Wrapper scripts

© December 2006 Anthony Lawrence

2006/12/05

I saw a news group post this morning that was trying to do something fairly complicated with "wget". Basically he had a list of servers that he wanted to get certain files from, but for one particular server, there was one file he did NOT want to get. He didn't say why, or why he couldn't just get it and deal with skipping or removing it from his machine later, but maybe it was exceptionally large or had other undesired side effects. It doesn't matter: the heart wants what the heart wants.

Now maybe there is some switch in wget that can do this: that's not my particular interest here. Here I want to discuss how you can use shell scripts to get what you want even though some other program doesn't want to play along.

For example, our "wget" issue. Let's look at it generically; given a file "servers" that contains:

server_one
server_two
server_three
 

And a file "files" containing:

file1
file2
file3
unwanted
file4
 

You can skip "unwanted" for server_two with a script like this:

 for i in `cat servers`
   do
   echo -n "$i "
   case $i in
      server_two) echo `cat files | sed '/unwanted/d'`;;
        *) echo `cat files`;;
    esac
    echo "  "
 done
 

When run, that produces:

server_one file1 file2 file3 unwanted file4
  
server_two file1 file2 file3 file4
  
server_three file1 file2 file3 unwanted file4

If we replace "echo" with "wget" and insert the appropriate flags, we have what we want.

Wrapping commands in shell scripts can solve difficult problems, but make sure you aren't ignoring capabilities of the command itself. For example at More reasons to love Unix/Linux, I show a shell wrapper that wants to find files NOT containing a certain pattern. That's a simple script:

grep -l "$1"  *.html >~/a
ls *.html > ~/b
diff ~/a ~/b | sort
 

But even more simple (assuming your "grep" supports this switch) is:

grep -h "$1"  *.html | sort -u
 

Often "old time" Unix folk will miss things like this because we have used "grep" and other commands for many, many years, but we haven't recently read the man pages. GNU versions of time honored Unix commands often have extended capabilities; make sure you check before you wrap.

Got something to add? Send me email.

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