From f0657cb84a6d3dea52cc95978c004fc8646805b1 Mon Sep 17 00:00:00 2001
From: Aaron LI <aly@aaronly.me>
Date: Sun, 1 Apr 2018 01:23:31 +0800
Subject: Add waitpid-change.sh: Find matching PID and waiting for its change

---
 unix/waitpid-change.sh | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100755 unix/waitpid-change.sh

diff --git a/unix/waitpid-change.sh b/unix/waitpid-change.sh
new file mode 100755
index 0000000..92d4b7a
--- /dev/null
+++ b/unix/waitpid-change.sh
@@ -0,0 +1,69 @@
+#!/bin/sh
+#
+# Find the PID of a command and wait until it changes.
+#
+# Aaron LI
+# 2018-04-01
+#
+
+self="$0"
+
+getpid() {
+    pattern="$1"
+    output=$(ps auxww | grep "${pattern}" | grep -v grep | grep -v ${self})
+    lines=$(echo "${output}" | wc -l)
+
+    if [ ${lines} -gt 1 ]; then
+        echo "Error: multiple processes matching the pattern: '${pattern}'" >&2
+        echo
+        echo "${output}" >&2
+        exit 2
+    elif [ -z "${output}" ]; then
+        echo "Error: no processes matching the pattern: '${pattern}'" >&2
+        exit 3
+    fi
+
+    echo "${output}" | awk '{ print $2 }'
+}
+
+usage() {
+    echo "usage: ${self##*/} [-h] [-d delay] <pattern>"
+    exit 1
+}
+
+while getopts :d:h opt; do
+    case "${opt}" in
+        d)
+            delay=${OPTARG}
+            ;;
+        h)
+            usage
+            ;;
+        \?)
+            echo "Invalid option -${OPTARG}" >&2
+            usage
+            ;;
+        :)
+            echo "Option -${OPTARG} requires an argument" >&2
+            usage
+            ;;
+    esac
+done
+shift $((OPTIND - 1))
+[ $# -ne 1 ] && usage
+
+delay=${delay:-5}  # [second]
+pattern="$1"
+
+pid=$(getpid "${pattern}") || exit $?
+echo "Process ${pid} matches pattern: '${pattern}'"
+echo -n "waiting for pid change ..."
+sleep ${delay}
+while pid2=$(getpid "${pattern}"); do
+    [ ${pid2} -ne ${pid} ] && {
+        echo "changed!"; exit 0
+    }
+    echo -n "."
+    sleep ${delay}
+done
+exit $?
-- 
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