Typing on an Android Device straight from computer?

As Manuel said, you can use adb shell input text, but you need to replace spaces with %s, as well as handle quotes. Here's a simple bash script to make that very easy:

#!/bin/bash

text=$(printf '%s%%s' ${@})  # concatenate and replace spaces with %s
text=${text%%%s}  # remove the trailing %s
text=${text//\'/\\\'}  # escape single quotes
text=${text//\"/\\\"}  # escape double quotes
# echo "[$text]"  # debugging

adb shell input text "$text"

Save as, say, atext and make executable. Then you can invoke the script without quotes...

atext Hello world!

...unless you need to send quotes, in which case you do need to put them between the other type of quotes (this is a shell limitation):

atext "I'd" like it '"shaken, not stirred"'

Here is a Bash-based solution that works for arbitrary/complex strings (e.g. random passwords). The other solutions presented here all failed for me in that regard:

#!/usr/bin/env bash
read -r -p "Enter string: " string    # prompt user to input string
string="${string// /%s}"              # replace spaces in string with '%s'
printf -v string "%q" "${string}"     # quote string in a way that allows it to be reused as shell input
adb shell input text "${string}"      # input string on device via adb

The following code may be used for repeated/continuous input:

#!/usr/bin/env bash
echo
echo "Hit CTRL+D or CTRL+C to exit."
echo
while true; do
    read -r -p "Enter string: " string || { echo "^D"; break; }
    string="${string// /%s}"
    printf -v string "%q" "${string}"
    echo "Sending string via adb..."
    adb shell input text "${string}"
done

This allowed me to easily set up all necessary accounts on an Android device from my notebook.

input does not support UTF-8 or other encodings, you will see something like this if you try it

$ adb shell input text ö
Killed

therefore if these are your intention you need something more robust.

The following script uses AndroidViewClient/culebra with CulebraTester2-public backend to avoid input limitations.

#! /usr/bin/env python3
# -*- coding: utf-8 -*-

from com.dtmilano.android.viewclient import ViewClient

vc = ViewClient(*ViewClient.connectToDeviceOrExit(), useuiautomatorhelper=True)

oid = vc.uiAutomatorHelper.ui_device.find_object(clazz='android.widget.EditText').oid
vc.uiAutomatorHelper.ui_object2.set_text(oid, '你好世界 ')

it finds an EditText and then enters some Chinese characters and a emoji.

You can achieve the same using bash and curl if entering text is the only you need.

#! /bin/bash
#
# simple-input-text
# - Finds an EditText
# - Enters text
#
# prerequisites:
# - adb finds and lists the device
# - ./culebratester2 start-server
# - jq installed (https://stedolan.github.io/jq/)
#

set -e
set +x

base_url=http://localhost:9987/v2/

do_curl() {
    curl -sf -H "accept: application/json" -H "Content-Type: application/json" "$@"
}

oid=$(do_curl -X POST "${base_url}/uiDevice/findObject" \
    -d "{'clazz': 'android.widget.EditText'}" | jq .oid)

do_curl -X POST "${base_url}/uiObject2/${oid}/setText" \
    -d "{'text': '你好世界 '}"

When using zsh, here is a more robust function to feed text to Android:

function adbtext() {
    while read -r line; do
        adb shell input text ${(q)${line// /%s}}
    done
}

While Zsh quoting may be slightly different from a regular POSIX shell, I didn't find anything it wouldn't work on. Dan's answer is missing for example > which also needs to be escaped.

I am using it with pass show ... | adbtext.