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I am learning the cat command of linux, and I found this command : 

$ echo 'Text through stdin' | cat - file.txt

What does "-" mean here? If I don't type it , then 'Text through stdin' will not be shown.

Yu Hao
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huashui
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  • possible duplicate of [Does " - " mean stdout in bash?](http://stackoverflow.com/questions/3797795/does-mean-stdout-in-bash) (admittedly for stdin/stdout, depending on the context...) – Bruno Aug 22 '13 at 13:23

3 Answers3

15

it is common to write stdin as dash (-).

even man cat mentions that:

With no FILE, or when FILE is -, read standard input.

and the manpage even has an example illustrating the use of dash and ordinary filenames (which is quite close to your original question, but includes the answer):

   cat f - g
          Output f's contents, then standard input, then g's contents.
umläute
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    It is important to realise that after `stdin` has been read the file `file.txt` will then be displayed. – PP. Aug 22 '13 at 13:23
  • @PP. right, i added two more lines of the manpage (incidentally they come just after my original quote), that should make this explicit. – umläute Aug 22 '13 at 13:38
5

- tells cat to read from stdin. This is quite common, a lot of apps read from stdin if you pass - to them.

Some apps use - as stdout.

Here is an example of downloading blender and instead of writing it to a file we write it directly to stdout and pipe it to tar, which expands it on the fly during download.

wget -c https://download.blender.org/source/blender-2.90.1.tar.xz -O - | tar -xzv

Here the -O - tells wget to write directly to stdout

hashier
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3
$ echo 'Text through stdin' | cat - file.txt

- tells cat to read from standard input, in this case, from the pipe, i.e, what echo 'Text through stdin' outputs.

Yu Hao
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