Shell Process Control Statements

I. The existence and judgment of documents

-D to determine if the file exists and whether it is a directory file (the directory is true)

-E to determine if the file exists (exists as true)

-F to determine if the file exists and is a normal file (the normal file is true)

Second, the file permission to judge

-R to determine if the file exists and whether it has Read permission (Read permission is true)

-W to determine if the file exists and has Write permission (Write permission is true)

-X determines whether the file exists and has execute permission (with Execute permission true)

Third, integer judgment

The shell is a weakly typed language, and all of the default contents are strings, but the following comparison commands automatically convert the contents of both sides to numeric types, so no (())

-eq equal

-ne Not Equal

-ge greater than or equal to

-le less than or equal to

-GT Greater than

-lt less than

Iv. type of string

-Z string is empty, NULL returns True

-n string is not NULL, non-null return True

= = equals, equality is true (= can also be used to determine whether a string is equal, but is easily confused with assignment operations)

! = range, not equal to True

V. Multiple conditions of judgment

A is really true

-O One true is True

! Non -

VI. If statement

Single branch:

If [conditional judgment]

Then program

Fi

Dual Branch:

If[]

Then

echo "1"

Else

echo "2"

Fi

Multi-branch:

If[]

Then

echo "1"

Elif[]

Then

echo "2"

Else

echo "3"

Fi

Vii. Case Statements

Case $a in

"Yes")

echo "AA"

;;

"No")

echo "BB"

;;

*)

echo "CC"

;;

Esac

Viii. for Loop

For I in 1 2 3 4 5

Do

Echo $i

Done

For I in $ (cat a.txt)

Do

Echo $i

Done

S=0

For ((i=1;i<=100;i=i+1))

Do

s=$ (($s +$2))

Done

Nine, while loop

While [$i-le 100]

Do

.....

Done

While Read line

Do

......

Done < cat A.txt

Shell Process Control Statements