Conditional control in shell scripts and the use of loops

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If conditional statement syntax:

If [conditional expression]

Then

Code

Else

Code

Fi

Note : In the IF condition statement above, there should be a space between if and [, and the conditional expression and [] on both sides should have spaces, and the conditional expression cannot be next to [and].

An IF condition expression ends with fi.

Eg.

As a result, the following issues occur:

The reason is because there is a/after the beginning of the #!, plus/After the interpreter error is not, the code is as follows:

However, there are still no errors for that file or directory. For this error I think echo after the expression has a problem, so write-off the expression, change output small and bigger, the result program runs correctly, output small or bigger.

So for the echo expression, I put the expression in double quotes, the result runs successfully, no problem.

Conditional judgments are made using case expressions:

Case variable in

STR1)

command string;;

STR2)

command string;;

*)

Eg.

The error has just occurred because the code does not end correctly, and the case condition statement should end in Esca in the shell.

The error appears as follows:

Reason:

* later) for the Chinese symbol, change it to an English symbol, and the end of the case condition statement should be ESCA instead of EASC, corrected as follows:

Right, the correct result is as follows:

Using a For loop in a shell script

There are three types of loops that can be used in the shell:

A, for Loop

The first form of syntax:
for Var in list

Do

Command string

Done

Note that the list above can be separated by a space of $1, or a space-delimited value of 2 3.

The second form of syntax:

for Var

Do

Command string

Done

In this form, the list is omitted, then the list refers to the command line parameters, each cycle of Var is a command-line parameter.

eg. output command-line arguments and

Note that the above shell program in the echo command parameters-E is for \c and \b to take effect, where \c is the output addend after the line is not wrapped, \b is the meaning of the backspace delete the extra + number.

An error occurred after running the script:

Reason:

Expr can not drink test instruction-gt after marriage together, otherwise will expr change to test, otherwise will-GT changed to >=

, it is worth noting that >= is used in front of \.

The error remains:

The reasons are:

When sum is summed, there must be no spaces at both ends .

Switch

After running the result is:

B, while loop

Syntax format:
While-conditional expression

Do

command line string;

Done

eg. output command line each parameter

The shift command shifts the command-line arguments to the left, which is the first parameter in the second position and then the output.

The result is:

Note that the conditional expression for the while loop also separates the conditional expression from [and] with a space.

If you want to use another variable n in the while, represent the unary expression used in the loop body in the form of-N:

Of course, if you do not add-N in the while, there is the same result:

C, until cycle

The syntax format is as follows:

Until conditional expression

Do

command string;

Done

Conditional control in shell scripts and the use of loops