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How can I *only* get the number of bytes available on a disk in bash?

df does a great job for an overview. But what if I want to set a variable in a shell script to the number of bytes available on a disk?

Example:

$ df
Filesystem            1K-blocks     Used Available Use% Mounted on
/dev/sda             1111111111  2222222  33333333  10% /
tmpfs                  44444444      555  66666666   1% /dev/shm

But I just want to return 33333333 (bytes available on /), not the whole df output.

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Answer

Portably:

df -P /dev/sda1 | awk 'NR==2 {print $4}'

The -P option ensures that df will print output in the expected format, and will in particular not break the line after the device name even if it’s long. Passing the device name as an argument to df removes any danger from parsing, such as getting information for /dev/sda10 when you’re querying /dev/sda1. df -P just prints two lines, the header line (which you ignore) and the one data line where you print the desired column.

There is a risk that df will display a device name containing spaces, for example if the volume is mounted by name and the name contain spaces, or for an NFS volume whose remote mount point contains spaces. In this case, there’s no fully portable way to parse the output of df. If you’re confident that df will display the exact device name you pass to it (this isn’t always the case), you can strip it:

df -P -- "$device" | awk -vn=${#device} 'NR==2 {$0 = substr($0, n+1); print $3}'
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