How to pipe output of command to another command while also displaying it on the console?

Question

I want to write part of the results of a stream to a file, but I want the entire contents of the stream printed to the console. Is there some command that would help with this?

Minimal example:

Say I had a file foo.txt with contents:

bat
dude
rude

And I wanted to write all lines in that file which contain the letter 'a' to bar.txt. I could write

cat foo.txt | grep 'a' > bar.txt

Which would result in bar.txt containing bat. But that wouldn't give me the console output that I want.

Instead I would prefer something like:

cat foo.txt | output-stdin-to-console-and-pass-to-stdout | grep 'a' > bar.txt

Which would not only write bat to bar.txt but also write the following to the console:

bat
dude
rude

Is there any command I can run to do that?

Answer 1

Explicit examples with tee:

• tee writing to the tty

  < foo.txt tee /dev/tty | grep 'a' > bar.txt
  

  This is portable, works in sh.

• tee writing to process substitution, its standard output goes to the console:

  < foo.txt tee >(grep 'a' > bar.txt)
  

  This is not portable, works in Bash and few other shells.

Note I got rid of the cat command (useless use of cat).

Answer 2

You could simply use this:

cat foo.txt && cat foo.txt | grep 'a' > bar.txt

Otherwise, a one liner is possible using tee

From https://www.geeksforgeeks.org/tee-command-linux-example/

tee command reads the standard input and writes it to both the standard output and one or more files. The command is named after the T-splitter used in plumbing. It basically breaks the output of a program so that it can be both displayed and saved in a file. It does both the tasks simultaneously, copies the result into the specified files or variables and also display the result.