Android - Is it possible to write to a device's clipboard using ADB?

Yes, you actually can do this. It's kind of kludgy looking when you inspect the clipboard, but it works just fine.

First off, you can inspect the current clipboard contents with service call clipboard 1 from an adb shell (or, without shelling in first, adb shell service call clipboard 1). It may start out initially blank after a reboot, for example:

# service call clipboard 1
Result: Parcel(
  0x00000000: 00000000 00000001 00000000 00000000 '................'
  0x00000010: 00000000 00000000                   '........        ')
#

You can put text into the clipboard using service call clipboard 2, which basically takes 3 parameters - two ints and the string you want to put on the clipboard:

# service call clipboard 2 i32 1 i32 0 s16 "Hi there"
Result: Parcel(00000000    '....')

To be honest, I'm not sure what the first two parameters are. One answer on Stack Overflow has suggested the first int is "number of items in the parcel" (one in this case) and that the second is the length of the string. However, I've used 0 for the second parameter and it works fine, and I can't find any documentation that matches up with this particular function...so take that for what it's worth.

In any case, it's basically creating a Parcel object with 3 fields, then passing it into the clipboard. The clipboard then unpacks the Parcel and sets the string value passed in as the clipboard's contents. You can see this when you go to retrieve the value afterwards:

# service call clipboard 1
Result: Parcel(
  0x00000000: 00000000 00000001 00000000 00000008 '................'
  0x00000010: 00690048 00740020 00650068 00650072 'H.i. .t.h.e.r.e.'
  0x00000020: 00000000 00000000                   '........        ')
#

Similarly, if you long-press on a text entry field and hit "Paste" after doing this, you will get the text that was set via the call service clipboard 2 line above (and it will look completely normal).

(The above examples come from my HTC EVO, running CyanogenMod 7)

I used this methodology, and it worked fine in 4.x, but failed for me in lollipop. While looking for alternative solution, I found this: https://stackoverflow.com/questions/3391160/paste-text-on-android-emulator

it is not exactly as you wanted it, but for myself, most time I want to copy text to clipboard is because I want to paste it into password field.

as an additional bonus, here's my script (edited 2015-04-24 to allow spaces in text):

#!/bin/bash

if [[ "$1" != "" ]]
then
    TEXT=$1
else
    read -s -p "Enter text you want to insert: " TEXT
fi

ESCAPED_TEXT=`echo $TEXT | sed "s/\s/\%s/g"`
adb shell input text "$ESCAPED_TEXT"

Here's an improved version of the script by galets above. As with that script, it does not write to the clipboard, but sends input to the currently focused input field, which is often what you want anyway.

This script is improved to escape special characters to prevent confusing the Android shell with them. This should allow any string to be sent unchanged. To prevent the read command from interpreting a (trailing) backslash, I added -r there.

This was tested using the following list of special characters:

./adb-send-string ' \`~!@#$%^&*()-_=+[{]}|;:",<.>/?'\'

Here's the script:

#!/bin/bash
# Send text to android device using adb, emulating keyboard input.
# Based on a script from https://android.stackexchange.com/a/105881/223695
# extended to support special characters

if [[ "$1" != "" ]]
then
    TEXT="$1"
else
    read -s -r -p "Enter text you want to insert: " TEXT
fi

escape() {
    # Encapsulate the string in $'', which enables interpretation of
    # \xnn escapes in the string. This is not POSIX-sh, but an extension
    # documented by bash and also supported by the Android sh.
    echo -n "$'"

    # Process each character in $1 one by one
    for (( i=0 ; i<${#1}; i++ )); do
        # Extract the i'th character
        C="${1:$i:1}"
        if [ "$C" = ' ' ]; then
            # Encode spaces as %s, which is needed for Android's
            # "input text" command below 6.0 Marshmellow
            # See https://stackoverflow.com/documentation/android/9408/adb-shell/3958/send-text-key-pressed-and-touch-events-to-android-device-via-adb
            echo -n '%s'
        else
            # Encode everything else as \xnn, to prevent them from being
            # interpreted by the Android shell
            printf '\\x%02x' "'$C"
        fi
    done
    # Terminate the $''
    echo -n "'"
}

ESCAPED_TEXT=`escape "$TEXT"`
adb shell input text "$ESCAPED_TEXT"