How to test an Internet connection with bash?

Question

How can an internet connection be tested without pinging some website? I mean, what if there is a connection but the site is down? Is there a check for a connection with the

Answer 1

Similarly to @Jesse's answer, this option might be much faster than any solution using ping and perhaps slightly more efficient than @Jesse's answer.

find /sys/class/net/ -maxdepth 1 -mindepth 1 ! -name "*lo*" -exec sh -c 'cat "$0"/carrier 2>&1' {} \; | grep -q '1'

Explenation:

This command uses find with -exec to run command on all files not named *lo* in /sys/class/net/. These should be links to directories containing information about the available network interfaces on your machine.

The command being ran is an sh command that checks the contents of the file carrier in those directories. The value of $interface/carrier has 3 meanings - Quoting:

It seems there are three states:

• ./carrier not readable (for instance when the interface is disabled in Network Manager).
• ./carrier contain "1" (when the interface is activated and it is connected to a WiFi network)
• ./carrier contain "0" (when the interface is activated and it is not connected to a WiFi network)

The first option is not taken care of in @Jesse's answer. The sh command striped out is:

# Note: $0 == $interface
cat "$0"/carrier 2>&1

cat is being used to check the contents of carrier and redirect all output to standard output even when it fails because the file is not readable. If grep -q finds "1" among those files it means there is at least 1 interface connected. The exit code of grep -q will be the final exit code.

Usage

For example, using this command's exit status, you can use it start a gnubiff in your ~/.xprofile only if you have an internet connection.

online() {
    find /sys/class/net/ -maxdepth 1 -mindepth 1 ! -name "*lo*" -exec sh -c 'cat "$0"/carrier 2>&1 > /dev/null | grep -q "1" && exit 0' {} \;
}
online && gnubiff --systemtray --noconfigure &

Reference

• Help testing special file in /sys/class/net/
• find -exec a shell function?