Is it possible to add some text at beginning of a file in CLI without making new file?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

all=$( cat file1 file2 )
echo "$all" > file2

only works with small-ish files.

Answer 4

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