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export、unset的生效范围

一个子进程的环境变量决定于两个方面：

1. 父进程在启动子进程那一刻，父进程的环境变量的快照作为基础。
2. 父进程在启动子进程时注入的环境变量（e.g., NAN=5 ./env_test.sh）。

export、unset等操作仅对当前进程生效，也仅对命令执行后启动的子进程生效，而在之前启动的进程则无法生效。

脚本样例

#!/bin/bash

sub_proc() {
    while true; do
        sleep 1
        echo "[sub process $1] exec\"env | grep NAN\" get $(env | grep NAN)"
    done
}

graceful_exit() {
    echo "start graceful exit"

    if [ $sub_proc_a_pid ]; then
        echo "killing sub_proc_a_pid=$sub_proc_a_pid"
        kill -SIGTERM $sub_proc_a_pid
    fi
    if [ $sub_proc_b_pid ]; then
        echo "killing sub_proc_b_pid=$sub_proc_b_pid"
        kill -SIGTERM $sub_proc_b_pid
    fi

    echo "exiting..."
    exit 0
}

trap graceful_exit SIGTERM
trap graceful_exit SIGINT

sub_proc a &
sub_proc_a_pid=$!

count=0

while (($count < 10)); do
    count=$(expr $count + 1)
    echo "[main process] is waiting" "$count"

    if [ $count == 5 ]; then
        echo "[main process] exec\"export NAN=1\""
        export NAN=1
        echo "[main process] exec\"env | grep NAN\""
        env | grep NAN

        sub_proc b &
        sub_proc_b_pid=$!
    fi

    sleep 1
done

graceful_exit

执行结果

root@DESKTOP-6MVDBD1:~/samples-of-shell-scripts# NAN=5 ./env_test.sh
[main process] is waiting 1
[main process] is waiting 2
[sub process a] exec"env | grep NAN" get NAN=5
[main process] is waiting 3
[sub process a] exec"env | grep NAN" get NAN=5
[main process] is waiting 4
[sub process a] exec"env | grep NAN" get NAN=5
[main process] is waiting 5
[main process] exec"export NAN=1"
[main process] exec"env | grep NAN"
NAN=1
[sub process a] exec"env | grep NAN" get NAN=5
[main process] is waiting 6
[sub process b] exec"env | grep NAN" get NAN=1
[sub process a] exec"env | grep NAN" get NAN=5
[main process] is waiting 7
[sub process b] exec"env | grep NAN" get NAN=1
[sub process a] exec"env | grep NAN" get NAN=5
[main process] is waiting 8
[sub process a] exec"env | grep NAN" get NAN=5
[sub process b] exec"env | grep NAN" get NAN=1
[main process] is waiting 9
[sub process a] exec"env | grep NAN" get NAN=5
[sub process b] exec"env | grep NAN" get NAN=1
[main process] is waiting 10
[sub process b] exec"env | grep NAN" get NAN=1
[sub process a] exec"env | grep NAN" get NAN=5
start graceful exit
killing sub_proc_a_pid=43829
killing sub_proc_b_pid=43866
exiting...

参考

• https://github.com/popomen/samples-of-shell-scripts/blob/main/env_test.sh

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