linux - Bash循环中的计数器增量不起作用

Question

我有以下简单的脚本，我正在运行一个循环并想要维护一个COUNTER. 我无法弄清楚为什么计数器没有更新。是因为创建了子shell吗？我怎样才能解决这个问题？

#!/bin/bash

WFY_PATH=/var/log/nginx
WFY_FILE=error.log
COUNTER=0
grep 'GET /log_' $WFY_PATH/$WFY_FILE | grep 'upstream timed out' | awk -F ', ' '{print $2,$4,$0}' | awk '{print "http://domain.com"$5"&ip="$2"&date="$7"&time="$8"&end=1"}' | awk -F '&end=1' '{print $1"&end=1"}' |
(
while read WFY_URL
do
    echo $WFY_URL #Some more action
    COUNTER=$((COUNTER+1))
done
)

echo $COUNTER # output = 0

score 170 · Accepted Answer

首先，您没有增加计数器。更改COUNTER=$((COUNTER))为COUNTER=$((COUNTER + 1))或COUNTER=$[COUNTER + 1]将增加它。

其次，正如您推测的那样，将子shell 变量反向传播给被调用者会比较棘手。子shell 中的变量在子shell 之外不可用。这些是子进程的本地变量。

解决它的一种方法是使用临时文件来存储中间值：

TEMPFILE=/tmp/$$.tmp
echo 0 > $TEMPFILE

# Loop goes here
  # Fetch the value and increase it
  COUNTER=$[$(cat $TEMPFILE) + 1]

  # Store the new value
  echo $COUNTER > $TEMPFILE

# Loop done, script done, delete the file
unlink $TEMPFILE

score 93 · Accepted Answer

COUNTER=1
while [ Your != "done" ]
do
     echo " $COUNTER "
     COUNTER=$[$COUNTER +1]
done

TESTED BASH: Centos, SuSE, RH

score 49 · Accepted Answer

COUNTER=$((COUNTER+1))

is quite a clumsy construct in modern programming.

(( COUNTER++ ))

looks more "modern". You can also use

let COUNTER++

if you think that improves readability. Sometimes, Bash gives too many ways of doing things - Perl philosophy I suppose - when perhaps the Python "there is only one right way to do it" might be more appropriate. That's a debatable statement if ever there was one! Anyway, I would suggest the aim (in this case) is not just to increment a variable but (general rule) to also write code that someone else can understand and support. Conformity goes a long way to achieving that.

HTH

score 17 · Accepted Answer

17

尝试使用

COUNTER=$((COUNTER+1))

代替

COUNTER=$((COUNTER))

于 2012-05-09T12:23:37.233 回答

score 12 · Accepted Answer

Instead of using a temporary file, you can avoid creating a subshell around the while loop by using process substitution.

while ...
do
   ...
done < <(grep ...)

By the way, you should be able to transform all that grep, grep, awk, awk, awk into a single awk.

Starting with Bash 4.2, there is a lastpipe option that

runs the last command of a pipeline in the current shell context. The lastpipe option has no effect if job control is enabled.

bash -c 'echo foo | while read -r s; do c=3; done; echo "$c"'

bash -c 'shopt -s lastpipe; echo foo | while read -r s; do c=3; done; echo "$c"'
3

score 12 · Accepted Answer

I think this single awk call is equivalent to your grep|grep|awk|awk pipeline: please test it. Your last awk command appears to change nothing at all.

The problem with COUNTER is that the while loop is running in a subshell, so any changes to the variable vanish when the subshell exits. You need to access the value of COUNTER in that same subshell. Or take @DennisWilliamson's advice, use a process substitution, and avoid the subshell altogether.

awk '
  /GET \/log_/ && /upstream timed out/ {
    split($0, a, ", ")
    split(a[2] FS a[4] FS $0, b)
    print "http://example.com" b[5] "&ip=" b[2] "&date=" b[7] "&time=" b[8] "&end=1"
  }
' | {
    while read WFY_URL
    do
        echo $WFY_URL #Some more action
        (( COUNTER++ ))
    done
    echo $COUNTER
}

score 12 · Accepted Answer

12

count=0   
base=1
(( count += base ))

于 2013-07-22T13:06:41.050 回答

score 9 · Accepted Answer

9

minimalist

counter=0
((counter++))
echo $counter

于 2015-11-05T04:59:46.243 回答

score 3 · Accepted Answer

This is all you need to do:

$((COUNTER++))

Here's an excerpt from Learning the bash Shell, 3rd Edition, pp. 147, 148:

bash arithmetic expressions are equivalent to their counterparts in the Java and C languages.[9] Precedence and associativity are the same as in C. Table 6-2 shows the arithmetic operators that are supported. Although some of these are (or contain) special characters, there is no need to backslash-escape them, because they are within the $((...)) syntax.

..........................

The ++ and - operators are useful when you want to increment or decrement a value by one.[11] They work the same as in Java and C, e.g., value++ increments value by 1. This is called post-increment; there is also a pre-increment: ++value. The difference becomes evident with an example:

$ i=0
$ echo $i
0
$ echo $((i++))
0
$ echo $i
1
$ echo $((++i))
2
$ echo $i
2

See http://www.safaribooksonline.com/a/learning-the-bash/7572399/

score 3 · Accepted Answer

There were two conditions that caused the expression ((var++)) to fail for me:

If I set bash to strict mode (set -euo pipefail) and if I start my increment at zero (0).
Starting at one (1) is fine but zero causes the increment to return "1" when evaluating "++" which is a non-zero return code failure in strict mode.

I can either use ((var+=1)) or var=$((var+1)) to escape this behavior

score 1 · Accepted Answer

This is a simple example

COUNTER=1
for i in {1..5}
do   
   echo $COUNTER;
   //echo "Welcome $i times"
   ((COUNTER++));    
done

score 1 · Accepted Answer

Source script has some problem with subshell. First example, you probably do not need subshell. But We don't know what is hidden under "Some more action". The most popular answer has hidden bug, that will increase I/O, and won't work with subshell, because it restores couter inside loop.

Do not fortot add '\' sign, it will inform bash interpreter about line continuation. I hope it will help you or anybody. But in my opinion this script should be fully converted to AWK script, or else rewritten to python using regexp, or perl, but perl popularity over years is degraded. Better do it with python.

Corrected Version without subshell:

#!/bin/bash
WFY_PATH=/var/log/nginx
WFY_FILE=error.log
COUNTER=0
grep 'GET /log_' $WFY_PATH/$WFY_FILE | grep 'upstream timed out' |\
awk -F ', ' '{print $2,$4,$0}' |\
awk '{print "http://example.com"$5"&ip="$2"&date="$7"&time="$8"&end=1"}' |\
awk -F '&end=1' '{print $1"&end=1"}' |\
#(  #unneeded bracket
while read WFY_URL
do
    echo $WFY_URL #Some more action
    COUNTER=$((COUNTER+1))
done
# ) unneeded bracket

echo $COUNTER # output = 0

Version with subshell if it is really needed

#!/bin/bash

TEMPFILE=/tmp/$$.tmp  #I've got it from the most popular answer
WFY_PATH=/var/log/nginx
WFY_FILE=error.log
COUNTER=0
grep 'GET /log_' $WFY_PATH/$WFY_FILE | grep 'upstream timed out' |\
awk -F ', ' '{print $2,$4,$0}' |\
awk '{print "http://example.com"$5"&ip="$2"&date="$7"&time="$8"&end=1"}' |\
awk -F '&end=1' '{print $1"&end=1"}' |\
(
while read WFY_URL
do
    echo $WFY_URL #Some more action
    COUNTER=$((COUNTER+1))
done
echo $COUNTER > $TEMPFILE  #store counter only once, do it after loop, you will save I/O
)

COUNTER=$(cat $TEMPFILE)  #restore counter
unlink $TEMPFILE
echo $COUNTER # output = 0

score 0 · Accepted Answer

0

看来您没有更新counter脚本，请使用counter++

于 2012-05-09T12:26:40.147 回答

linux - Bash循环中的计数器增量不起作用

13 回答 13

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