I'm learning bash redirections and wonder if there a way to emulate tee command with cat using redirections. Is that possible? Or may be there are some other ways to do this.
Before asking about tee I just sought for a possibility to split one streem into two identical ones using bash expression. But it seems to be impossible.
Provided that the Here is a bash function with the same functionality as tee and tee -a, provided the file does not contain ¤¤¤¤¤¤¤¤¤¤! (you can adjust this string to your liking, to make it highly unlikely that a file will ever contain it):
tee(){
[ "$1" = '-a' ] && shift || rm -f "$1"
sed 's/\x0/¤¤¤¤¤¤¤¤¤¤!/g' - | while read line; do
echo "$line" | sed 's/¤¤¤¤¤¤¤¤¤¤!/\x0/g' >> "$1"
done
}
Tested with bash and zsh.
Example use:
echo asdf | tee -a file
</dev/stdin is also wrong, especially on Linux.
Commented
Dec 16, 2021 at 9:40
-a, the tee command truncates and overwrites the destination rather than removing it. (You lose permissions, ownerships, etc. if you rm.) I'd change rm -f "$1" to a simple >"$1"
Commented
Dec 16, 2021 at 13:26
tee file1 file2 file3... nor does it copy its stdin to stdout.
Commented
Dec 16, 2021 at 13:27
printf over echo when dealing with arbitrary data like we are here. There's also good advice in there about the while read construct.
You can not do it in bash with redirections, but you can do something like this:
$ cat file1
europe|EU
australia|AU
china|CN
$ cat file1 | { IFS=$'\n' read -r -d '' -a a; printf '%s\n' "${a[@]}"; printf '%s\n' "${a[@]}" >teetest; }
europe|EU
australia|AU
china|CN
$ cat teetest
europe|EU
australia|AU
china|CN
Alternative:
$ { IFS=$'\n' read -r -d '' -a a; printf '%s\n' "${a[@]}"; printf '%s\n' "${a[@]}" >teetest; } <file1
Or you can built a function:
$ function teetest { v="$(</dev/stdin)";printf '%s\n' "$v";printf '%s\n' "$v">teetest; }
$ cat file1 |teetest
europe|EU
australia|AU
china|CN
$ cat teetest
europe|EU
australia|AU
china|CN
With zsh, yes, not with bash.
With zsh:
cat file.in > file1.out > file2.out
or even:
< file.in > file1.out > file2.out
or with brace expansion to generate the output files:
< file.in > file{1..2}.out
(running $NULLCMD: cat by default)
Note that cat has only one stdout which it writes the content of file.in to. Above, cat's stdout is a pipe. And zsh reads that output from the other end to write it to both file1.out and file2.out.
In other words, there's no magic there, zsh is just implementing tee internally.
cat; you would I believe have to write your own tool that reads from standard input and writes simultaneously to standard output and to a specified list of files. But then you will end up with a roll-your-own implementation oftee.MULTIOSoption set, but I don't think you can hack it into bash.