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    This is worse than ilkkachu's very similar answer from 4 hours ago: cat prefix data > data.new && mv data.new data, as yours will clobber file.txt if the initial cat fails (full filesystem?). Commented Jan 17, 2018 at 17:08
  • You have a point there, but I did start the answer by assuming a normal, sane environment. A full filesystem is a rare condition, yet very easily noticed when it happens (and as such you can take special precautions, such as when working with a system out of memory). Otherwise, inserting an explanatory header into a regular text config file is rather unlikely to be the tipping point. Commented Jan 18, 2018 at 9:10
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    The thing is that for scripted solutions, you don't get to assume a sane operating environment. So at the very least an answer like this should have a big bold warning message that it should only be used interactively—and in that case, it really shouldn't be on one line (i.e. the semicolon should be a literal newline). The percentage of Production outages that are caused by lazy scripting that assumes a sane operating environment is probably in excess of 75%. Commented Jan 18, 2018 at 13:42