adb shell cmgs AT command in pdu mode on Redmi 7 not working

I run these commands:

cat /dev/smd7 & echo "AT+CSCS=\"GSM\";\r" > /dev/smd7
cat /dev/smd7 & echo "AT+CMGF=0;\r" > /dev/smd7
cat /dev/smd7 & echo "AT+CMGS=24;\r" > /dev/smd7

Then I enter the PDU message:

07...985C369F01

I get this output:

/system/bin/sh: 07...985C369F01: not found

Let's analyze the two commands you provide to your shell:

cat /dev/smd7 &
echo "some_data" > /dev/smd7

1. cat /dev/smd7 &: Listen to device /dev/smd7: from now on all data coming from that device will be redirected to the stdout (the shell you are writing in). Do it in background (&) in order to be able to send further commands

2. echo "some_data" > /dev/smd7: send some_data to device /dev/smd7

   1. The connection with the device is open
   2. The data is sent
   3. The connection is closed and the control comes back to the shell

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When you send echo "AT+CMGS=24;\r" > /dev/smd7

1. AT+CMGS=24;\r is sent to the device
2. The connection is closed
3. ... in the meanwhile the device sends back > prompt character telling you that it is waiting for the PDU message
4. ... but the shell has the control. The > prompt is just a print on the shell, so any sent data will be directly sent to the shell!
5. Since the sent data is not a shell command, the not found error is shown

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In conclusion, in order to send correctly the PDU message to the device, just keep sending it through echo command:

echo "07...985C369F01" > /dev/smd7

Note: Make sure to terminate the sequence with CTRL+Z character (ASCII 0x1A).