Skip first 3 bytes of a file

Question

I am using AIX 6.1 ksh shell.

I want to use one liner to do something like this:

cat A_FILE | skip-first-3-bytes-of-the-file

I want to skip the first 3 bytes of the first line; is there a way to do this?

Solution 1

Old school — you could use dd:

dd if=A_FILE bs=1 skip=3

The input file is A_FILE, the block size is 1 character (byte), skip the first 3 'blocks' (bytes). (With some variants of dd such as GNU dd, you could use bs=1c here — and alternatives like bs=1k to read in blocks of 1 kilobyte in other circumstances. The dd on AIX does not support this, it seems; the BSD (macOS Sierra) variant doesn't support c but does support k, m, g, etc.)

There are other ways to achieve the same result, too:

sed '1s/^...//' A_FILE

This works if there are 3 or more characters on the first line.

tail -c +4 A_FILE

And you could use Perl, Python and so on too.

Solution 2

Instead of using cat you can use tail as such:

tail -c +4 FILE

This will print out the entire file except for the first 3 bytes. Consult man tail for more information.

Comments

[squiguy]

@BobDuell It's hard to post something that is compatible with every OS.

[Manishkumar]

Yes, it works in AIX 6.1

[Manishkumar]

Thanks for your help. Both the sed and the tail commands work in AIX 6.1. For the dd command, it should be dd if=A_FILE bs=1 skip=3 in AIX 6.1

[squiguy]

@AlvinSIU Good to know. Glad I could help.

[MUY Belgium]

You may want to use standard input as such cat A_FILE | tail -c +4 with gnu.

[jimbobmcgee]

If only because I'm in that kind of mood, and don't like coding against the output of ls; have you considered using stat -c'%s' "${IFILE}" instead of that ls|awk combo? That is, assuming GNU coreutils...

[Admin]

Thank you, this is a much better choice for working with large files with a tiny amount of garbage at the beginning. I used dd over an ssh connection to get a file image and I needed to remove the "[sudo] password for X:" at the beginning of the resulting file.