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awk: seeking to bytes

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# 1  
Old 02-07-2005
Question awk: seeking to bytes
can I seek to a particular byte in a file and replace it using awk?
if so, how?
# 2  
Old 02-07-2005
substitute the THIRD character in the record/line with 'A"
Code:
nawk '{$0=substr($0,2,1) "A" substr($0,4); print}'

# 3  
Old 02-07-2005
Quote:
Originally Posted by vgersh99
substitute the THIRD character in the record/line with 'A"
Code:
nawk '{$0=substr($0,2,1) "A" substr($0,4); print}'

What if I want to go to byte 24 and replace it, instead of knowing which character?
# 4  
Old 02-07-2005
Using sed, you can navigate to a particular offset in a line and make a substitution....

sed 's/./A/24' myfile

This'll replace the 24th character on each line with 'A'

If you want to change a particular byte offset within a multiline file online once, try something like
Code:
#!/bin/sh

replace="B"
offset=24

awk -vr=$replace -vo=$offset '
   BEGIN {
      s=o-1;
      e=o+1;
      RS="";
   } {
      printf("%s%s%s",substr($0,1,s), r, substr($0,e) );
   }
' testfile

exit 0

But beware, each newline is also a byte in itself.

Cheers
ZB
# 5  
Old 02-07-2005
Quote:
Originally Posted by zazzybob
Using sed, you can navigate to a particular offset in a line and make a substitution....

sed 's/./A/24' myfile

This'll replace the 24th character on each line with 'A'

If you want to change a particular byte offset within a multiline file online once, try something like
Code:
#!/bin/sh

replace="B"
offset=24

awk -vr=$replace -vo=$offset '
   BEGIN {
      s=o-1;
      e=o+1;
      RS="";
   } {
      printf("%s%s%s",substr($0,1,s), r, substr($0,e) );
   }
' testfile

exit 0

But beware, each newline is also a byte in itself.

Cheers
ZB
I believe the above 'awk':
1. will not work for the multi-byte character sets as 'substr' is a character based operation [not a byte oriented one].

2. will not work on LARGE files as most awks [at least Solaris' stock ones] have inherited limits on the lenght of records [RS], number of fields [NF] etc....

It might work in some isolated cases, but it is not a generic solution.
# 6  
Old 02-07-2005
Quote:
Originally Posted by zazzybob
Using sed, you can navigate to a particular offset in a line and make a substitution....

sed 's/./A/24' myfile

This'll replace the 24th character on each line with 'A'

If you want to change a particular byte offset within a multiline file online once, try something like
Code:
#!/bin/sh

replace="B"
offset=24

awk -vr=$replace -vo=$offset '
   BEGIN {
      s=o-1;
      e=o+1;
      RS="";
   } {
      printf("%s%s%s",substr($0,1,s), r, substr($0,e) );
   }
' testfile

exit 0

But beware, each newline is also a byte in itself.

Cheers
ZB

I tried the sed command, but it did not output anything???
# 7  
Old 02-07-2005
Quote:
Originally Posted by vgersh99
I believe the above 'awk':
1. will not work for the multi-byte character sets as 'substr' is a character based operation [not a byte oriented one].

2. will not work on LARGE files as most awks [at least Solaris' stock ones] have inherited limits on the lenght of records [RS], number of fields [NF] etc....

It might work in some isolated cases, but it is not a generic solution.
Do you know the limit on awks for length of records? I am only interested in the first 80 bytes of the line, does that make a difference? Also, can't I equate a byte = 1 character?
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