As an easy example, consider the following command:
$ sort file.txt
This will output the file's data in sorted order. How do I put that data right back into the same file? I want to update the file with the sorted results.
This is not the solution:
$ sort file.txt > file.txt
... as it will cause the file to come out blank. Is there a way to update this file without creating a temporary file?
Sure, I could do something like this:
sort file.txt > temp.txt; mv temp.txt file.txt
But I would rather keep the results in memory until processing is done, and then write them back to the same file. sort actually has a flag that will allow this to be possible:
sort file.txt -o file.txt
...but I'm looking for a solution that doesn't rely on the binary having a special flag to account for this, as not all are guaranteed to. Is there some kind of linux command that will hold the data until the processing is finished?
For sort, you can use the -o option.
For a more general solution, you can use sponge, from the moreutils package:
sort file.txt | sponge file.txt
As mentioned below, error handling here is tricky. You may end up with an empty file if something goes wrong in the steps before sponge.
This is a duplicate of this question, which discusses the solutions above: How do I execute any command editing its file (argument) "in place" using bash?
You can do it with sed (with its r command), and Process Substitution:
sed -ni r<(sort file) file
In this way, you're telling sed not to print the (original) lines (-n option) and to append the file generated by <(sort file).
The well known -i option is the one which does the trick.
Example
$ cat file
b
d
c
a
e
$ sed -ni r<(sort file) file
$ cat file
a
b
c
d
e
Try vim-way:
$ ex -s +'%!sort' -cxa file.txt
Related
I need to copy the contents of a file to the end of the same file.
I wrote the following code.
#!/bin/bash
cat file.txt >> file1.txt
cat file1.txt >> file.txt
rm file1.txt
But it creates an additional file. How can this be done without creating an additional file?
You could do something like:
input=file1.txt
dd bs="$(wc -c < "$input")" count=1 if="$input" >> "$input"
but....why? There's nothing wrong with using auxiliary files, and if you remove it shortly after it is created there is almost zero chance it will ever cause any actual disk operations. Just create the temp file and remove it. Note that this has an inherent race condition, since the computation of the file size (and wc -c is a terrible way to do that!) may produce a value that is wrong if any other process is mutating the file, but this issue is inherent in your problem description.
You can create a variable that as the value of you file and put it to the end :)
Or use sponge - that's exactly what it's there for.
cat file1.txt file1.txt | sponge file1.txt
I'm trying to figure out how to efficiently copy-paste from X application to the terminal. Specifically I want to highlight a text section in my web browser, then paste this commented to a file after the shebang line.
the code I have so far is this:
xclip -o | sed 's/^/#/' | sed '2n' myscript.pl
the first command takes the text that I have highlighted in my browser
the second command comments the lines by adding #
the last bit does not work..
what I am trying to do here is append the text after line number 2 to my script. But obviously I am doing this wrong.. Does anyone have a helpful suggestion?
You can use sed read for safely handling all types of input, including input with special characters and multiple lines. This requires an intermediate file:
xclip -o | sed -e 's/^/#/g' -e '$s/$/\n/' > TMP && sed -i '1r TMP' den && rm TMP
sed only operates on one input stream (either a pipe or a file), if you are using the output of xclip as the data stream then you can't also tell sed to read from a file. Instead you could use command substitution to store the modified output, and use that in a separate command. How about:
sed "2i$(xclip -o | sed 's/^/#/')" myscript.pl
This will print the amended file to stdout, if you want to edit the file itself then use the -i flag.
I am using bash with linux to accomplish adding content to the top of a file.
Thus far i know that i am able to get this done by using a temporary file. so
i am doing it this way:
tac lines.bar > lines.foo
echo "a" >> lines.foo
tac lines.foo > lines.bar
But is there a better way of doing this without having to write a second file?
echo a | cat - file1 > file2
same as shellter's
and sed in one line.
sed -i -e '1 i<whatever>' file1
this will insert to file1 inplace.
the sed example i referred to
tac is very 'expensive' solution, especially as you need to use it 2x. While you still need to use a tmp file, this will take less time:
edit per notes from KeithThompson, now using '.$$' filename and condtional /bin/mv.
{
echo "a"
cat file1
} > file1.$$ && /bin/mv file1.$$ file1
I hope this helps
Using a named pipe and in place replacement with sed, you could add the output of a command at the top of a file without explicitly needing a temporary file:
mkfifo output
your_command >> output &
sed -i -e '1x' -e '1routput' -e '1d' -e '2{H;x}' file
rm output
What this does is buffering the output of your_command in a named pipe (fifo), and inserts in place this output using the r command of sed. For that, you need to start your_command in the background to avoid blocking on output in the fifo.
Note that the r command output the file at the end of the cycle, so we need to buffer the 1st line of file in the hold space, outputting it with the 2nd line.
I write without explicitly needing a temporary file as sed might use one for itself.
When I am trying to sort a file and save the sorted output in itself, like this
sort file1 > file1;
the contents of the file1 is getting erased altogether, whereas when i am trying to do the same with 'tee' command like this
sort file1 | tee file1;
it works fine [ed: "works fine" only for small files with lucky timing, will cause lost data on large ones or with unhelpful process scheduling], i.e it is overwriting the sorted output of file1 in itself and also showing it on standard output.
Can someone explain why the first case is not working?
As other people explained, the problem is that the I/O redirection is done before the sort command is executed, so the file is truncated before sort gets a chance to read it. If you think for a bit, the reason why is obvious - the shell handles the I/O redirection, and must do that before running the command.
The sort command has 'always' (since at least Version 7 UNIX) supported a -o option to make it safe to output to one of the input files:
sort -o file1 file1 file2 file3
The trick with tee depends on timing and luck (and probably a small data file). If you had a megabyte or larger file, I expect it would be clobbered, at least in part, by the tee command. That is, if the file is large enough, the tee command would open the file for output and truncate it before sort finished reading it.
It doesn't work because '>' redirection implies truncation, and to avoid keeping the whole output of sort in the memory before re-directing to the file, bash truncates and redirects output before running sort. Thus, contents of the file1 file will be truncated before sort will have a chance to read it.
It's unwise to depend on either of these command to work the way you expect.
The way to modify a file in place is to write the modified version to a new file, then rename the new file to the original name:
sort file1 > file1.tmp && mv file1.tmp file1
This avoids the problem of reading the file after it's been partially modified, which is likely to mess up the results. It also makes it possible to deal gracefully with errors; if the file is N bytes long, and you only have N/2 bytes of space available on the file system, you can detect the failure creating the temporary file and not do the rename.
Or you can rename the original file, then read it and write to a new file with the same name:
mv file1 file1.bak && sort file1.bak > file1
Some commands have options to modify files in place (for example, perl and sed both have -i options (note that the syntax of sed's -i option can vary). But these options work by creating temporary files; it's just done internally.
Redirection has higher precedence. So in the first case, > file1 executes first and empties the file.
The first command doesn't work (sort file1 > file1), because when using the redirection operator (> or >>) shell creates/truncates file before the sort command is even invoked, since it has higher precedence.
The second command works (sort file1 | tee file1), because sort reads lines from the file first, then writes sorted data to standard output.
So when using any other similar command, you should avoid using redirection operator when reading and writing into the same file, but you should use relevant in-place editors for that (e.g. ex, ed, sed), for example:
ex '+%!sort' -cwq file1
or use other utils such as sponge.
Luckily for sort there is the -o parameter which write results to the file (as suggested by #Jonathan), so the solution is straight forward: sort -o file1 file1.
Bash open a new empty file when reads the pipe, and then calls to sort.
In the second case, tee opens the file after sort has already read the contents.
You can use this method
sort file1 -o file1
This will sort and store back to the original file. Also, you can use this command to remove duplicated line:
sort -u file1 -o file1
How can we append text in a file via a one-line command without using io redirection?
If you don't mind using sed then,
$ cat test
this is line 1
$ sed -i '$ a\this is line 2 without redirection' test
$ cat test
this is line 1
this is line 2 without redirection
As the documentation may be a bit long to go through, some explanations :
-i means an inplace transformation, so all changes will occur in the file you specify
$ is used to specify the last line
a means append a line after
\ is simply used as a delimiter
If you just want to tack something on by hand, then the sed answer will work for you. If instead the text is in file(s) (say file1.txt and file2.txt):
Using Perl:
perl -e 'open(OUT, ">>", "outfile.txt"); print OUT while (<>);' file*.txt
N.B. while the >> may look like an indication of redirection, it is just the file open mode, in this case "append".
You can use the --append feature of tee:
cat file01.txt | tee --append bothFiles.txt
cat file02.txt | tee --append bothFiles.txt
Or shorter,
cat file01.txt file02.txt | tee --append bothFiles.txt
I assume the request for no redirection (>>) comes from the need to use this in xargs or similar. So if that doesn't count, you can mute the output with >/dev/null.
You can use Vim in Ex mode:
ex -sc 'a|BRAVO' -cx file
a append text
x save and close
On Linux/GNU systems, the simplest and cleanest solution is:
dd of=oldfile oflag=append conv=notrunc
Simple and clean, because no quoting or backslashitis is required.
Unfortunately, this also doesn't work on BSD (and so, on Darwin), because their dd has no oflag . Argh! Can anyone suggest how to do it with the BSD dd ?