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On my Linux machine - I do the following:

Create a script named script.sh

 #!/bin/bash variable=`df -hP` echo < my_file.txt

Then I create an external file called my_file.txt

The disk usage is: $variable

When I execute the script.sh I get nothing printed.

Expected output is:

The disk usage is: Filesystem Size Used Avail Use% Mounted on udev 3.2G 0 3.2G 0% /dev tmpfs 651M 9.2M 642M 2% /run /dev/sda1 218G 9.5G 197G 5% / tmpfs 3.2G 62M 3.2G 2% /dev/shm tmpfs 5.0M 4.0K 5.0M 1% /run/lock tmpfs 3.2G 0 3.2G 0% /sys/fs/cgroup /dev/sda4 266G 1.1G 252G 1% /home /dev/sda2 923M 137M 723M 16% /boot tc 308G 158G 151G 52% /media/sf_tc tmpfs 651M 52K 651M 1% /run/user/1000
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You could export your variable and use envsubst:

#!/bin/bash export variable=$(df -hP) envsubst '$variable' < my_file.txt

If you use the form envsubst < my_file.txt, then all exported variables are substituted. It's safer to explicitly list all your variables you want to substitute, like

envsubst '$var1 $var2 $var3' < file

Related:

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  • Also, echo does not read from standard input. Commented Nov 27, 2019 at 15:31

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