[英]Bash shell scripting: How to replace characters at specific byte offsets
I'm looking to replace characters at specific byte offsets. 我正在寻找替换特定字节偏移的字符。
Here's what is provided: An input file that is simple ASCII text. 这是提供的内容:一个简单的ASCII文本输入文件。 An array within a Bash shell script, each element of the array is a numerical byte-offset value. Bash shell脚本中的数组,数组的每个元素都是一个数字字节偏移值。
The goal: Take the input file, and at each of the byte-offsets, replace the character there with an asterisk. 目标:获取输入文件,并在每个字节偏移集中,用星号替换字符。
So essentially the idea I have in mind is to somehow go through the file, byte-by-byte, and if the current byte-offset being read is a match for an element value from the array of offsets, then replace that byte with an asterisk. 所以基本上我想到的想法是以某种方式逐个字节地浏览文件,如果读取的当前字节偏移量与偏移量数组中的元素值匹配,则将该字节替换为星号。
This post seems to indicate that the dd command would be a good candidate for this action, but I can't understand how to perform the replacement multiple times on the input file. 这篇文章似乎表明dd命令是这个动作的一个很好的候选者,但我无法理解如何在输入文件上多次执行替换。
Input file looks like this: 输入文件如下所示:
00000
00000
00000
The array of offsets looks this: 偏移数组看起来像这样:
offsetsArray=("2" "8" "9" "15")
The output file's desired format looks like this: 输出文件的所需格式如下所示:
0*000
0**00
00*00
Any help you could provide is most appreciated. 非常感谢您提供的任何帮助。 Thank you! 谢谢!
Please check my comment about about newline offset. 请检查我关于换行偏移的评论。 Assuming this is correct (note I have changed your offset array), then I think this should work for you: 假设这是正确的(注意我已经改变了你的偏移数组),那么我认为这应该适合你:
#!/bin/bash
read -r -d ''
offsetsArray=("2" "8" "9" "15")
txt="${REPLY}"
for i in "${offsetsArray[@]}"; do
txt="${txt:0:$i-1}*${txt:$i}"
done
printf "%s" "$txt"
Explanation: 说明:
read -d ''
reads the whole input (redirected file) in one go into the $REPLY
variable. read -d ''
读取整个输入(重定向文件)进入$REPLY
变量。 If you have large files, this can run you out of memory. 如果你有大文件,这可能会让你内存不足。 i
to grab i-1
characters from the beginning of the string, then insert a *
character, then add the remaining bytes from offset i
. 我们使用每个索引i
从字符串的开头抓取i-1
字符,然后插入一个*
字符,然后从offset i
添加剩余的字节。 This is done with bash parameter expansion . 这是通过bash参数扩展完成的 。 Note that while your offsets are one-based, bash strings use zero-based indexing. 请注意,虽然您的偏移量是基于一的,但是bash字符串使用从零开始的索引。 In use: 正在使用:
$ ./replacechars.sh < input.txt
0*000
0**00
00*00
$
Caveat: 警告:
This is not really a very efficient solution, as it causes the sting containing the whole file to be copied for every offset. 这不是一个非常有效的解决方案,因为它会导致包含整个文件的sting被复制到每个偏移量。 If you have large files and/or a large number of offsets, then this will run slowly. 如果您有大文件和/或大量偏移,那么这将运行缓慢。 If you need something faster, then another language that allows modification of individual characters in a string would be much better. 如果你需要更快的东西,那么允许修改字符串中的单个字符的另一种语言会好得多。
The usage of dd
can be a bit confusing at the time, but it's not that hard: dd
的使用在当时可能有点令人困惑,但并不难:
outfile="test.txt"
# create some test data
echo -n 0123456789abcde > "$outfile"
offsetsArray=("2" "7" "8" "13")
for offset in "${offsetsArray[@]}"; do
dd bs=1 count=1 seek="$offset" conv=notrunc of="$outfile" <<< '*'
done
cat "$outfile"
Important for this example is to use conv=notrunc
, otherwise dd truncates the file to the length of blocks it seeks over. 对于此示例重要的是使用conv=notrunc
,否则dd会将文件截断为它所寻找的块的长度。 bs=1
specifies that you want to work with blocks of size 1, and seek
specifies the offset to satart writing count
blocks to. bs=1
指定您要使用大小为1的块,并且seek
指定要将写入count
块的satart设置为。
The above produces 01*3456**9abc*e
以上产生01*3456**9abc*e
With the same offset considerations as @DigitalTrauma's superior solution, here's a GNU awk-based alternative. 与@ DigitalTrauma的卓越解决方案具有相同的偏移考虑因素,这是基于GNU awk的替代方案。 This assumes your file contains no null bytes 假设您的文件不包含空字节
(IFS=','; awk -F '' -v RS=$'\0' -v OFS='' -v offsets="${offsetsArray[*]}" \
'BEGIN{split(offsets, o, ",")};{for (k in o) $o[k]="*"; print}' file)
0*000
0**00
00*00
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