linux - how to replace unknown substring of line with sed
2013-09
I want to replace a substring in a line, but I don't know what that substring is. I do know how the line starts though. For example, DNS1=[...to be replaced....].
I tried:
sed -i 's/DNS1=*/DNS1="8.8.8.8"/' /etc/sysconfig/network-scripts/ifcfg-eth0
and the result was:
Before: DNS1="10.10.2.2"
After: DNS1="8.8.8.8""10.10.2.2"
What am I doing wrong...
Try:
sed -i 's/DNS1=.*/DNS1="8.8.8.8"/' /etc/sysconfig/network-scripts/ifcfg-eth0
The *
operator means "match the preceding character 0 or more times". In your example the preceding character is =
, therefore it will match DNS1=
and nothing more, since there are no more sequential =
characters, and replace that portion with DNS1="8.8.8.8"
, which generates the string DNS1="8.8.8.8""10.10.2.2"
, just as you said.
The .
operator means "any character" in sed
context, which is probably what you want here. That variation will match DNS1=
and then "any character 0 or more times" until the end of the line and then replace that with DNS1="8.8.8.8"
.
I have a very large file ~400G, and I need to remove the last 2 lines from it. I tried to use sed, but it ran for hours before I gave up. Is there a quick way of doing this, or am I stuck with sed?
I haven't tried this on a large file to see how fast it is, but it should be fairly quick.
To use the script to remove lines from the end of a file:
./shorten.py 2 large_file.txt
It seeks to the end of the file, checks to make sure the last character is a newline, then reads each character one at a time going backwards until it's found three newlines and truncates the file just after that point. The change is made in place.
Edit: I've added a Python 2.4 version at the bottom.
Here is a version for Python 2.5/2.6:
#!/usr/bin/env python2.5
from __future__ import with_statement
# also tested with Python 2.6
import os, sys
if len(sys.argv) != 3:
print sys.argv[0] + ": Invalid number of arguments."
print "Usage: " + sys.argv[0] + " linecount filename"
print "to remove linecount lines from the end of the file"
exit(2)
number = int(sys.argv[1])
file = sys.argv[2]
count = 0
with open(file,'r+b') as f:
f.seek(0, os.SEEK_END)
end = f.tell()
while f.tell() > 0:
f.seek(-1, os.SEEK_CUR)
char = f.read(1)
if char != '\n' and f.tell() == end:
print "No change: file does not end with a newline"
exit(1)
if char == '\n':
count += 1
if count == number + 1:
f.truncate()
print "Removed " + str(number) + " lines from end of file"
exit(0)
f.seek(-1, os.SEEK_CUR)
if count < number + 1:
print "No change: requested removal would leave empty file"
exit(3)
Here's a Python 3 version:
#!/usr/bin/env python3.0
import os, sys
if len(sys.argv) != 3:
print(sys.argv[0] + ": Invalid number of arguments.")
print ("Usage: " + sys.argv[0] + " linecount filename")
print ("to remove linecount lines from the end of the file")
exit(2)
number = int(sys.argv[1])
file = sys.argv[2]
count = 0
with open(file,'r+b', buffering=0) as f:
f.seek(0, os.SEEK_END)
end = f.tell()
while f.tell() > 0:
f.seek(-1, os.SEEK_CUR)
print(f.tell())
char = f.read(1)
if char != b'\n' and f.tell() == end:
print ("No change: file does not end with a newline")
exit(1)
if char == b'\n':
count += 1
if count == number + 1:
f.truncate()
print ("Removed " + str(number) + " lines from end of file")
exit(0)
f.seek(-1, os.SEEK_CUR)
if count < number + 1:
print("No change: requested removal would leave empty file")
exit(3)
Here is a Python 2.4 version:
#!/usr/bin/env python2.4
import sys
if len(sys.argv) != 3:
print sys.argv[0] + ": Invalid number of arguments."
print "Usage: " + sys.argv[0] + " linecount filename"
print "to remove linecount lines from the end of the file"
sys.exit(2)
number = int(sys.argv[1])
file = sys.argv[2]
count = 0
SEEK_CUR = 1
SEEK_END = 2
f = open(file,'r+b')
f.seek(0, SEEK_END)
end = f.tell()
while f.tell() > 0:
f.seek(-1, SEEK_CUR)
char = f.read(1)
if char != '\n' and f.tell() == end:
print "No change: file does not end with a newline"
f.close()
sys.exit(1)
if char == '\n':
count += 1
if count == number + 1:
f.truncate()
print "Removed " + str(number) + " lines from end of file"
f.close()
sys.exit(0)
f.seek(-1, SEEK_CUR)
if count < number + 1:
print "No change: requested removal would leave empty file"
f.close()
sys.exit(3)
you can try GNU head
head -n -2 file
The problem with sed is that it is a stream editor -- it will process the entire file even if you only want to make modifications near the end. So no matter what, you are creating a new 400GB file, line by line. Any editor that operates on the whole file will probably have this problem.
If you know the number of lines, you could use head
, but again this creates a new file instead of altering the existing one in place. You might get speed gains from the simplicity of the action, I guess.
You might have better luck using split
to break the file into smaller pieces, editing the last one, and then using cat
to combine them again, but I'm not sure if it will be any better. I would use byte counts rather than lines, otherwise it will probably be no faster at all -- you're still going to be creating a new 400GB file.
I see my Debian Squeeze/testing systems (but not Lenny/stable) include a "truncate" command as part of the "coreutils" package.
With it you could simply do something like
truncate --size=-160 myfile
to remove 160 bytes from the end of the file (obviously you need to figure out exactly how many characters you need to remove).
Try VIM...I'm not sure if it will do the trick or not, as I've never used it on such a big file, but I've used it on smaller larger files in the past give it try.
What kind of file and in what format? May be easier to use something like Perl dependent on what kind of file it is - text, graphics, binary? How is it formatted - CSV, TSV...
If you know the size of the file to the byte (400000000160 say) and you know that you need to remove exactly 160 characters to strip the last two lines, then something like
dd if=originalfile of=truncatedfile ibs=1 count=400000000000
should do the trick. It's been ages since I used dd in anger though; I seem to remember things go faster if you use a bigger block size, but whether you can do that depends on whether the lines you want to drop are at a nice multiple.
dd has some other options to pad text records out to a fixed size which might be useful as a preliminary pass.
If "truncate" command isn't available on your system (see my other answer), look at the "man 2 truncate" for the system call to truncate a file to a specified length.
Obviously you need to know how many characters you need to truncate the file to (size minus the length of the problem two lines; don't forget to count any cr/lf characters).
And make a backup of the file before you try this!
#!/bin/sh ed "$1" << HERE $ d d w HERE
changes are made in place. This is simpler and more efficient than the python script.
Modified the accepted answer to solve a similar problem. Could be tweaked a little bit to remove n lines.
import os
def clean_up_last_line(file_path):
"""
cleanup last incomplete line from a file
helps with an unclean shutdown of a program that appends to a file
if \n is not the last character, remove the line
"""
with open(file_path, 'r+b') as f:
f.seek(0, os.SEEK_END)
while f.tell() > 0: ## current position is greater than zero
f.seek(-1, os.SEEK_CUR)
if f.read(1) == '\n':
f.truncate()
break
f.seek(-1, os.SEEK_CUR) ## don't quite understand why this has to be called again, but it doesn't work without it
And the corresponding test:
import unittest
class CommonUtilsTest(unittest.TestCase):
def test_clean_up_last_line(self):
"""
remove the last incomplete line from a huge file
a line is incomplete if it does not end with a line feed
"""
file_path = '/tmp/test_remove_last_line.txt'
def compare_output(file_path, file_data, expected_output):
"""
run the same test on each input output pair
"""
with open(file_path, 'w') as f:
f.write(file_data)
utils.clean_up_last_line(file_path)
with open(file_path, 'r') as f:
file_data = f.read()
self.assertTrue(file_data == expected_output, file_data)
## test a multiline file
file_data = """1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b
1362358458954466,2013-03-03 16:54:18,34.5,3.0,b
1362358630923094,2013-03-03 16:57:10,34.5,50.0,b
136235"""
expected_output = """1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b
1362358458954466,2013-03-03 16:54:18,34.5,3.0,b
1362358630923094,2013-03-03 16:57:10,34.5,50.0,b
"""
compare_output(file_path, file_data, expected_output)
## test a file with no line break
file_data = u"""1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b"""
expected_output = "1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b"
compare_output(file_path, file_data, expected_output)
## test a file a leading line break
file_data = u"""\n1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b"""
expected_output = "\n"
compare_output(file_path, file_data, expected_output)
## test a file with one line break
file_data = u"""1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b\n"""
expected_output = """1362358424445914,2013-03-03 16:53:44,34.5,151.16345879,b\n"""
compare_output(file_path, file_data, expected_output)
os.remove(file_path)
if __name__ == '__main__':
unittest.main()