What generates the "text file busy" message in Unix?

It's a while since I've seen that message, but it used to be prevalent in System V R3 or thereabouts a good couple of decades ago. Back then, it meant that you could not change a program executable while it was running.

For example, I was building a make workalike called rmk, and after a while it was self-maintaining. I would run the development version and have it build a new version. To get it to work, it was necessary to use the workaround:

gcc -g -Wall -o rmk1 main.o -L. -lrmk -L/Users/jleffler/lib/64 -ljl
if [ -f rmk ] ; then mv rmk rmk2 ; else true; fi ; mv rmk1 rmk

So, to avoid problems with the 'text file busy', the build created a new file rmk1, then moved the old rmk to rmk2 (rename wasn't a problem; unlink was), and then moved the newly built rmk1 to rmk.

I haven't seen the error on a modern system in quite a while...but I don't all that often have programs rebuilding themselves.

This error means you are trying to modify an executable while it is executing. The "Text" here refers to the fact that the file being modified is the text segment for a running program. Use lsof to check what other processes are using it. You can use kill command to kill it if needed.

This occurs when you try and write to a file that is currently being executed by the kernel, or execute a file that is currently open for writing.

Source: http://wiki.wlug.org.nz/ETXTBSY