Shell: How to check available space and exit if not enough?

I want to check if I do have enough space before executing my script. I thought about

#!/bin/bash
set -e
cd
Home=$(pwd)
reqSpace=100000000
if [ (available Space) < reqSpace ]
    echo "not enough Space"
    exit 1
fi
do something

The space I want to check via

df | grep $Home

Any suggestions?

Answer

Bringing it all together (set -e and cd omitted for brevity; Bash-specific syntax – not POSIX-compliant):

#!/bin/bash

# Note: Numbers are 1024-byte units on Linux,
#       and 512-byte units on macOS.
reqSpace=100000000
availSpace=$(df "$HOME" | awk 'NR==2 { print $4 }')
if (( availSpace < reqSpace )); then
  echo "not enough Space" >&2
  exit 1
fi

df "$HOME" outputs stats about disk usage for the $HOME directory’s mount point, and awk 'NR==2 { print $4 }' extracts the (POSIX-mandated) 4th field from the 2nd output line (the first line is a header line), which contains the available space in 1024-byte units on Linux^[1], and 512-byte units on macOS; enclosing the command in $(...) allows capturing its output and assigning that to variable availSpace.

Note the use of (( ... )), an arithmetic conditional, to perform the comparison, which allows:

• referencing variables without their $ prefix, and

• use of the customary arithmetic comparison operators, such as < (whereas you’d have to use -le if you used [ ... ] or [[ ... ]])

Finally, note the >&2, which redirects the echo output to stderr, which is where error messages should be sent.

---

^{[1] If you define the POSIXLY_CORRECT environment variable (with any value), the GNU implementation of df too uses the POSIX-mandated 512-byte units; e.g., POSIXLY_CORRECT= df "$HOME".}