Write null to a file unix

I watched some examples and i believe if checks if an argument is NULL. If so, how can i add the checks for argument 2 and 3 in the statement? RedHat Commands. OpenSolaris Commands. Linux Commands. SunOS Commands. FreeBSD Commands. Full Man Repository. Advanced Search. Contact Us. Forum Rules. Mark Forums Read. Thread Tools. View Public Profile for april. Find all posts by april.

View Public Profile for aigles. Find all posts by aigles. View Public Profile for vgersh Find all posts by vgersh So my question is, Are the single and double quotes a substitution for the escape backslash character? The point is that the command has to understand what to do with backslash-escaped characters. Last more check that we really print null character before trying to create file, let's pass the printed value to the command which will throw the error, like xargs :.

Notice how only a was printed at the end. Of course xargs -0 works fine:. The result is exactly the same as in touch '' , it seems null is just ignored all together.

What if we skip double quotes around command substitution? Yet another result is if we surround null with text:. Double quotes are unique, though. The bash manual doesn't spend much time on how to use all the features it describes, just how they work individually.

It's literally not possible to pass a string containing a zero-byte as a command line argument, or to a system call. C strings are character arrays where the end-of-string is marked by a zero-byte. There is no way to "escape" a zero byte to indicate that it's not the end of a string. It wouldn't make it out of the argv array in the exevce 2 system call that ran touch. System calls that need to handle arbitrary data, like read 2 and write 2 , take a length argument as well as a pointer to the buffer.

It's not even possible to do much of anything with null bytes with bash. I guess this means bash stores strings internally as C strings, not with an explicit length. So we can't use this syntax to send a null byte anywhere. We'd have to use tr or something. And printf already processes such escapes in its format string.

The reason for why the different options work vary:. Also there is the test command [ and the quotes we already used ' " , as well as recalling previous commands with an exclamation mark! Also note that the current and parent dir are. I hope I forgot none as there are quite a bunch of them. That is the only reason. The NUL character cannot be used as part of the filename, because the relevant system calls use C language string-passing conventions, and NUL is the terminator for such a string.

So it cannot be interpreted as part of the name. Sign up to join this community. So don't bother and only use fd's If you need more the 9 file descriptors in a bash script you should use a different language anyways :. For environments where here documents are unavailable Makefile , Dockerfile , etc you can often use printf for a reasonably legible and efficient solution.

I thought there were a few perfectly fine answers, but no concise summary of all possibilities; thus:. The core principal behind most answers here is redirection. Two are important redirection operators for writing to files:. Others mentioned, rather than from a fixed input source like echo 'text' , you could also interactively write to files via a "Here Document", which are also detailed in the link to the bash manual above.

Those answers, e. In the above syntax, you write to the FILE. The writing only takes place after the interactive input is given some specific string, in this case 'EOF', but this could be any string, e. Here Documents also look for various delimiters and other interesting parsing characters, so have a look at the docs for further info on that. A bit convoluted, and more of an exercise in understanding both redirection and Here Documents syntax, but you could combine Here Document style syntax with standard redirect operators to become a Here String:.

Can also use here document and vi, the below script generates a FILE. Then file will have 3 lines as below. How are we doing? Please help us improve Stack Overflow. Take our short survey. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Open and write data to text file using Bash? Ask Question. Asked 9 years, 6 months ago. Active 11 months ago. Viewed k times. How can I write data to a text file automatically by shell scripting in Linux? I was able to open the file. However, I don't know how to write data to it. Improve this question.