I have two files; file1 containing -
hello world
hello bangladesh
and file2 containing -
Dhaka in Bangladesh
Dhaka is capital of Bangladesh
I want to update file2 as -
hello world
hello bangladesh
Dhaka in Bangladesh
Dhaka is capital of Bangladesh
This is done by -
cat file1 file2 >> file3
mv file3 file2
But, I don't want to create a new file. I guess using sed it may be possible.
Sure it's possible.
(Unless you are also concerned about temporary files being potentially created "under the hood," as virtually all text editors work that way. I don't say it is flatly impossible to avoid any possibility under-the-hood temp file creation, but it's not covered in this answer.)
printf '%s\n' '0r file1' x | ex file2
This is a POSIX-compliant command using ex, the POSIX-specified non-visual predecessor to vi.
printf is only used here to feed a command to the editor. What printf outputs is:
0r file1
x
x is save and exit.
r is "read in the contents of the named file."
0 specifies the line number after which the read-in text should be placed.
file1 your hard disk usage will have a net increase (although even that's not actually true in all cases). But will you have a temporary increase in hard disk utilization? That's a different layer of abstraction, not answerable from the data given.
– Wildcard
Jan 03 '18 at 01:55
file1/file2/? then it donesn't create temporary file. Am I right? or Do you mean any complex thing?
– alhelal
Jan 03 '18 at 02:03
ex implementations to give a definitive answer to that—but what you describe is what I would guess, as well. Don't take it as gospel truth, but that is probably the case. :)
– Wildcard
Jan 03 '18 at 02:11
ex is usually implemented as part of vim. vim creates a swap file. I'm not sure if ex does same but if you add -n to a ex/vim invocation that will prevent swap file creation. So you might want to do that depending on what your requirements are.
– B Layer
Jan 03 '18 at 02:22
There aren't a lot of ways to modify files in place using standard tools. Even if they appear to do so they may be using temporary files (i.e. GNU sed -i).
ex on the other hand, will do the trick:
ex -n -c '0r file2' -c wq file1
ex is a line editor and vim evolved from it so these commands may look familiar. 0r filename does the same thing as :0r filename in vim: insert the specified file after the given address (line number). The line number here is 0 which is a kind of virtual line that represents the line before line 1. So the file will be inserted before any existing text.
Then we have wq which saves and quits.
If you notice the comment below about this being "very brittle" take it with a grain of salt. If you want to use this in a script just be sure to do proper pre-condition validation (files exist, are readable/writable, etc.). Good scripters would be doing that anyways.
Update: I've added -n to address OP's concerns about avoiding any temporary file creation. Assuming that your ex is implemented as part of vim (as is the case on most modern systems) this flag will suppress the normal swap file creation that vim does.
ex is not mandated to support +command syntax, and also (b) it is very brittle if used in scripting as the non-existence of file2 will hang ex up waiting on user input. See my answer for a version that avoids these pitfalls.
– Wildcard
Jan 03 '18 at 01:32
file2 is not readable ex will still wait forever. If ex is unable to write the changes to file1 it will wait forever. Any error will make ex wait for user input. For a scripted use of ex, you should never do otherwise than feeding it commands in a way that guarantees it will receive an EOF (such as using a pipe or heredoc).
– Wildcard
Jan 03 '18 at 02:22
Similar to Jasen's answer, this is a basis for a utility.
The code sponge in package moreutils is designed to allow an in-place capability to almost any program, albeit externally. Essentially it soaks up input from STDIN and then writes the collected output to a filename, but NOT with re-direction.
We liked the idea, so we created a work-alike that collects data in memory, as opposed to a file as the real sponge does.
Here is a snippet of shell script to illustrate this:
# Utility functions: print-as-echo, print-line-with-visual-space.
pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
pl " Input files data2, data3:"
head data[23]
pe
ls -gGli data[23]
pl " Results, re-write file from memory, preserving inode:"
cat data[23] | ./absorb-memory-public data3
head -v data3
pe
ls -gGli data3
Producing:
-----
Input files data2, data3:
==> data2 <==
hello world
hello bangladesh
==> data3 <==
Dhaka in Bangladesh
Dhaka is capital of Bangladesh
1051395 -rw-r--r-- 1 30 Jan 7 08:58 data2
1052221 -rw-r--r-- 1 53 Jan 7 08:58 data3
-----
Results, re-write file from memory, preserving inode:
==> data3 <==
hello world
hello bangladesh
Dhaka in Bangladesh
Dhaka is capital of Bangladesh
1052221 -rw-r--r-- 1 83 Jan 7 08:58 data3
The core perl code is quite short (especially without error checking, the addition of which may be desirable):
use warnings;
use strict;
use Carp;
# Avoid hang for argument matching "-version","--version", etc.
exit(0) if @ARGV && $ARGV[0] =~ /-version/;
my ( $file, $f, $memory );
$file = shift || croak("Need a filename.");
$/ = 0777; # Slurp the entire STDIN
$memory = do { local $/; <> };
open( $f, ">", $file ) || die " Cannot open file \"$file\" for write.\n";
print $f "$memory";
Run on a system like:
OS, ker|rel, machine: Linux, 3.16.0-4-amd64, x86_64
Distribution : Debian 8.9 (jessie)
bash GNU bash 4.3.30
perl 5.20.2
Best wishes ... cheers, drl
file2tofile1and then move it over the top offile2. That way you don't create a new file, you just overwrite the old. – Wildcard Jan 03 '18 at 01:57