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Hi I want to prepend text to a file. For example I want to add tasks to the beginning of a todo.txt file. I am aware of echo 'task goes here' >> todo.txt but that adds the line to the end of the file (not what I want).

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11 Answers 11

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453

Linux :

echo 'task goes here' | cat - todo.txt > temp && mv temp todo.txt

or

sed -i '1s/^/task goes here\n/' todo.txt

or

sed -i '1itask goes here' todo.txt

Mac os x :

sed -i '.bak' '1s/^/task goes here\'$'\n/g' todo.txt

or

echo -e "task goes here\n$(cat todo.txt)" > todo.txt

or

echo 'task goes here' | cat - todo.txt > temp && mv temp todo.txt
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  • 6
    the first one works great! would you mind explaining the logic? im not particularly sure how to interpret the syntax. – user479534 Feb 18 '11 at 4:24
  • 57
    @user8347: Pipe (|) the message (echo '...') to cat which uses - (standard input) as the first file and todo.txt as the second. cat conCATenates multiple files. Send the output (>) to a file named temp. If there are no errors (&&) from cat then rename (mv) the temp file back to the original file (todo.txt). – Dennis Williamson Feb 18 '11 at 4:51
  • 1
    @itaifrenkel: I'd have to see what you did, but if cat receives a literal backslash n, it won't convert it to a newline. Something else must have done that. Instead of cat, try piping into hexdump -C to see if you're actually sending backslash and n or if it's a newline. You could also try cat -e to show line endings. – Dennis Williamson Jan 26 '14 at 21:58
  • 1
    Using 2 and 3 (3 seems simpler to me) allows you to prepend text to many files at once. – Felix Jan 27 '14 at 15:24
  • 4
    @Kira: The 1 means do the next command only on line one of the file and the i command is insert. Look in the man page under the "Addresses" section and in the "Zero- or One- address commands" section. – Dennis Williamson Oct 23 '15 at 16:33
86

A simpler option in my opinion is :

echo -e "task goes here\n$(cat todo.txt)" > todo.txt

This works because the command inside of $(...) is executed before todo.txt is overwritten with > todo.txt

While the other answers work fine, I find this much easier to remember because I use echo and cat every day.

EDIT: This solution is a very bad idea if there are any backslashes in todo.txt, because thanks to the -e flag echo will interpret them. Another, far easier way to get newlines into the preface string is...

echo "task goes here
$(cat todo.txt)" > todo.txt

...simply to use newlines. Sure, it isn't a one-liner anymore, but realistically it wasn't a one-liner before, either. If you're doing this inside a script, and are worried about indenting (e.g. you're executing this inside a function) there are a few workarounds to make this still fit nicely, including but not limited to:

echo 'task goes here'$'\n'"$(cat todo.txt)" > todo.txt

Also, if you care about whether a newline gets added to the end of todo.txt, don't use these. Well, except the second-to-last one. That doesn't mess with the end.

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  • 1
    I don't get $(...) executed at all. – SCL Feb 22 '13 at 17:10
  • 2
    That might work better (or at all) with double quotes instead of single… – ℝaphink May 22 '13 at 7:25
  • 5
    Won't the -e also convert escape sequences inside todo.txt? – mk12 Sep 2 '13 at 21:23
  • 2
    Workarounds yield --> cat: <destination filename>: input file is output file ... – ingyhere Jan 9 '15 at 2:10
  • printf would be a lot more consistent across platforms and should generally work more smoothly than echo -e – Peter Berg Feb 17 '18 at 20:10
31

The moreutils have a nice tool called sponge:

echo "task goes here" | cat - todo.txt | sponge todo.txt

It'll "soak up" STDIN and then write to the file, which means you don't have to worry about temporary files and moving them around.

You can get moreutils with many Linux distros, through apt-get install moreutils, or on OS X using Homebrew, with brew install moreutils.

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  • I would go for (echo 'ble'; cat todo.txt) :-) – Ciro Santilli TRUMP BAN IS BAD Dec 4 '18 at 9:37
  • I would use tee instead of sponge which come by default on most distro echo "task goes here" | cat - todo.txt | tee todo.txt – Angel115 Apr 28 '20 at 17:12
  • 1
    The tee example is not correct askubuntu.com/a/752451 – Steven Penny May 1 '20 at 14:51
  • @stevenpenny Good point. Now I remember why I didn't suggest it originally. I came back to this answer not noticing that the output file name was the same as the input, which is why you need sponge to buffer it all before writing it back out. – slhck May 2 '20 at 12:07
  • @Angel115 Please see the above comments. – slhck May 2 '20 at 12:08
16

You can use the POSIX tool ex:

ex a.txt <<eof
1 insert
Sunday
.
xit
eof

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ex.html

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5

You can create a new, temporary file.

echo "new task" > new_todo.txt
cat todo.txt >> new_todo.txt
rm todo.txt
mv new_todo.txt todo.txt

You might also use sed or awk. But basically the same thing happens.

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  • 1
    Say you're out of disk space so that new_todo.txt gets written only partially. Your solution appears to lose the original file. – NPE Feb 17 '11 at 10:31
  • Who runs out of disk space? ;-) It's just a simple example. – Keith Feb 17 '11 at 10:33
  • 2
    @Keith Someone working on a VM who didn't expect to need a particularly large virtual drive. Or someone moving a large file. In any case, the real argument against this is directory permissions; if you don't have permission to create new files in the given directory, the only command that will successfully execute in your script is the rm of the original file. – Parthian Shot Jul 1 '14 at 20:23
3

If the text file is small enough to fit in memory, you don't have to create a temporary file to replace it with. You can load it all into memory and write it back out to the file.

echo "$(echo 'task goes here' | cat - todo.txt)" > todo.txt

It's impossible to add lines to the beginning of the file without over writing the whole file.

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  • Just to ask the obvious question: Where's the character limit of shell variables? – nixda Jan 9 '13 at 22:59
  • As far as I'm aware, it's only limited by the amount of memory available. I've filled up variables well over 100MB into memory. text=$(cat file). Be careful to only use text though, because shell variables aren't binary clean mywiki.wooledge.org/BashFAQ/058 – Rucent88 Jan 14 '13 at 1:48
3

You cannot insert content at the beginning of a file. The only thing you can do is either replace existing content or append bytes after the current end of file.

Any solution to your question then requires a temporary file (or buffer) to be created (on memory or on disk) which will eventually overwrite the original file.

Beware of not losing data by preserving the original file while creating the new one, should the file system happen to be full during the process. eg:

cat <(echo task go there) todo.txt > todo.txt.new && mv todo.txt.new todo.txt
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  • Downvoters are welcome to explain their motivation. None of the remaining answers, including the accepted one, do contradict anything in my reply. – jlliagre Apr 12 '16 at 13:13
  • This is difficult to parse as the < ... > look like brackets, which I assume they are not. A space between the < and the ( might help? – dumbledad Jun 1 '18 at 9:20
  • This is not working for me. echo HOME=\"/g/Users/timregan/\" | cat - 'F:\Program Files\Git\etc\profile' works but cat <echo HOME=\"/g/Users/timregan/\" 'F:\Program Files\Git\etc\profile' gives the error "echo: No such file or directory" – dumbledad Jun 1 '18 at 9:35
  • @dumbledad You are overthinking my reply. There is nothing for you to parse in it. A space between the < and the ( would break the syntax. Try cat <(echo HOME=\"/g/Users/timregan/\") 'F:\Program Files\Git\etc\profile' – jlliagre Jun 2 '18 at 1:11
2

You can use tee:

echo 'task goes here' | cat - todo.txt | tee todo.txt
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1

Final answer

cat <<< "prepended text
$(cat test.txt)" > test.txt

Context

I wasn't too satisfied with the answers as they felt like too much typing. I liked John Alberts his answer but couldn't stand to type -e. Unfortunately, I accidentally read over John Alberts his echo 2 liner as well (significantly reducing the value of this answer and me 30 minutes playing around, but oh well, it happens).

In any case, I was focused on finding something that meant you only needed to type the filename and text you want to prepend.

Moreover, I was searching for something that looked aesthetically intuitive. With that I mean: the preprend needs to physically show, even if it'd be an illusion it'd have the effect of a mnemonic.

So I tried an approach with herestrings since in the right context they reduce cognitive strain (i.e. typing < 3 times doesn't require too much thinking power).

I created a file test.txt with the word "monkeys".

And I typed:

cat <<< "prepend
> $(< test.txt)"

Output:

prepend
monkeys

A bit of clarification:

You need to manually press enter yourself.

On the second line the > is from the shell itself, you don't need to type that.

Notes:

(1) What I couldn't manage was a one liner. There seems to be no herestring combination in which I could use $() and \n. Which is why you need to press the newline manually yourself.

(2) $(< test.txt) has the same effect as $(cat test.txt). The Bash Reference Manual states:

The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

So you could also do: 

cat <<< "prepend
> $(cat test.txt)"

More typing, but I admit a bit less cognitive strain since cat is being typed twice and is more well-known than the trick of the Bash Reference Manual.

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0

GitBash + Windows10 + Multline:

Here is a version that lets you use multi-line strings.

##############################################
## This section for demo purpose only,      ##
## So you can save entire file as           ##
## whatever.sh and run it.                  ##
##                                          ##
##############################################
> MY_TARGET_FILE.txt ##Make Or Clear File
echo "[STARTER_CONTENT]" >> MY_TARGET_FILE.txt
##############################################

## Below is main code:

##################################################
TARGET_FILE_VARIABLE="MY_TARGET_FILE.txt"
ADD_TO_HEAD_VARIABLE=$(cat << "HEREDOC_HEAD_TEXT"
//|  +-------------------------------------+   |//
//|  |                                     |   |//
//|  |     MESSAGE_FOR_HEAD_OF_FILE        |   |//
//|  |                                     |   |//
//|  +-------------------------------------+   |//
HEREDOC_HEAD_TEXT
)
##################################################
TAR=$TARGET_FILE_VARIABLE                       ##
TEX=$ADD_TO_HEAD_VARIABLE                       ##
echo "$TEX" | cat - $TAR > TEMP && mv TEMP $TAR ##
##################################################

## Expected contents of MY_TARGET_FILE.txt :
## //|  +-------------------------------------+   |//
## //|  |                                     |   |//
## //|  |     MESSAGE_FOR_HEAD_OF_FILE        |   |//
## //|  |                                     |   |//
## //|  +-------------------------------------+   |//
## [STARTER_CONTENT]
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0

Similar solution with awk

wolf@linux:~$ cat todo.txt 
a
b
wolf@linux:~$

wolf@linux:~$ awk 'BEGIN {print "task goes here"} {print}' todo.txt 
task goes here
a
b
wolf@linux:~$
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  • But you haven’t shown how to put the updated contents back into the file,  which is the hard/interesting part of the question. – Scott Jan 17 at 1:55

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