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< 9 ]]thenecho $akfi==============================================================================|| 逻辑符号这个会时常看到，代表 or 逻辑的符号。&& 逻辑符号这个也会常看到，代表 and 逻辑的符号。& 后台工作单一个& 符号，且放在完整指令列的最后端，即表示将该指令列放入后台中工作。tar cvfz data.tar.gz data >

/ dev/null&\ word boundary is defined as the meaning of "boundary" in regular expressions. For example, when we look for the word the, if we use grep the FileA, you will find that words like there are also treated as matching words. Because the happens to be part of there. If we want to avoid this situation, we have to add the "boundary" symbol grep'\ 'FileA+ plus sign (plus) in the expression, which she uses to mean "addition". Expr 1 + 2 + 3 in addition, in regular expressions, it is used to express the meaning of "many" preceding characters. The symbol # grep'10\ + 9 'fileB109100910000910000931010009# must be preceded by an escape character. -the minus sign (dash) is used in the expression to mean "subtraction". Expr 10-2 is also an option symbol for system instructions. Ls-expr 10-2 in the GNU instruction, if you use the-symbol alone, without any additional file name, it means "standard input". This is a common option for GNU instructions. For example, the following example tar xpvf-the-symbol here, both represents reading data from standard input. However, the cd directive is more special cd-this means changing the working directory to the "last" working directory. Division (Modulo) is used in the expression to denote "division". Expr 10% 2 in addition, the following ${parameter%word} ${parameter%%word}, which is also used in regular expressions about variables, represents the shortest word match and two represents the longest word match. The symbol commonly seen when setting variables is the equal sign (Equals). Vara=123echo "vara= $vara" or settings like PATH, or even for such purposes as operations or judgments. The equal sign (Equals) is often seen in conditional judgments and stands for "equal". If [$vara = = $varb]... The following! = does not mean that it is often seen in the conditional judgment formula, which means "not equal to". If [$vara! = $varb]... The symbol below ^ represents the "beginning" position of a line in a regular expression and is also associated with "!" in []. (exclamation mark) also means "not" = output / input redirection > >

< &>

2 & > 2 > & > & 2 file descriptor (File Descriptor), using a number (usually 0-9) to represent a file. The commonly used file descriptors are as follows: file descriptor name commonly abbreviated default value 0 standard input stdin keyboard 1 standard output stdout screen 2 standard error output stderr screen when we simply use it, it is equivalent to using 0

< 或 1>

(described in more detail below). * cmd > file redirects the output of the cmd command to the file file. If file already exists, empty the original file and use the noclobber option of bash to prevent the original file from being overwritten. * cmd > > file redirects the output of the cmd command to the file file, and if the file already exists, add the information to the original file. * cmd

< file使cmd命令从file读入* cmd &n把输出送到文件描述符ncmd m>

& n redirects the information output to the file character m to the file descriptor ncmd > &-turning off standard output cmd & 1 is not the same as cmd 2 > & 1 > file.

Summary

The above is the complete book of linux special characters and their functions introduced by the editor to you. I hope it will be helpful to you. If you have any questions, please leave me a message and the editor will reply you in time. Thank you very much for your support to the website!

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