Waiting for a network interface before executing a command

Question

I have a couple of ideas on how I will achieve this. Not sure how I could script.

Method 1: (probably the best choice)

Create a loop that pings the server before receiving a response, then run the command if the response is not received within X time / runs, continue the script.

Method 2:

Check if the network interface has a valid IP address, then continue the script

How can I add this functionality to the script. Will awk or grep be used in this situation? Thank you in advance for ANY input.

Answer 1

This command must wait until it can contact Google or it tries 50 times:

for i in {1..50}; do ping -c1 www.google.com &> /dev/null && break; done

for i in {1..50} 50 , break. ping -c1 www.google.com 1 ping- google, &> /dev/null , . && break , , , ping .

Answer 2

, DHCP.

, ( eth0), , IP- (, , ) DHCP, , .

, , / , (.. 8.8.8.8 www.google.com ). IP- , , IP .

while ! ip route | grep -oP 'default via .+ dev eth0'; do
  echo "interface not up, will try again in 1 second";
  sleep 1;
done

Answer 3

- Geofferey

, ...

await-ipv4-address.sh

#!/usr/bin/env bash


## Lists IP addresses for given interface name
## @returns {number|list}
## @param {string} _interface      - Name of interface to monitor for IP address(es)
## @param {number} _sleep_intervel - Number of seconds to sleep between checks
## @param {number} _loop_limit     - Max number of loops before function returns error code
## @author S0AndS0
## @copyright AGPL-3.0
## @exampe As an array
##     _addresses_list=($(await_ipv4_address 'eth0'))
##     printf 'Listening address: %s\n' "${_addresses_list[@]}"
##     #> Listening address: 192.168.0.2
##     #> Listening address: 192.168.0.4
## @example As a string
##     _addresses_string="$(await_ipv4_address 'eth0' '1' '3')"
##     printf 'Listening address(es): %s\n' "${_addresses_string}"
##     #> Listening address(es): 192.168.0.2 192.168.0.4
await_ipv4_address(){
    local _interface="${1:?# Parameter_Error: ${FUNCNAME[0]} not provided an interface}"
    local _sleep_interval="${2:-1}"
    local _loop_limit="${3:-10}"
    if [ "${_sleep_interval}" -lt '0' ] || [ "${_loop_limit}" -le '0' ]; then
        printf 'Parameter_Error: %s requires positive numbers for second and third parameters\n' "${FUNCNAME[0]}" >&2
        return 1
    fi

    local _loop_count='0'
    local -a _ipv4_addresses
    while true; do
        for _address in $({ ip addr show ${_interface} | awk '/inet /{print $2}'; } 2>/dev/null); do
            _ipv4_addresses+=("${_address}")
        done

        if [ "${#_ipv4_addresses[@]}" -gt '0' ]; then
            printf '%s\n' "${_ipv4_addresses[*]}"
            break
        elif [ "${_loop_count}" -gt "${_loop_limit}" ]; then
            break
        fi

        let _loop_count+=1
        sleep "${_sleep_interval}"
    done

    [[ "${#_ipv4_addresses[@]}" -gt '0' ]]; return "${?}"
}

GitHub bash-utilities/await-ipv4-address, ReadMe Git .

...

source "await-ipv4-address.sh"

... ...

#!/usr/bin/env bash


## Enable sourcing via absolute path
__SOURCE__="${BASH_SOURCE[0]}"
while [[ -h "${__SOURCE__}" ]]; do
    __SOURCE__="$(find "${__SOURCE__}" -type l -ls | sed -n 's@^.* -> \(.*\)@\1@p')"
done
__DIR__="$(cd -P "$(dirname "${__SOURCE__}")" && pwd)"


source "${__DIR__}/modules/await-ipv4-address/await-ipv4-address.sh"

---

awk grep ? - Geofferey

; , echo , grep ... awk , , printf .

awk IPv4 IPv6 ....

    # ... trimmed for brevity
        for _address in $({ ip addr show ${_interface} | awk '/inet/{print $2}'; } 2>/dev/null); do
            # ... things that get done with an address
        done

... , .

---

-,

1. IP eth0

_ip_addresses_list=($(await_ipv4_address 'eth0'))

2. IP tun0

_ip_addresses_list=($(await_ipv4_address 'tun0' '1' '29'))

3. IP wlan0 3

_ip_addresses_list=($(await_ipv4_address 'wlan0' '3' '19'))

---

, await_ipv4_address , , ...

_ip_addresses_list=($(await_ipv4_address 'wlan0' '3' '19' || true))

... , , - .

---

IP-...

for _ip_address in "${_ip_addresses_list[@]}"; do
    printf 'IP -> %s\n' "${_ip_address}"
done

---

, , ? - user207421

, , , , - Geofferey

, IP- /AP ; , IP- ... , , .

, , dig curl, ping DNS , .

---

, - , , ? - Geofferey

... ...

await_ipv4_address(){
    local _interface="${1:?# Parameter_Error: ${FUNCNAME[0]} not provided an interface}"
    local _sleep_interval="${2:-1}"
    local _loop_limit="${3:-10}"
    # ...
}

• local , help local help declare,

• "${something:?Error message}" Error message something

• "${another_thing:-1}" "${another_thing:-1}" 1 another_thing

, man --pager='less -p ^"PARAMETERS"' bash Special Parameters man --pager='less -p "Parameter Expansion"' bash , .

---

if [ "${_sleep_interval}" -lt '0' ] || [ "${_loop_limit}" -le '0' ]; then
    printf 'Parameter_Error: %s requires positive numbers for second and third parameters\n' "${FUNCNAME[0]}" >&2
    return 1
fi

• , _sleep_interval _loop_count - -lt (-lt) (-le)

• , if true, || , false , as && &&, true

man operator

• printf 'something\n' >&2 something ; ,

• ,

---

while true; do
    # ... stuff
done

• , , .

---

    for _address in $({ ip addr show ${_interface} | awk '/inet /{print $2}'; } 2>/dev/null); do
        _ipv4_addresses+=("${_address}")
    done

• $({ command | parser; } 2>/dev/null) - ,

  • $(something) something
  • { one_thing | another_thing; } { one_thing | another_thing; }

  , man --pager='less -p "Compound Commands"' bash

  • 2>/dev/null ,

• _preexisting_list+=("element") element _preexisting_list +=

---

        if [ "${#_ipv4_addresses[@]}" -gt '0' ]; then
            printf '%s\n' "${_ipv4_addresses[*]}"
            break
        elif [ "${_loop_count}" -gt "${_loop_limit}" ]; then
            break
        fi

• if part , _ipv4_addresses 0 #, ${#_list_name[@]}

• elif part ,

break while , .

---

        let _loop_count+=1
        sleep "${_sleep_interval}"

• let _counter+=1 1 _counter _counter

• sleep ,

---

    [[ "${#_ipv4_addresses[@]}" -gt '0' ]]; return "${?}"

• A semicolon with check brackets ( [[ is_it_true ]]) instead of ||or &&calls returnto return the status if the number of IP addresses found is greater, 0regardless of the reliability of the test

---

If after all this there is something dubious, do not hesitate to leave comments to improve the answer.