Determining Parameters in the if statement for linux shell programming (1)

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]

Note the use of quotation marks for string comparison operators. This 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]