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This article mainly introduces the syntax of script sentences commonly used in linux shell. In daily operation, I believe that many people have doubts about the syntax of script sentences commonly used in linux shell. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts of script sentence syntax commonly used in linux shell. Next, please follow the editor to study!

If statement syntax in 1shell

I find that sometimes it is equivalent to using eq, sometimes it is expressed as = standard, I don't understand.

=

Eq value

= string!

=

You can take a look at man test

=

Process control

The if expression executes the following part of the then if the condition is true:

If....

Then

....

Elif....

Then

....

Else

....

Fi

In most cases, you can use test commands to test conditions. For example, you can compare strings, determine whether the file exists and whether it is readable and so on.

Conditional tests are usually represented by []. Notice that the spaces here are very important. Make sure the square brackets are blank.

[- f somefile]: determine whether it is a file or not

[- x / bin/ls]: determine whether / bin/ls exists and has executable permission

[- n $var]: determine whether the $var variable has a value

[$a = $b]: judge whether $an and $b are equal

=

I saw a book today, which says that if you use the =, etc. Comparison symbol in the if sentence, it should be written in parentheses, and if you use-eq,-lt, it should be written in [], right?

=

This should be aimed at ksh/bash, because they all extend the original sh and can be used directly

= to make a numerical judgment, but as you mentioned, write it in ().

=

The judgment after if is test syntax, while

If [expression] and

If test expression

Is equivalent.

+ +

Test command usage:

1) judgment expression

If test (expression is true)

If test! The expression is false

Test expression 1-an expression 2 both expressions are true

Test expression 1-o expression 2 one of the two expressions is true

2) judge string

The length of test-n string string is non-zero

The length of the test-z string string is zero

Test string 1 = string 2 string equal

Test string 1! = string 2 string is not equal

3) judge integers

Test integers 1-eq integers 2 integers equal

Test Integer 1-ge Integer 2 Integer 1 greater than or equal to Integer 2

Test Integer 1-gt Integer 2 Integer 1 is greater than Integer 2

Test Integer 1-le Integer 2 Integer 1 less than or equal to Integer 2

Test Integer 1-lt Integer 2 Integer 1 is less than Integer 2

Test Integer 1-ne Integer 2 Integer 1 is not equal to Integer 2

4) judgment document

Both test File1-ef File2 files have the same device number and I node number

Test File1-nt File2 file 1 is newer than file 2

Test File1-ot File2 file 1 is older than file 2

The test-b File file exists and is a block device file

Test-c File file exists and is a character device file

The test-d File file exists and is a directory

Test-e File file exists

Test-f File file exists and is a regular file

The test-g File file exists and the group ID is set

Test-G File file exists and belongs to valid group ID

The test-h File file exists and is a symbolic link (same as-L)

The test-k File file exists and the sticky bit is set

The test-b File file exists and is a block device file

Test-L File file exists and is a symbolic link (same as-h)

The test-o File file exists and belongs to a valid user ID

The test-p File file exists and is a named pipe

The test-r File file exists and is readable

The test-s File file exists and is a socket

The test-t FD file descriptor is opened at a terminal and reproduced. Please indicate the article.

Command line inner loop:

Do echo 0 & & while [$a-lt 10]; do echo $a & & a = `expr $a + 1`; done

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