Shell_compress

Compress & Uncompress in Unix-liked OS

• Common Compress Tools

  • compress and its object file is .z in extension name.

  • gzip and its object file is .gz in extension name.

  • bzip2 and its object file is .bz2 in extension name.

• Common Usage for sed

  • sed -f

    把結果輸出到另一個檔案。

  • sed -s

    取代結果輸出，不改變檔案內容。

  • echo old old | sed s/old/new/2

    可使用 flag 如 g 或是數字，讓同行字針對特定第幾比符合比對的結果可以進行替換。

  • sed -i

    直接在檔案內容中進行操作。

  • delimeter ; , - , |

  內文剛好出現分隔符的時侯，就可以派上用場。例如可以把

    $ echo /A/ | sed 's/\/A\//\/B\//' #escape using one backslash \ in suffix
  

  改寫成

    $ echo /A/ | sed 's:/A/:/B/:' 。
  

  https://blog.codylab.com/sed-substitute-s-command/

  https://charleslin74.pixnet.net/blog/post/419884144

• Script

   #!bin/bash/
   #vivycompressor
  
   # zcat, zmore, zgrep-- 此腳本使用三組符號（軟連結）或是硬連結
  
   Z='compress'; uz='uncompress'; zlist=''
   gz='gzip'; ugz='ungzip'; gzlist=''
   bz='bzip2'; ubz='unbzip2'; blist=''
  
   # 隔離檔案名稱
   # 藉由副檔名檢查是否為壓縮檔
   # 對所有檔案逐一解壓縮
   # 完成後，再對所有解壓縮檔案進行壓縮
  
   for arg
   do
    if [ -f "$arg" ]; then
       case "$arg" in
           *.Z) $uz "$arg" # to uncompress file
                arg="$(echo $arg | sed 's/\.Z$//')" # (a)
                zlist="$zlist \"$arg\"" # (b)
                 ;;
           *.gz) $ugz "$arg"
                 arg="$(echo $arg | sed 's/\.gz$//')"
                 gzlist="$gzlist \"$arg\""
                 ;;
           *.bz2) $ubz "$arg"
                  arg="$(echo $arg | sed 's/\.bz2$//')"
                  bzlist="bzlist \"$arg\""
                 ;;
       esac
    fi
  
    newargs = "${newargs:-""} \"$arg\""
  
   done
  
   case $0 in
       *zcat*) eval cat $newargs ;;
       *zmore*) eval more $newargs ;;
       *zgrep*) eval grep $newargs ;;
       *) echo "$0 has an unkown file extention name." >&2  # (c)
           exit 1
  
   esac
  
   if [ ! -z "$zlist"]; then # (d)
       eval $Z $zlist # (e)
  
   fi 
  
   if [ ! -z "$gzlist" ]; then
       eval $gz $gzlist
   fi
  
   if [ ! -z "$bzlist"]; then
       eval $bz $bzlist
   fi
  
   exit 0
  

• Syntax

  • (a) sed 's / '

     sed 's/\.Z$//'
    
     use sed 's/ replaced / replace /' cmd to do string replacement.
    

  • (b) $zlist "$arg"

     "bzlist \"$arg\""  
     the inside-backslash is to escape. 
    

  • (c) >&2

    https://github.com/QueenieCplusplus/Shell_echo/blob/master/README.md#--

      1 means stdout
      2 means stderr
      
      & is reference to file_val
    
      command > output is just a shortcut for command 1> output ; You can use &[FILE_DESCRIPTOR] to reference a file descriptor value; 
    
      Using 2>&1 will redirect stderr to whatever value is set to stdout (and 1>&2 will do the opposite).
    

  • (d) -z

      When -z is used, a zero byte (the ascii ' NUL ' character) is added between the lines (instead of a new line).
    

  • (e) eval

• Execution

    $ compress < file name >
    
    $ zcat < file name >
    
    $ zgrep < word_u_wanna_search_for > < file name >
  
    $ ls -l < file name>*
  

• Related Info

  • Link, 軟連結與硬連結

    https://linuxize.com/post/how-to-create-symbolic-links-in-linux-using-the-ln-command/

  為了適當作業，腳本有三個名稱，如何產生彼此之間可以互相合作的關係，可藉由呼叫連結符號，他們是一種儲存著目的檔案的特殊檔案，亦可使用硬連結，是實際儲存著和被連結的檔案擁有相同的 inode，如上腳本為 zcat，經由連結符號 ln -s ，利用連結符號將 zmore 和 zgrep 都指向 zcat，一旦完成連結，這三種指令就擁有相同內容了。

    $ ln -s zcat zmore
    $ ln -s zcat zgrep