0

I have this bash script that I use with i3wm to toggle between screen modes (external screen, internal screen, all or mirror). The first part of the script grabs the name of the screens. If I copy the following part and runit in a terminal:

#!/bin/bash

xrandr|grep " connected primary"|IFS=" " read INTERNAL_OUTPUT con
xrandr|awk '/ connected/ && !/ primary/'|IFS=" " read EXTERNAL_OUTPUT con
echo internal monitor is $INTERNAL_OUTPUT
echo external monitor is $EXTERNAL_OUTPUT

...

I get something like

$ ./script.sh
internal monitor is eDPI
external monitor is DPI-1

But when I run the actual script, the variables are empty and all I see is 

$ ./script.sh
internal monitor is 
external monitor is

What is the diference when setting the variables inside a script?

Improve this question
6
  • 1
    If the echo commands are in the same script as the read commands they should work, but they won't work outside the script unless you call it with the ./source command. Commented Sep 11, 2017 at 11:22
  • I don't understand what you mean. The whole thing in inside the script, including the echos, but it doesn't work. The variables are still empty. Commented Sep 11, 2017 at 11:37
  • In that case I don't see how it can work from the terminal: the only way I could get it to work is with xrandr|grep " connected primary"|{ IFS=" " read INTERNAL_OUTPUT con; echo internal monitor is $INTERNAL_OUTPUT; }, and similarly for the other monitor. This works in both the terminal and a script. Commented Sep 11, 2017 at 11:59
  • 2
    They don't work for me in a terminal either... Hypothesis: this won't work in bash, and your console isn't bash (so they work there)(what says ls -l /proc/$$/exe?) but your script requires bash (due to shebang) so uses bash rules and this makes the code fail. Commented Sep 11, 2017 at 12:01
  • If you want to set a variable in bash, you need something like INTERNAL_OUTPUT=$(xrandr|grep " connected "|grep -o '^[^ ]*'). This makes INTERNAL_OUTPUT available on subsequent lines in the script, though not on returning to the terminal unless called with .. If you want a script to set variables in the current shell without calling . you need to code it as a function. Commented Sep 11, 2017 at 12:18

1 Answer 1

Reset to default
2

You can't set variables in a pipeline and have the value available outside the pipe. When running a pipeline each part is run its own subshell, so the read command is in its own subshell. A subshell cannot modify the parent's environment, so the variables you set there are not available to the parent.

To get around this you could do as @AFH says and change the command so only what you want is printed then assign that to a variable in the parent shell.

Alternately, you could restructure the commands so you aren't piping into read. I'd prefer AFH's approach, but reordering could look like:

IFS=" " read INTERNAL_OUTPUT con < <(xrandr|grep " connected primary")

where we redirect the input of read to be from a process substitution that does the rest of the command, so read is still in the parent shell.

Improve this answer

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.