为什么在 bash 中使用下划线作为参数来读取？

Question

我最近发现了一篇关于如何从 GitHub 中的组织克隆所有存储库的post。最佳答案如下：

gh repo list myorgname --limit 4000 | while read -r repo _; do
  gh repo clone "$repo" "$repo"
done

我理解下划线之前和之后的所有内容。是什么意思 _;在上面的命令中？

谢谢！

Answer 1

变量

_

被用作垃圾

来自

man bash

（或

man -P'less +"/^ *Shell Variables"' bash

）：

Shell Variables
    The following variables are set by the shell:

    _      At  shell  startup, set to the pathname used to invoke the shell
           or shell script being executed as passed in the  environment  or
           argument  list.   Subsequently,  expands to the last argument to
           the previous simple command executed in  the  foreground,  after
           expansion.   Also  set  to the full pathname used to invoke each
           command executed and placed in the environment exported to  that
           command.   When  checking mail, this parameter holds the name of
           the mail file currently being checked.

此变量将保存上一行的最后一个参数。

小实用案例

我经常用这个：

mkdir -p /some/path/somewhere
cd $_

无法以其他方式使用

如果你尝试将它们用作变量，你可能会失望

_='Hello world!'
echo "I said: '$_'"
I said: ''

用作垃圾桶

但在您的示例中，它们被用作垃圾：

因此，

while read -r repo _;...

将通过将第一个（空格分隔）单词存储到

$repo

并将其余行存储到（不可用）

$_

来解析每一行输入。

示例，显示密码文件中的 GECOS 字段：

while IFS=: read -r user _ uid _ gecos ;do
    (( UID == uid )) && echo $user ${gecos%%,*}
done < /etc/passwd

关于您使用的样本

gh repo list

我更喜欢用另一种方式写这个，这将允许使用overall变量来创建统计数据和其他可重用的东西：

declare -i cloned=0 total=0
while read -r repo _; do
    total+=1
    gh repo clone "$repo" "$repo" &&
        cloned+=1
done < <(
    gh repo list myorgname --limit 4000
)
printf 'There was %d repo successfully cloned, over %d total\n' \
    $cloned $total

您可能会在如何将变量设置为命令的输出

找到更多示例

但是作为TIMTOWTDI

在处理元素之前，您可以将 gh repo list

读取

放入数组中：

mapfile -t rList < <(gh repo list myorgname --limit 4000)
declare -i cloned=0 total=${#rList[@]}
printf 'There are %d objects to clone.\n' $total
for repo in "${rList[@]/%$'\11'*}"; do
    gh repo clone "$repo" "$repo" &&
        cloned+=1
done
printf 'There was %d repo successfully cloned, over %d total\n' \
    $cloned $total

例如，您可以计划一次同时下载最多 5 个进程：

并行

gh

克隆完整的演示脚本。

（复制自通过同时/并发文件传输加速 rsync？。）

将其另存为

gh_parClone.sh

并赋予他执行权：

#!/bin/bash

maxProc=5
[[ $1 == -v ]] && verbose=1 && shift || verbose=0
orgName="$1"

declare -ai start elap results order
declare -a succeeded failed messages
wait4oneTask() {
    printf 'Running task: %d\r' ${#running[@]}
    wait -np epid
    results[epid]=$?
    local _i _line
    elap[epid]=" ${EPOCHREALTIME/.} - ${start[epid]} "
    if ((results[epid])); then
        failed[epid]="${repos[epid]}"
    else
        succeeded[epid]="${repos[epid]}"
    fi
    
    while read -ru "${ghFds[epid]}" -t .02 _line; do
        messages[epid]+="$_line"$'\n'
    done
    exec {ghFds[epid]}<&-
    unset "ghFds[epid]"
    unset "running[$epid]"
    while [[ -v elap[${order[0]}] ]]; do
        _i=${order[0]}
        printf " - %(%a %d %T)T.%06.0f %-36s %4d %12d\n" "${start[_i]:0:-6}" \
               "${start[_i]: -6}" "${repos[_i]}" "${results[_i]}" "${elap[_i]}"
        order=("${order[@]:1}")
    done
}

mapfile -t rList < <(LANG=C gh repo list "$orgName")
printf 'GH: %d object in \47%s\47. Start clone with max %d instances.\n' \
       ${#rList[@]} "$orgName" $maxProc

printf "   %-22s %-36s %4s %12s\n" Started Repo Rslt 'microseconds'
for repo in "${rList[@]/%$'\11'*}"; do
    exec {ghFd}<> <(:)
    gh repo clone "$repo"{,} >&$ghFd 2>&1 &
    lpid=$!
    ghFds[lpid]=$ghFd
    repos[lpid]="${repo#$orgName/}" 
    start[lpid]=${EPOCHREALTIME/.}
    running[lpid]=''
    order+=("$lpid")
    ((${#running[@]}>=maxProc)) && wait4oneTask
done
while ((${#running[@]})); do
    wait4oneTask
done
printf 'Succeeded: %d.\n%s\n' ${#succeeded[@]} "${succeeded[*]}"
if ((verbose)); then
    for pid in "${!succeeded[@]}"; do
    printf ' - %s\n' "${succeeded[pid]}"
    printf '       %s\n' "${messages[pid]//$'\n'/$'\n'       }"
    done
fi
printf 'Failed: %d.\n' ${#failed[@]}
if ((${#failed[@]})); then
    for pid in "${!failed[@]}"; do
        printf ' - %s\n' "${failed[pid]}"
        printf '       %s\n' "${messages[pid]//$'\n'/$'\n'       }"
    done
fi

示例运行：

$ mkdir -p demos/mixxx/foo
$ ./gh_parClone.sh demos
GH: 9 object in 'demos'. Start clone with max 5 instances.
   Started                Repo                                 Rslt microseconds
 - Fri 12 10:41:59.476735 php-logical-filter                      0      2100719
 - Fri 12 10:41:59.477372 mixxx                                   1       403229
 - Fri 12 10:41:59.477718 Cache                                   0      2062995
 - Fri 12 10:41:59.478078 developer_skins                         0      3713021
 - Fri 12 10:41:59.478470 Qt-Inspector                            0      1949781
 - Fri 12 10:41:59.901984 pytaglib                                0      2325354
 - Fri 12 10:42:01.449372 gmap3                                   0      2269148
 - Fri 12 10:42:01.561883 Motif                                   0      4105144
 - Fri 12 10:42:01.600549 offlineimap                             0      4493532
Succeeded: 8.
php-logical-filter Cache developer_skins Qt-Inspector pytaglib gmap3 Motif offlineimap
Failed: 1.
 - mixxx
       fatal: destination path 'demos/mixxx' already exists and is not an empty directory.
       failed to run git: exit status 128

如果使用

-v

标志运行，输出会更长：

$ ./gh_parClone.sh -v demos