Reading and writing to the same file

If you try to modify a file (removing all empty lines for example) using a command like:

 cat file.txt | sed '/^$/d' > file.txt 

you will end up with and empty file.txt. The reason is that bash parses the command line looking for “metacharacters” (  “|” , “>” and “space”  in this case) that separate words, then groups and executes those words according to their precedence. This means that “> file.txt”  get executed FIRST. This creates an empty file.txt (overwriting any existing file) and a “process”  to redirect standard output to that file. Then “cat file.txt” get executed, but by now file.txt is empty. So “cat file.txt” outputs 0 lines,  “sed ‘/^$/d’ ” deletes all 0 empty lines, and 0 lines get written to file.txt . This works as “intended” and bash outputs no error.

You can get around this using a temporal file.

 cat file.txt | sed '/^$/d' > tmp_file.txt
mv tmp_file.txt file.txt

But, as file.txt is technically a new file you might lose some information, in particular permissions and whether file.txt was originally a symbolic link or not.

Other options is to use sponge, which is part of moreutils and sadly not standard in many systems.

 cat file.txt | sed '/^$/d' | sponge file.txt