Unix: add tee command notes

src/content/unix/tee.md

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+---
+title: 'Tee Command'
+description: 'The powerful shell-stream controller'
+updateDate: 'December 29 2023'
+---
+
+# Unix Tee
+
+The `tee` command is available on all Unix systems. It takes in any number of
+files as arguments and copies the `stdin` to all these files and the `stdout`.
+This can be a very helpful command when using Unix pipes.
+
+## Multi-file Output
+
+It can be helpful to duplicate the output of a command across multiple files. By
+default, shells only provide one stdout stream, so it'll take several commands
+or a for-loop to write to several files.
+
+`tee` provides an alternative. The command below copies the output across 3
+files:
+
+```bash
+echo "something" | tee file1 | tee file2 > file3
+```
+
+`tee` can also take in multiple file name arguments, where the output is copied
+to all of them. The following command is equivalent to the above:
+
+```bash
+echo "something" | tee file1 file2 file3 > /dev/null
+```
+
+This can further be further exploited with bash for loops. The following will
+copy the message across 100 different files:
+
+```bash
+echo "something" | tee $(for x in {1..100}; do echo file$x; done) >/dev/null
+```
+
+You can similarly append to files using the `-a` option. This is analogous to
+the `bash` syntax `>>`.
+
+The following command will append to files 1 and 3, but overwrite file 2:
+
+```bash
+echo "something" | tee -a file1 | tee file2 >> file3
+```
+
+### Writing Stderr
+
+Consider this C program, which will write "something" to the `stderr`:
+
+```c
+#include <stdio.h>
+
+int main(void) {
+    fprintf(stderr, "something\n");
+}
+```
+
+`tee` only reads from the `stdin`. To use this output with `tee`, we'll need to
+first redirect the `stderr` to `stdout`. Suppose the C program above is compiled
+into an executable called `a.out`:
+
+```bash
+./a.out 2>&1 | tee file1 file2
+```
+
+## Elevating Permissions
+
+It's common on a single-user system, like your personal computer, to need to
+write to files that are write-only for the root user. For example consider the
+following command:
+
+```bash
+sudo echo "something" > root_file
+```
+
+This won't work, as the `sudo` only applies to `echo "something"` not `>
+root_file`. One alternative would be to run the entire shell as root:
+
+```bash
+sudo bash -c 'echo "something" > root_file'
+```
+
+But now you've given root access to the entire command! We only want to give
+root access to the "write" operation. `tee` can do just that!
+
+Both of the following will use the root user to write to the file `root_file`,
+but the `echo` command itself will still be run by your user.
+
+```bash
+echo "something" | sudo tee root_file
+echo "something" | sudo tee root_file &>/dev/null
+```
+
+`echo` isn't a very serious application of `tee`, though there are many cases
+where we only want to elevate permissions for the write and not the whole
+command.
+
+### Elevating Permissions for Vim
+
+If you open a file that's inaccessible for writing by your user with Vim, `tee`
+can be used to switch to the root user just for writing!
+
+The following vim command writes your current buffer to the file as the root
+user:
+
+```vim
+:w ! sudo tee %
+```
+
+To deconstruct that command a bit, we start with `:w` which is the write
+command. We use `!` to invoke a shell. `:w` will provide the current buffer as
+through the stdin to the shell. We run the command `sudo tee %`, where Vim
+expands `%` to the current file name. `tee` will overwrite the file name expand
+from `%` with the stdin provided by the `:w` command.