如何在 shell 脚本中“回显”另一个函数中声明/初始化的变量？

Question

在 Ubuntu 14.04.1 中运行此命令：

./script.sh MyVM take

为什么echo函数中的行可以list_snapshot工作（它打印这 3 个变量的值）但是echo中的行take_snapshot不能打印变量的值$output而$old_snapshot_name只能打印的值$machine_name？

请注意，当我运行时脚本会起作用./script.sh MyVM take，它会删除旧快照并创建新快照。

#! /bin/sh

if [ ! "$1" ]; then
    echo "Usage: $0 <vmname> list|take"
    exit 1
else
    machine_name=$1
fi

list_snapshot () {
    output=$(VBoxManage snapshot "$machine_name" list)
    count=$(echo "$output" | grep -c 'UUID')

    echo "Machine '$machine_name' has '$count' snapshots: \n '$output'"
    return "$count"
}


take_snapshot () {
    echo "$output"
    while (list_snapshot; [ $? -gt 3 ])  
    do
        old_snapshot_name=$(echo "$output" | grep -o 'UUID: (.{36})' | head -1)
        echo "Deleting old snapshot '$old_snapshot_name' for machine '$machine_name'..."

        VBoxManage snapshot "$machine_name" delete "$old_snapshot_name"

    done

    current_time=$(date "+%Y.%m.%d-%H.%M.%S")
    snapshot_name=$machine_name.$current_time

    echo "Taking new snapshot '$snapshot_name' for machine '$machine_name'..."
    VBoxManage snapshot "$machine_name" take "$snapshot_name" --live 
}

case "$2" in
  list)
    list_snapshot
    ;;
  take)
    take_snapshot
    ;;
  *)
    echo "Usage: $0 <vmname> list|take"
    exit 1
    ;;
esac

输出不含-x：./snapshotVM.sh RancherOS-tools take

Machine 'RancherOS-tools' has '4' snapshots: 
 '   Name: RancherOS-tools.2015.11.03-11.25.12 (UUID: 7afef8ee-1915-4494-9634-add82e1a613f)
      Name: RancherOS-tools.2015.11.03-11.25.40 (UUID: 79a53543-377e-4a74-80ca-751883a7787b)
         Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
            Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051) *'
Deleting old snapshot '' for machine 'RancherOS-tools'...
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Machine 'RancherOS-tools' has '3' snapshots: 
 '   Name: RancherOS-tools.2015.11.03-11.25.40 (UUID: 79a53543-377e-4a74-80ca-751883a7787b)
      Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
         Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051) *'
Taking new snapshot 'RancherOS-tools.2015.11.03-11.35.20' for machine 'RancherOS-tools'...
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Snapshot taken. UUID: 5195881b-113b-448a-81e3-98a121c8fbfe

输出为-x：./snapshotVM.sh RancherOS-tools take

dy@dy:~$ ./snapshotVM.sh RancherOS-tools take
+ [ ! RancherOS-tools ]
+ machine_name=RancherOS-tools
+ take_snapshot
+ echo 

+ list_snapshot
+ VBoxManage snapshot RancherOS-tools list
+ output=   Name: RancherOS-tools.2015.11.03-11.25.40 (UUID: 79a53543-377e-4a74-80ca-751883a7787b)
      Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
         Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
            Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *
+ echo    Name: RancherOS-tools.2015.11.03-11.25.40 (UUID: 79a53543-377e-4a74-80ca-751883a7787b)
      Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
         Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
            Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *
+ grep -c UUID
+ count=4
+ echo Machine 'RancherOS-tools' has '4' snapshots: \n '   Name: RancherOS-tools.2015.11.03-11.25.40 (UUID: 79a53543-377e-4a74-80ca-751883a7787b)
      Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
         Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
            Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *'
Machine 'RancherOS-tools' has '4' snapshots: 
 '   Name: RancherOS-tools.2015.11.03-11.25.40 (UUID: 79a53543-377e-4a74-80ca-751883a7787b)
      Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
         Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
            Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *'
+ return 4
+ [ 4 -gt 3 ]
+ echo 
+ head -1
+ grep -o UUID: (.{36})
+ old_snapshot_name=
+ echo Deleting old snapshot '' for machine 'RancherOS-tools'...
Deleting old snapshot '' for machine 'RancherOS-tools'...
+ VBoxManage snapshot RancherOS-tools delete 
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
+ list_snapshot
+ VBoxManage snapshot RancherOS-tools list
+ output=   Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
      Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
         Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *
+ echo    Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
      Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
         Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *
+ grep -c UUID
+ count=3
+ echo Machine 'RancherOS-tools' has '3' snapshots: \n '   Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
      Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
         Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *'
Machine 'RancherOS-tools' has '3' snapshots: 
 '   Name: RancherOS-tools.2015.11.03-11.32.10 (UUID: 25a942fe-2d0a-4290-a98e-7d16ec4c60bd)
      Name: RancherOS-tools.2015.11.03-11.34.58 (UUID: 02af926b-396c-4ce6-bfb3-5e953de5a051)
         Name: RancherOS-tools.2015.11.03-11.35.20 (UUID: 5195881b-113b-448a-81e3-98a121c8fbfe) *'
+ return 3
+ [ 3 -gt 3 ]
+ date +%Y.%m.%d-%H.%M.%S
+ current_time=2015.11.03-11.37.46
+ snapshot_name=RancherOS-tools.2015.11.03-11.37.46
+ echo Taking new snapshot 'RancherOS-tools.2015.11.03-11.37.46' for machine 'RancherOS-tools'...
Taking new snapshot 'RancherOS-tools.2015.11.03-11.37.46' for machine 'RancherOS-tools'...
+ VBoxManage snapshot RancherOS-tools take RancherOS-tools.2015.11.03-11.37.46 --live
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Snapshot taken. UUID: 0e11c8d0-130c-4508-a2be-8692fc9316bd

更新：看起来我遇到了一些变量范围问题，重新设计我的代码后，删除 while 循环，现在它可以工作了。

#! /bin/sh

# usage: 
# $0 <vmname> list : list all snapshot
# $0 <vmname> take : delete old snapshot name startwith "Backup" and leave 3 "Backup" snapshot

#set -x

if [ ! "$1" ]; then
    echo "Usage: $0 <vmname> list|take"
    exit 1
else
    machine_name=$1
fi

list_snapshot () {
    output=$(VBoxManage snapshot "$machine_name" list)
    count=$(echo "$output" | grep -c 'UUID')

    echo "Machine $machine_name has $count snapshots: "
    echo "$output"
}


take_snapshot () {
    output=$(VBoxManage snapshot "$machine_name" list)
    count=$(echo "$output" | grep -c 'Backup')

    echo "Machine $machine_name has $count snapshots: "
    echo "$output"

    if [ "$count" -ge 3 ]; then
        old_snapshots_uuid=$(echo "$output" | grep "Backup" | grep -Eo 'UUID: .{36}' | cut -c 7-42 | head -$((count-2)))

        for u in $old_snapshots_uuid; do
          echo "Deleting old snapshot $u for machine $machine_name..."
          VBoxManage snapshot "$machine_name" delete "$u"
        done    
    fi

    current_time=$(date "+%Y.%m.%d-%H.%M.%S")
    snapshot_name="Backup".$machine_name.$current_time

    echo "Taking new snapshot $snapshot_name for machine $machine_name..."
    VBoxManage snapshot "$machine_name" take "$snapshot_name"     
}

case "$2" in
  list)
    list_snapshot
    ;;
  take)
    take_snapshot
    ;;
  *)
    echo "Usage: $0 <vmname> list|take"
    exit 1
    ;;
esac

Answer 1

你不能；在函数中声明/初始化的变量仅在该函数的范围内可见。

同样，在您的特定情况下，如果您运行./script.sh MyVM take，list_snapshot()甚至不会运行；output应该如何声明/初始化（以及应该如何old_snapshot_name声明/初始化，因为它的声明/初始化依赖于output尚未声明/初始化的？）

为了使其output对所有函数可见，而不管实际运行的是哪些函数，请将其声明/初始化移到 之外list_snapshot()，例如在take_snapshot()的定义之后：

#! /bin/sh

if [ ! "$1" ]; then
    echo "Usage: $0 <vmname> list|take"
    exit 1
else
    machine_name=$1
fi

list_snapshot () {
    count=$(echo "$output" | grep -c 'UUID')

    echo "Machine '$machine_name' has '$count' snapshots: \n '$output'"
    return "$count"
}


take_snapshot () {
    echo "$output"
    while (list_snapshot; [ $? -gt 3 ])  
    do
        old_snapshot_name=$(echo "$output" | grep -o 'UUID: (.{36})' | head -1)
        echo "Deleting old snapshot '$old_snapshot_name' for machine '$machine_name'..."

        VBoxManage snapshot "$machine_name" delete "$old_snapshot_name"

    done

    current_time=$(date "+%Y.%m.%d-%H.%M.%S")
    snapshot_name=$machine_name.$current_time

    echo "Taking new snapshot '$snapshot_name' for machine '$machine_name'..."
    VBoxManage snapshot "$machine_name" take "$snapshot_name" --live 
}

output=$(VBoxManage snapshot "$machine_name" list)

case "$2" in
  list)
    list_snapshot
    ;;
  take)
    take_snapshot
    ;;
  *)
    echo "Usage: $0 <vmname> list|take"
    exit 1
    ;;
esac