46

I know that to capture a pipeline's contents at an intermediate stage of processing, we use tee as ls /bin /usr/bin | sort | uniq | tee abc.txt | grep out , but what if i don't want to redirect the contents after uniq to abc.txt but to screen(through stdout, ofcourse) so that as an end result , i'll have on screen, the intermediate contents after uniq as well as the contents after grep.

Aman
  • 1,013
  • 2
  • 10
  • 18
  • 1
    The problem is `stdout` is a pipe to `grep out`. It's not your terminal anymore. Do you have any sort of guarantee that this will **always** be run from a terminal? – Joe Sewell Jan 12 '15 at 17:50
  • @JoeSewell In my case, yes from a terminal, but please tell what are the other sources from apart from terminal(are you talking about simply shell-scripting or something else). Just curious. I'm new to Linux and would love to explore what you are talking in detail. – Aman Jan 12 '15 at 17:53
  • One of my comments to an answer below gives a few examples. Running it from within `gvim` may leave you without a terminal at all. Having this line in a `gmake` makefile may cause problems if the `-j` option is used, and the subshell in use doesn't have access to the terminal. `cron` jobs wouldn't have a terminal, either. A shell script, on the other hand, **might** have a terminal, unless it's run without one. – Joe Sewell Jan 12 '15 at 17:58
  • @JoeSewell thanks, i'll do some research on them :) – Aman Jan 12 '15 at 18:18

7 Answers7

67

sometimes /dev/tty can be used for that...

ls /bin /usr/bin | sort | uniq | tee /dev/tty | grep out | wc
JJoao
  • 11,887
  • 1
  • 22
  • 44
  • 3
    This assumes the command is run from a terminal. – Joe Sewell Jan 12 '15 at 17:48
  • 1
    `ls /bin /usr/bin | sort | uniq | tee /dev/tty | grep out | wc > out` only contains the output from `wc`. – user7543 Feb 12 '21 at 20:14
  • 1
    @user7543 tee -o out will give you tee output you can dan do wc >> out to append output to the file (depending on nonglobering settings that is). I would use file descriptor for both tee and grep and cat it. – nethero Apr 16 '21 at 14:58
  • On Linux at least you can also use `tee /dev/stdout`. – Mike May 08 '22 at 18:08
  • @Mike, tee /dev/stdout is fine but it would duplicate the files that "enter" the `grep out` command (wc numbers would be the double) – JJoao Jan 10 '23 at 14:19
11
ls /bin /usr/bin | sort | uniq | tee /dev/fd/2 | grep out | wc

On a linux system you can use the the /dev/fd/[num] links like named pipes in many cases. This will duplicate stdout to stderr, which, typically, is your terminal screen, but doesn't need to be.

mikeserv
  • 57,448
  • 9
  • 113
  • 229
  • Only works with bash, not dash /bin/sh on Ubuntu. I use this command for docker build: `ID=$(docker build "$dockercontext" 2>&1 | tee /dev/fd/3 - | tail -1)` – Daniel Alder Oct 15 '15 at 13:15
  • @DanielAlder - it works fine with `dash`. but you need to direct it somewhere. where does `&3` go? the `fd/2` above assumes the command is run from a terminal and `&2` goes to the `tty`. for example: `dash -c '{ echo hey; read v; echo "${v#?}" >/dev/fd/2; } <>/dev/fd/1 |:'` – mikeserv Oct 16 '15 at 05:24
  • You are right. Didn't realize that /dev/fd/* are now also supported in dash. fd/3 is redirected to stdout in my case. I use this command line now, the other variant was overkill: `ID=$(docker build "$1" 2>&1 | tee /dev/fd/2 - | tail -1)` – Daniel Alder Oct 16 '15 at 07:51
  • @DanielAlder - those dont really have anything to do with the shell. theyre just files. or... links to file-descriptors anyway. but the shell doesnt put them there or control their behavior at all - those are handled by the kernel. the shell just does `open()` and `dup2()` with file-descriptors and it treats those exactly the same as it does any other file. – mikeserv Oct 16 '15 at 07:59
  • You don't know the history, and I didn't know the current situation. Not many years ago, there was no /dev/fd/ in the system. It was compiled directly in bash like /dev/tcp/ too which can be used to open tcp connections. Actually, bash still supports the emulation in case it can't find /dev/fd. Obviously I missed some developments of /proc/self, udev and the kernel so I didn't know that this is now system default. – Daniel Alder Oct 16 '15 at 08:30
  • @DanielAlder - it was `ksh`, actually, not `bash` that started all that - especially the `/dev/tcp` socket biz. in fact `|` isnt even a pipe at all in `ksh` - its a socket, and `/dev/fd/[anything]` fails for it. guess they came full circle. – mikeserv Oct 16 '15 at 08:41
2

This command worked for me. 

ls /bin /usr/bin | sort | uniq | tee /dev/pts/0 | grep out

You could check what is your terminal using the command tty and replace the tee to redirect the output to that terminal.

References

https://stackoverflow.com/a/18025293/1742825

Community
  • 1
Ramesh
  • 38,687
  • 43
  • 140
  • 215
  • That assumes the command runs on the terminal /dev/pts/0. – Joe Sewell Jan 12 '15 at 17:48
  • @JoeSewell, you could see which is the running terminal using `tty`. I have updated with that information. – Ramesh Jan 12 '15 at 17:49
  • 1
    Nice update, but why not use `/dev/tty` instead? That assumes, of course, that the command even has access to the terminal. If it's run from, for example, inside `gvim`, there may be no terminal involved at all. Doing this within `gmake` could become even more complex with multiple jobs, since not all processes can get to "the terminal." – Joe Sewell Jan 12 '15 at 17:53
  • Or use the output of the `tty` command? `tee \`tty\`` etc. – user7543 Feb 10 '21 at 14:08
2
mkfifo myfifo
cat myfifo& ls /bin /usr/bin | sort | uniq | tee myfifo | grep out

mkfifo creates a FIFO (first in, first out) special file, a.k.a. a named pipe.  Start an asynchronous cat to read from the fifo, and then run your pipeline, teeing the intermediate result to the fifo.

This will produce a [1]+ Done cat myfifo message at the end.  You can suppress that with this magic trick:

(cat myfifo&); ls /bin /usr/bin | sort | uniq | tee myfifo | grep out

For a long term, robust solution, you might want to create a permanent fifo (e.g., $HOME/myfifo) rather than creating a new one every time.  But that will fail if you may be running multiple instances of this simultaneously.  Alternatively,

  • Generate a unique name (e.g., with mktemp).
  • Create the fifo in a directory that's guaranteed to be writable (e.g., /tmp).
  • Remove the fifo at the end of the command.
Scott - Слава Україні
  • 10,332
  • 10
  • 39
  • 68
2

How to do it (example):

exec 3>&1; ( ls |( tee >&3 ) >/dev/null ); exec 3>&-

Which will show the ls result and send it to nirvanah.

To grok the key part, 3>&1, you may read I/O Redirection and esp. this example.

In short: >somefileis short for 1>somefile, which in turn means Assign the file handle of somefile to file descriptor 1 (and drop the former value of that descriptor, for the scope of this process).

So, 3>&1 means: Assign the file descriptor 1 (which may but need not be tty) to the (until now unused) file descriptor 3. We're effectively using &3 as a temporary variable.

geek-merlin
  • 121
  • 4
0

In the case this is being run from a terminal you could

  • start a terminal multiplexer such as tmux or screen
  • split your terminal - in tmux Ctrl B"
  • in the first window touch abc.txt then tail -f abc.txt
  • switch to the second window and run your command. You'll see the file created by tee update.
muru
  • 69,900
  • 13
  • 192
  • 292
Nick B.
  • 31
  • 3
-2

http://tldp.org/LDP/abs/html/io-redirection.html

cat stuff | 3>&1 tee 3 | xsel -i

Edit: unfortunately, the above ↑ is incorrect. An earlier answer of

cat stuff  | tee >(cat) | xsel -i

is good though.

Gis Ochre
  • 1
  • 1
  • 1
    (1) It seems to me that this does not do what the question asks for.  (2) TLDP is not a good source of information; please do not use it.  (3) I don’t see much correlation between the page you linked to and your answer.  Why are you giving TLDP credit (/ blame)?  (4) The question is about a pipeline involving `ls`, `sort`, `uniq` and `grep`.  It’s good manners to answer the question that was asked — and yet your answer shows a pipeline involving `cat` (a UUOC, BTW) and `xsel`. … (Cont’d) – Scott - Слава Україні Mar 16 '20 at 03:15
  • 1
    (Cont’d) … (5) Please don’t clutter our site with answers that you know to be wrong, unless you have a good reason to do so.  Instead of *adding* a second answer (and, inexplicably, calling it an “earlier answer”), just delete the wrong answer and replace it with the one that’s less bad. – Scott - Слава Україні Mar 16 '20 at 03:15