bash - 正确计算 bash 变量的行数

Question

我需要计算给定变量的行数。例如，我需要找到有多少行VAR，在哪里VAR=$(git log -n 10 --format="%s")。

我试过 with echo "$VAR" | wc -l)，确实有效，但如果VAR为空，则为 prints 1，这是错误的。有解决方法吗？比使用if子句检查变量是否为空更好...（也许添加一行并从返回值中减去 1？）。

Answer 1

wc计算换行符的数量。您可以grep -c '^'用于计算行数。您可以通过以下方式看到差异：

#!/bin/bash

count_it() {
    echo "Variablie contains $2: ==>$1<=="
    echo -n 'grep:'; echo -n "$1" | grep -c '^'
    echo -n 'wc  :'; echo -n "$1" | wc -l
    echo
}

VAR=''
count_it "$VAR" "empty variable"

VAR='one line'
count_it "$VAR" "one line without \n at the end"

VAR='line1
'
count_it "$VAR" "one line with \n at the end"

VAR='line1
line2'
count_it "$VAR" "two lines without \n at the end"

VAR='line1
line2
'
count_it "$VAR" "two lines with \n at the end"

什么产生：

Variablie contains empty variable: ==><==
grep:0
wc  :       0

Variablie contains one line without \n at the end: ==>one line<==
grep:1
wc  :       0

Variablie contains one line with \n at the end: ==>line1
<==
grep:1
wc  :       1

Variablie contains two lines without \n at the end: ==>line1
line2<==
grep:2
wc  :       1

Variablie contains two lines with \n at the end: ==>line1
line2
<==
grep:2
wc  :       2

Answer 2

你总是可以有条件地写它：

[ -n "$VAR" ] && echo "$VAR" | wc -l || echo 0

这将检查是否$VAR有内容并采取相应措施。

Answer 3

对于纯 bash 解决方案：不要将git命令的输出放入变量（可以说是丑陋的），而是将其放入数组中，每个字段一行：

mapfile -t ary < <(git log -n 10 --format="%s")

然后你只需要计算数组中的字段数ary：

echo "${#ary[@]}"

如果您需要检索第 5 条提交消息，此设计还将使您的生活更简单：

echo "${ary[4]}"

Answer 4

尝试：

echo "$VAR" | grep ^ | wc -l