1. Special location variables
About the above related variables introduction , You can also refer to the man manual, execute the command “man bash”, and then search for the keyword “Special Parameters” to see the explanation of the relevant variables.
2, bash shell built-in variable commands
bash shell contains some built-in commands. These built-in commands are invisible in the directory listing, they are provided by the Shell itself. Commonly used internal commands are: echo, eval, exec, export, read, shift.
(1) echo——output information on the screen
Common parameters are as follows:
(2) export——Convert custom local variables into global variables.
(3) Shift——Offset the position variable
If shift is written in the script, it will change the original corresponding to $1, $2… The variable content of is shifted backward by one by default, the original $2 becomes $1, and the original $1 is gone. You can also write “”shift 3” in the script, which means that it is shifted by three places. The original $4 will become $1, and $5 will become $2… Then, what if my positional variables are only two? The offset will be cyclically. Assuming that $1 corresponds to “a” and $2 corresponds to b, I wrote a line of “shift 5” in the script, then it will offset the cycles of $1 and $2, and the final result is $1 Corresponds to b.
For other infrequently used commands, if necessary, use Baidu on your own. I won’t explain it here.
3. Shell variable string
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You can use the following table to understand the variable strings of the shell, or you can execute the “man bash” command and search for “Parameter Expansion” to find the relevant introduction.
Summary about the deletion of matches in the table above:
- # means deleting the shortest match from the beginning;
- ## means deleting the longest match from the beginning;
- % means deleting the shortest match from the end;
- %% means removing the longest match from the end;
- A “/” means replacing the matched first string;
- Two “/” means replacing the matched All strings.