如何查看文件的大小是否是 4 字节的倍数 [英] How to see if the size of a file is multiple of 4 bytes

问题描述

我正在为大学练习编写 bash 脚本，该脚本读取作为参数传递给它的文件的字节大小并将它们相加.如果大小是 4 字节的倍数，则添加大小.如果不是，我必须把它做成4的倍数.特别是练习的正文是这样写的:如果文件的大小D不是4字节的倍数，则填写最后的Dmod 4 字节，带零."

I'm writing a bash script for a university exercise, which reads the byte size of the files passed to it as arguments and adds them up. The size is added if it is a multiple of 4 bytes. If not, I have to make it a multiple of 4. In particular, the text of the exercise says like this: "If the size D of the file is not a multiple of 4 bytes, fill in the last D mod 4 bytes with zeros. "

现在，为了检查文件的大小是否是 4 字节的倍数，我这样做了，但我不知道这是否正确:

Now, to check if the size of a file is a multiple of 4 bytes I do this, but i don't know if this is right:

D=`stat -c '%s' file.txt`  #command sostitution
if (( ($D/4)*4 == $D ))
then 
    echo it's ok
else
    echo it isn't ok
fi

我也想知道，给定文件的大小D(以字节为单位)，如何填写最后的D如果 D 不是 4 字节的倍数，则对 4 个字节取零.

Also I would like to know, given the size D of the file (in bytes), how to fill in the last D mod 4 bytes with zeros, if D is not a multiple of 4 bytes.

问题是我不明白填写最后一个D"是什么意思mod 4 字节带零.

The problem is that I don't understand what it means to "fill in the last D mod 4 bytes with zeros ".

推荐答案

不是停在模数处，而是直接计算 null 填充量以与 truncate 一起使用:

Instead of stopping at the modulo, directly compute the null padding amount to use with truncate:

#!/usr/bin/env sh

# Pad files with null to the alignment
# align ALIGNMENT FILE...
# $1: The alignemnt in bytes
# $@: The filenames to get aligned
####
# Example usage:
# align 4 file1.txt file2.txt file3.txt

# Take alignment as first argument
alignment=$1

# Shift first argument out
shift

# Iterate all remaining arguments as filename
for filename; do
  # If filename is a real file and has write permission
  if [ -f "$filename" ] && [ -w "$filename" ]; then
    # Get file size in bytes
    size=$(stat -c '%s' "$filename")

    # Compute how much padding would be need for alignment
    padding=$((alignment - size % alignment))

    # If file need padding
    if [ $padding -gt 0 ]; then
      # Pad file with padding nulls
      truncate -s +$padding "$filename"
    fi
  fi
done

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