Uses the Gitlab API and jq to get the Gitlab User ID by supplying the username.

get-gitlab-user-id-by-name.sh

#!/bin/bash
# requires curl and jq
# assumes usage of Gitlab API v4
# hella hacky

url="https://git.example.net"
token="abcdefghijklmnop"
username="$1"

# output is always paginated - get total number of pages from http header
numpages=$(curl -sSl -D - "$url/api/v4/users?order_by=id&per_page=50" --header "PRIVATE-TOKEN: $token" | grep -i 'x-total-pages' | awk -F ' ' '{print $2}' | cut -c1 )

# iterate to all pages (each requires one API call)
for p in $(seq $numpages); do
    # create temporary file to store json in and get data
    now=$(date +%Y%m%d_%H%M%S%z)-$p
    curl -s "$url/api/v4/users?per_page=50&page=$p" --header "PRIVATE-TOKEN: $token" >> $now-gitlab-users.json
    # get number of entries on page using jq
    entries=$(cat $now-gitlab-users.json | jq '. | length')
    # off by one, dirty as all heck. frick you
    entries=$(($entries-1))

    # iterate through entries on page
    for e in $(seq 0 $entries); do
        if [[  $(cat $now-gitlab-users.json | jq .[$e].username | sed 's/\"//g') == "$username" ]]; then
            # echo ID from json if username matches
            cat $now-gitlab-users.json | jq .[$e].id
            rm $now-gitlab-users.json
            exit 0
        fi
        
    done
    rm $now-gitlab-users.json
done
exit 1