To get free space on / in a machine-readable single number (bytes):

df --output=avail -B1 /

This prints a header line, so to get only the value:

df --output=avail -B1 / | tail -n 1

Options:

• --output=avail — only the available-space column
• -B1 — block size of 1 byte, so the number is exact bytes (no human units like K/M)

Common variations:

# Kilobytes (1024-byte blocks, classic "df -k" style)
df -k --output=avail / | tail -n 1

# Human-readable only for eyeballing (not machine-safe)
df -h /

# Portable alternative (POSIX, works even without GNU df)
stat -f -c '%s * %f' / | bc
# %s = fundamental block size, %f = free blocks

You can also get used/total alongside if you need them:

df -B1 --output=size,used,avail /

The df --output=avail -B1 / | tail -n 1 form is the standard idiom: stable, unversioned output, safe to parse in scripts.