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How to replace all but the first 3 characters with sed?

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Top Forums Shell Programming and Scripting How to replace all but the first 3 characters with sed?
# 8  
Old 02-09-2015
Hi,
Beware with builtin 'read' of bash, because if you specified a variable, you lost spaces that begin a line, example:
Code:
$ read -r line <<<"    foobar"
$ echo "$line"
foobar
$ read -r <<<"    foobar"
$ echo "$REPLY"
    foobar
$

Is it a bug ? I don't know
Regards.
This User Gave Thanks to disedorgue For This Post:
wisecracker
# 9  
Old 02-09-2015
Good point. This is not a bug, it is because of the IFS internal field separator.
Code:
$ IFS= read -r line <<<"    foobar"
$ printf "%s\n" "$line" 
    foobar
$


To preserve content of a file including spaces while processing line by line in shell,
  1. Set IFS to "" and local to the read operation
  2. Use -r to avoid interpretation of \
  3. Use printf, rather than echo to make sure that a particular version of echo does not interpret the content of the variable, and the output will come out right.
  4. Prevent variable expansion from field splitting and globbing by using double quotes around it.

So:
Code:
while IFS= read -r line
do
  printf "%s\n" "$line"
done < file

Since the IFS variable is set local to the read operation, it will retain its original value after the fact..
Last edited by Scrutinizer; 09-17-2016 at 12:18 PM..
These 2 Users Gave Thanks to Scrutinizer For This Post:
disedorgue drl
# 10  
Old 02-10-2015
Code:
$ cat file
abababab
cdcdcdcdcd
efefefefe
$ sed "s/./*/4g" file
aba*****
cdc*******
efe******

This User Gave Thanks to anbu23 For This Post:
Scrutinizer
# 11  
Old 02-10-2015
Quote:
Originally Posted by anbu23
Code:
$ cat file
abababab
cdcdcdcdcd
efefefefe
$ sed "s/./*/4g" file
aba*****
cdc*******
efe******

Unfortunately, the standards say the results are unspecified if you have both a number and a g flag for a sed substitute command. On some systems, you will get what you showed above. On others, you'll get something like:
Code:
sed: 1: "s/./*/4g": more than one number or 'g' in substitute flags

as is the result on OS X.
This User Gave Thanks to Don Cragun For This Post:
anbu23
# 12  
Old 02-10-2015
Hi guys...

I did not forget IFS="" to account for leading spaces, I just worked on the OP's post to have alpha characters starting but allowed for spaces inside any line which worked.
Yes there are spaces at the end of one line.

However with the extra line added, (note IFS is not saved in this DEMO.)
Code:
AMIGA:barrywalker~> IFS=""
AMIGA:barrywalker~> echo 'kajhd(*&&#$%^ASDFGHJ{}][!\\|\00
123131,.,xv.,c.,.?><lksdlfk
     barry
A*b*C***Klom
abnm
n n n n n
 n n n n n' > /tmp/txt
AMIGA:barrywalker~> while read -r line; do pad="${line:3}"; echo $line; echo -E "${line:0:3}${pad//[' '-~]/*}"; done < /tmp/txt
kajhd(*&&#$%^ASDFGHJ{}][!\\|\00
kaj****************************
123131,.,xv.,c.,.?><lksdlfk
123************************
     barry
   *******
A*b*C***Klom
A*b*********
abnm
abn*
n n n n n
n n******
 n n n n n   
 n **********
AMIGA:barrywalker~> _

Last edited by wisecracker; 02-10-2015 at 05:18 AM.. Reason: Show spaces at the end of one line...
# 13  
Old 02-10-2015
With awk:
Code:
awk '{x=substr($0,N+1); gsub(".","*",x); print substr($0,1,N) x}' N=3 file

# 14  
Old 02-10-2015
Quote:
Originally Posted by anbu23
Code:
$ cat file
abababab
cdcdcdcdcd
efefefefe
$ sed "s/./*/4g" file
aba*****
cdc*******
efe******

The combination of a number flag and a g flag is a GNU sed extension. Still, it is a nice solution, when GNU sed is available...
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