Delete the first n bytes of files

Question

I've got an extreme problem, and all of the solutions I can imagine are complicated. According to my UNIX/Linux experience there must be an easy way.

I want to delete the first 31 bytes of each file in /foo/. Each file is long enough. Well, I'm sure somebody will deliver me a suprisingly easy solution I just can't imagine. Maybe awk?

Solution 1

for file in /foo/*
do
  if [ -f "$file" ]
  then
    dd if="$file" of="$file.truncated" bs=31 skip=1 && mv "$file.truncated" "$file"
  fi
done

or the faster, thanks to Gilles' suggestion:

for file in /foo/*
    do
      if [ -f $file ]
      then
        tail +32c $file > $file.truncated && mv $file.truncated $file
      fi
    done

Note: Posix tail specify "-c +32" instead of "+32c" but Solaris default tail doesn't like it:

   $ /usr/bin/tail -c +32 /tmp/foo > /tmp/foo1
    tail: cannot open input

/usr/xpg4/bin/tail is fine with both syntaxes.

If you want to keep the original file permissions, replace

... && mv "$file.truncated" "$file"

by

... && cat "$file.truncated" "$file" && rm "$file.truncated"

Solution 2

The following commands cut first 31 bytes from $file (using $file~ as a temp. copy):

dd if="$file" of="$file~" bs=1 skip=31
mv "$file~" "$file"

You only need to list or find all files under /foo/ and execute the two above for every $file found.

Solution 3

tail -c +32 outputs its input minus the first 31 bytes. (Yes, the argument is off by one.) To edit a file in place, use sponge in a loop, or if you don't have it and don't want to bother, do its job in the shell:

for x in /foo/*; do tail -c +32 "$x" | sponge "$x"; done
for x in /foo/*; do tail -c +32 "$x" >"$x.new" && mv "$x.new" "$x"; done

If the commands are interrupted for whatever reason (e.g. a power failure), it might be hard to figure out where you left off. Writing the new files to a separate directory would make things easier.

mkdir /foo.tmp
cd /foo
for x in *; do tail -c +42 -- "$x" >"/foo.tmp/$x" && rm -- "$x"; done
mv /foo.tmp/* /foo
rmdir /foo.tmp

If the files are really large (as in, large enough that having two copies of even a single one is a problem), you can use one of the techniques mentioned in this thread.

Solution 4

You can use Vim in Ex mode:

for each in /foo/*
do
  ex -sc '%!tail -c+32' -cx "$each"
done

1. % select all lines

2. ! run command

3. x save and close

View more solutions

Comments

[Angel Todorov]

Any awk/sed/ed solution will be line-oriented, so if you don't know the first line will be at least 31 characters then complications ensue.

[jlliagre]

Swapping bs and skip values will increase the performance.

[Gilles 'SO- stop being evil']

Suggesting dd here is overkill, tail is more appropriate (simpler, less risk of a killer typo, no spurious messages on stderr).

[jlliagre]

You are right. I usually avoid commands intended to process text files when processing possibly binary ones but "tail +32c" will work here.

[Peter.O]

@jlliagre: You have written cut (shouldn't that be tail? ... asis, it doesn't work for me...

[jlliagre]

Of course, it's tail. Sorry about the mismatch.

[Gilles 'SO- stop being evil']

@jlliagre: On Solaris, you should have /usr/xpg4/bin ahead of /usr/bin on your PATH, or you'll be stuck in the early 1990s. Many unices (e.g. GNU, BusyBox) no longer support the historical +32c syntax, and take it to mean a file called +32c (as POSIX requires).

[jlliagre]

@Gilles: I should indeed as I usually strongly support standard conformance but in that specific case, I'm afraid I won't switch soon. What refrains me to do it is that change would trigger a gazillion of issues with existing scripts, and would distance me from the community of Solaris users who in their very large majority are still using the traditional commands.

[Aryeh Armon]

tail: cannot open '+32c' for reading: No such file or directory

[jlliagre]

@AryehArmon As stated in my reply, you can use instead "tail -c +32 $file" on POSIX systems.

[Aryeh Armon]

@jlliagre this will give me last 32 bytes, correct?

[jlliagre]

@AryehArmon Incorrect, unless if the file size is exactly 64 bytes.

[Hameda169]

mv changes the file permissions. you can use cat instead of mv like this: tail +32c $file > $file.truncated && cat $file.truncated > $file && rm $file.truncated @jlliagre