Judgment structure in LinuxShell

IFTHEN structure determines whether a piece of code returns & amp; 20540; whether it is 0 (according to UNIX conventions, 0 indicates yes). if it is 0, it executes a specific piece of code; the IF/THEN structure determines whether the return value of a piece of code is 0 (according to UNIX conventions, 0 indicates yes). IF it is 0, a specific piece of code is executed. a dedicated command exists in the Shell. [(That is, left braces ). He and test are synonyms and introduced to improve efficiency. This command regards its parameters as a comparison statement or a file Test statement and returns the corresponding value. in version 2.2, Bash introduced the extended test command [[], the comparison method is similar to other languages. note: [is a keyword, not a command, and [[] is a command! Bash regards [[$ a-lt $ B] as an element and returns only one status value; () and let... the structure returns the status based on whether the mathematical expression calculates a non-zero value. IF can test any command, not just those expanded with braces.