How to determine parameters in the IF statement for Linux Shell Programming

Use the if statement to determine parameters in shell programming

-B. returns true if the file exists and is a block file.

-C: returns true if the file exists and is a character file.

-D. If pathname exists and is a directory, true is returned.

-E: returns true if the file or directory specified by pathname exists.

-F returns true if the file exists and is a regular file

-G returns true if the file or directory specified by pathname exists and the sgid bit is set.

-H returns true if the file exists and is a symbolic link file. This option is invalid in some old systems.

-K. If a file or directory specified by pathname exists and a "Sticky" bit is set, the system returns the true value.

-P: returns true if the file exists and is a command pipeline.

-R: returns true if the file or directory specified by pathname exists and is readable.

-S: returns true if the file size is greater than 0.

-U returns true if a file or directory specified by pathname exists and SUID is set.

-W returns true if the file or directory specified by pathname exists and is executable. A directory must be executable for its content access.

-O returns true if a file or directory specified by pathname exists and the user specified by the valid user ID of the current process is owned by it.

 

 

Comparison character writing in Unix shell:

-EQ equals

-Ne is not equal

-GT greater

-Lt is less

-Le is less than or equal

-Ge is greater than or equal

-Z empty string

= Two equal characters

! = Two characters

-N non-empty string

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For more details:

Operator Description Example

File comparison operator

-E filename if filename exists, it is true [-E/var/log/syslog]

-D filename if filename is a directory, it is true [-D/tmp/mydir]

-F filename if filename is a regular file, it is true [-F/usr/bin/grep]

-L filename if filename is a symbolic link, it is true [-L/usr/bin/grep]

-R filename if filename is readable, it is true [-r/var/log/syslog]

-W filename if filename is writable, it is true [-W/var/mytmp.txt]

-X filename if filename is executable, it is true [-L/usr/bin/grep]

Filename1-nt filename2 if filename1 is newer than filename2, it is true [/tmp/install/etc/services-nt/etc/services]

Filename1-ot filename2 if filename1 is earlier than filename2, true [/boot/bzimage-ot ARCH/i386/boot/bzimage]

String comparison operator (please note the use of quotation marks, which is a good way to prevent space from disturbing the code)

-Z string if the string length is zero, it is true [-Z $ myvar]

-N string if the string length is non-zero, it is true [-N $ myvar]

String1 = string2 if string1 is the same as string2, it is true [$ myvar = One Two Three]

String1! = String2 if string1 is different from string2, it is true [$ myvar! = One Two Three]

Arithmetic comparison operator

Num1-EQ num2 equals to [3-EQ $ mynum]

Num1-ne num2 is not equal to [3-ne $ mynum]

Num1-lt num2 is less than [3-lt $ mynum]

Num1-Le num2 is less than or equal to [3-Le $ mynum]

Num1-GT num2 is greater than [3-GT $ mynum]

Num1-ge num2 is greater than or equal to [3-ge $ mynum]

Script example:

#! /Bin/bash

# This script prints a message about your weight if you give it your

# Weight in kilos and hight in centimeters.

If [! $ #== 2]; then

Echo "Usage: $0 weight_in_kilos length_in_centimeters"

Exit

Fi

Weight = "$1"

Height = "$2"

Idealweight = $ [$ height-110]

If [$ weight-Le $ idealweight]; then

Echo "You shoshould eat a bit more fat ."

Else

Echo "You shoshould eat a bit more fruit ."

Fi

# Weight. Sh 70, 150

You shoshould eat a bit more fruit.

# Weight. Sh 70 150 33

Usage:./weight. Sh weight_in_kilos length_in_centimeters

The location parameter $1, $2,..., $ N, $ # indicates the number of parameters in the command line, and $0 indicates the Script Name,

The first parameter represents $1, the second parameter represents $2, and so on. The total number of parameters exists in $ #. The above example shows how to change the script, if there are fewer than or two additional parameters, a message is printed.

Run the command and check the status.

# Bash-x tijian. Sh 60 60 170

+ Weight = 60

+ Height = 170

+ Idealweight = 60

+ '['60-Le 60']'

+ Echo 'you should eat a bit more fat .'

You shoshould eat a bit more fat.

-X is used to check the execution of the script.