[SOLVED] how to replace a subtring with sed

password636 · 09-19-2010, 01:55 AM

Hi

If I have a line like AA=<value> in the file, and the <value> is not a constant. For example, it could be AA=BB or AA=CC. How can I replace the <value> with a specified string such as 'DD'? Thus, AA=BB can become AA=DD and AA=CC can also become DD. Don't want the whole line replaced, just the <value> part.

Can sed or awk work for this?

EricTRA · 09-19-2010, 02:13 AM

Hello,

Yes you can do that with both sed and awk. If you know the length of the value; if that's static (always two characters) then you could use sed like this:

Code:

sed -i 's/=[A-Z][A-Z]/=DD/g' yourfile

The above will change all instances of '=' followed by two capital characters from the alphabet to =DD. Of course if the value is outside this scope; for example including numbers or being larger then two characters, you'll have to adapt the regex.

Sed will execute the changes inside de document without output on the screen. If you want to view the results on screen remove the -i. If you want the result in another file remove the -i and redirect output to another file.

Hope that helps.

Kind regards,

Eric

sag47 · 09-19-2010, 02:16 AM

Sed can do this for sure. Is AA=value unique within the file? Is AA=value the only line in the file?

You would need something like

Code:

cat file | sed 's/^\(AA=\)\(.*\)/\1Insert Your New Value/' > file.tmp
mv file.tmp file

The above expression applies only to lines where the line starts with AA=. Change "Insert Your New Value" with whatever you want the new value of AA= to be.

Look up "regex sed" or "sed regex" for more information on how to form accurate expressions to suit your purpose.

quanta · 09-19-2010, 02:25 AM

@EricTRA: your solution lack of 'AA' pattern

@sag47:
- No need to 'cat' and pipe to 'sed'
- You should use "-i" (in-place) instead of redirect output to a file and move it back

@password636: you want the line begin with 'AA=' or including 'AA='? If I have the following input file:

Quote:

ijkAA=BB
AA=CC
AA=EF
AA=XYZ

What output do you want?

EricTRA · 09-19-2010, 02:28 AM

Quote:

Originally Posted by quanta

@EricTRA: your solution lack of 'AA' pattern

@sag47:
- No need to 'cat' and pipe to 'sed'
- You should use "-i" (in-place) instead of output to a file and move it back

Hello,

Quite right but I mentioned that in my post.

Quote:

The above will change all instances of '=' followed by two capital characters

I didn't state all istances of AA= followed by.....

I'm sure OP is able to 'fill in the blanks'.

Kind regards,

Eric

sag47 · 09-19-2010, 02:46 AM

Quote:

Originally Posted by quanta

@sag47:
- No need to 'cat' and pipe to 'sed'
- You should use "-i" (in-place) instead of redirect output to a file and move it back

I prefer backups for my backups. It's not like my command wouldn't work (there's a 1000 ways to build a house). Whatever the case may be, as Eric puts it, the OP can easily modify all the information we've given him/her to their liking. Thanks for the tips.

password636 · 09-19-2010, 04:43 AM

Hi guys really thank you all!
@quanta:
beginning with 'AA=' is OK.

kurumi · 09-19-2010, 04:57 AM

Code:

# cat file
AA=BB dog cat AA=CC dog cat
mouse rat AA=xxxxxAA=yyyyy

# ruby -e 's=readlines.join.split("AA=").each{|x| x.sub!(/^.[^\s]*/,"AA=DD")};print s.join' < file
AA=DD dog cat AA=DD dog cat
mouse rat AA=DDAA=DD

sed is greedy.

Code:

# sed 's/^\(AA=\)\(.*\)/\1DD/' file
AA=DD
mouse rat AA=xxxxxAA=yyyyy

Wrong output.

sag47 · 09-19-2010, 05:14 AM

Shouldn't matter if it is a settings/config file where usually it is one setting per line in a setting=value format. I designed the regex to be greedy for simplicity's sake. It can be refined to be less or more greedy you just have to modify the regex.

Using ruby is an interesting solution for it though.