop Statements

#!/bin/bash Var=1 While [$var-le 10] Do Echo $var var=$ (($var + 1)) Done

Note: It is necessary to determine whether the condition is true before the loop executes.

2-3.until Loop Statements

#!/bin/bash Var=1 Until [$var-GT 10] Do Echo $var var=$ (($myvar + 1)) Done

The method of self-increment of five kinds of variables

1. i= ' expr $i + 1 ';

2. Let i+=1;

3. ((i++));

4. i=$[$i +1];

5. i=$ (($i + 1))

Note points that require special attention in writing shell scripts