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This article mainly explains "how to use the set command of the Linux system". The content of the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "how to use the set command of the Linux system".

Set is a command used to set shell in the Linux system. It can set the execution mode of the shell used, which can be set according to different requirements. The parameters of the set command are described in linux as follows:

Parameter description:

-a marks the modified variable for output to the environment variable.

-b causes the aborted daemon to report the execution status immediately.

The file generated by the C turn cannot overwrite an existing file.

-d Shell presupposes to use hash tables to memorize used instructions to speed up the execution of instructions. Use the-d parameter to cancel.

-e if the instruction returns a value that is not equal to 0, exit shell immediately.

-f cancels the use of wildcards.

-h automatically records the location of the function.

-H Shell can take advantage of "!" Add the way to execute the instructions recorded in history.

The parameters given by the-k instruction are treated as environment variables for this instruction.

-l records the variable name of the for loop.

-m uses monitoring mode.

-n only reads instructions and does not actually execute them.

-p starts priority mode.

After-P starts the-P parameter, the symbolic link is replaced by the actual file or directory when the instruction is executed.

-t exits the shell after executing the following instructions.

-u when undefined variables are used during execution, an error message is displayed.

-v displays the input values read by shell.

-x after the instruction is executed, the instruction and the parameters are displayed first.

+ cancel the parameters that a set has started.

That is, the code that appears after "set-e" exits the entire script as soon as the return value is non-zero. Some people like to use this parameter for the sake of code security. But sometimes, this kind of good intention can also lead to serious problems.

Real case: the script a.sh starts with "set-e" and works properly. After a few months or more, three lines of hadoop actions were added to the script due to the need to upgrade:

The logic of the hadoop command lines #! / bin/bash set-e... / home/work/.../hadoop dfs-rmr / app/.../dir/ home/work/.../hadoop dfs-mkdir / app/.../dir/ home/work/.../hadoop dfs-put file_1 / app/.../dir/.. 123456 is simple: clear and create a new directory on hdfs, and push a local file to this directory for later use. Take these lines out and execute them under the command line. Except for prompting that the directory to be deleted does not exist, there is no problem, and the file will still be pushed to the specified place.

However, when the script was executed for the first time, it failed to exit, resulting in the overall exit of the program that called the script, with serious consequences. The reason is that there is no such directory on hdfs, and the line rmr will return 255. this value is captured by "set-e" in front of the script, which directly causes the script to exit.

There is no problem with the new code itself. First delete and then create a new directory. Instead, it is a relatively standard operation to ensure data security. Deleting the fault tolerance of the command itself can ensure the normal execution of subsequent commands. The fact is that the script has hundreds of lines and complex logic, and when adding these lines of code, developers can no longer remember that there is a "set-e" lurking in the script.

It can be seen that setting "set-e" may be very helpful in the script development process, but after the development is completed, especially for scripts that may have upgrades later, it may be a security hazard.

\ 2. Set-o pipefail for the pipefail option of the set command-o parameter, linux interprets it as: "If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status,or zero if all commands in the pipeline exit successfully. This option is disabled by default."

When this option is set, the return value of the statement containing the pipe command becomes the return value of the last pipe command that returns non-zero. It sounds like a circle, but it's actually very simple.

# test.sh set-o pipefail ls. / a.txt | echo "hi" > / dev/null echo $? 123 runs test.sh because there is no a.txt file in the current directory. Output: ls:. / a.txt: No such file or directory 1 # sets set-o pipefail and returns the first non-zero return value from right to left, that is, the return value of ls 1

Comment out the line set-o pipefail and run it again. Output: ls:. / a.txt: No such file or directory 0 # No set-o pipefail. The return value of the last pipe command is returned by default.

Thank you for reading, the above is the content of "how to use the set command of the Linux system". After the study of this article, I believe you have a deeper understanding of how to use the set command of the Linux system, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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