Unix command to prepend text to a file

Question

Is there a Unix command to prepend some string data to a text file?

Something like:

prepend "to be prepended" text.txt

Answer 1

sed -i.old '1s;^;to be prepended;' inFile

• -i writes the change in place and take a backup if any extension is given. (In this case, .old)
• 1s;^;to be prepended; substitutes the beginning of the first line by the given replacement string. 1 means act on the first line, s means replace, ; is the delimiter we've selected for s (/ is a common choice as in s/// but any character can be used) and ^ is the regular expression that matches the beginning of a line

If you want to add a new line to the beginning of the file, you need to add \n on the replacement as in:

sed -i.old '1s;^;to be prepended\n;' inFile

Answer 2

Solution for smaller files:

printf '%s\n%s' 'text to prepend' "$(cat file.txt)" > file.txt

Note that this is safe on all kind of inputs, because there are no expansions. For example, if you want to prepend !@#$%^&*()ugly text\n\t\n, it will just work:

printf '%s\n%s' '!@#$%^&*()ugly text\n\t\n' "$(cat file.txt)" > file.txt

UPDATE 2023: Re-reading my own answer. It's important to not that this solution is NOT SAFE if e.g. you have a power outage while the file was being written to disk. The reason is that file content filling is not atomic in the filesystem, so you may be interrupted while you've already read the file, started writing to the old address (thus beginning a full overwrite) but not finishing it. This problem also applies to all solutions that do not create the intermediate file, included the currently most upvoted solution.

The last part left for consideration is whitespace removal at end of file during command substitution "$(cat file.txt)". All work-arounds for this are relatively complex. If you want to preserve newlines at end of file.txt, see this: https://stackoverflow.com/a/22607352/1091436

Answer 3

With ex,

ex - $file << PREPEND
-1
i
prepended text
.
wq
PREPEND

The ex commands are

• -1 Go to the very beginning of the file
• i Begin insert mode
• . End insert mode
• wq Save (write) and quit

Answer 4

you can use variables

Even though a bunsh of answers here work pretty well, I want to contribute this one-liner, just for completeness. At least it is easy to keep in mind and maybe contributes to some general understanding of bash for some people.

PREPEND="new line 1"; FILE="text.txt"; printf "${PREPEND}\n`cat $FILE`" > $FILE

In this snippe just replace text.txt with the textfile you want to prepend to and new line 1 with the text to prepend.

example

$ printf "old line 1\nold line 2" > text.txt
$ cat text.txt; echo ""
old line 1
old line 2
$ PREPEND="new line 1"; FILE="text.txt"; printf "${PREPEND}\n`cat $FILE`" > $FILE 
$ cat text.txt; echo ""
new line 1
old line 1
old line 2
$

Answer 5

If it's acceptable to replace the input file:

Note:

• Doing so may have unexpected side effects, notably potentially replacing a symlink with a regular file, ending up with different permissions on the file, and changing the file's creation (birth) date.

• sed -i, as in Prince John Wesley's answer, tries to at least restore the original permissions, but the other limitations apply as well.

Here's a simple alternative that uses a temporary file (it avoids reading the whole input file into memory the way that shime's solution does):

{ printf 'to be prepended'; cat text.txt; } > tmp.txt && mv tmp.txt text.txt

^{Using a group command ({ ...; ...; }) is slightly more efficient than using a subshell ((...; ...)), as in 0xC0000022L's solution.}

The advantages are:

• It's easy to control whether the new text should be directly prepended to the first line or whether it should be inserted as new line(s) (simply append \n to the printf argument).

• Unlike the sed solution, it works if the input file is empty (0 bytes).

---

The sed solution can be simplified if the intent is to prepend one or more whole lines to the existing content (assuming the input file is non-empty):

sed's i function inserts whole lines:

With GNU sed:

# Prepends 'to be prepended' *followed by a newline*, i.e. inserts a new line.
# To prepend multiple lines, use '\n' as part of the text.
# -i.old creates a backup of the input file with extension '.old'
sed -i.old '1 i\to be prepended' inFile

A portable variant that also works with macOS / BSD sed:

# Prepends 'to be prepended' *followed by a newline*
# To prepend multiple lines, escape the ends of intermediate
# lines with '\'
sed -i.old -e '1 i\
to be prepended' inFile

Note that the literal newline after the \ is required.

---

If the input file must be edited in place (preserving its inode with all its attributes):

Using the venerable ed POSIX utility:

Note:

• ed invariably reads the input file as a whole into memory first.

To prepend directly to the first line (as with sed, this won't work if the input file is completely empty (0 bytes)):

ed -s text.txt <<EOF
1 s/^/to be prepended/
w
EOF

• -s suppressed ed's status messages.
• Note how the commands are provided to ed as a multi-line here-document (<<EOF\n...\nEOF), i.e., via stdin; by default string expansion is performed in such documents (shell variables are interpolated); quote the opening delimiter to suppress that (e.g., <<'EOF').
• 1 makes the 1st line the current line
• function s performs a regex-based string substitution on the current line, as in sed; you may include literal newlines in the substitution text, but they must be \-escaped.
• w writes the result back to the input file (for testing, replace w with ,p to only print the result, without modifying the input file).

To prepend one or more whole lines:

As with sed, the i function invariably adds a trailing newline to the text to be inserted.

ed -s text.txt <<EOF
0 i
line 1
line 2
.
w
EOF

• 0 i makes 0 (the beginning of the file) the current line and starts insert mode (i); note that line numbers are otherwise 1-based.
• The following lines are the text to insert before the current line, terminated with . on its own line.

Answer 6

Editor's note:

• This command will result in data loss if the input file happens to be larger than your system's pipeline buffer size, which is typically 64 KB nowadays. See the comments for details.

---

In some circumstances prepended text may available only from stdin. Then this combination shall work.

echo "to be prepended" | cat - text.txt | tee text.txt

If you want to omit tee output, then append > /dev/null.

Answer 7

% echo blaha > blaha
% echo fizz > fizz
% cat blaha fizz > buzz
% cat buzz 
blaha
fizz

Answer 8

For future readers who want to append one or more lines of text (with variables or even subshell code) and keep it readable and formatted, you may enjoy this:

echo "Lonely string" > my-file.txt

Then run

cat <<EOF > my-file.txt
Hello, there!

$(cat my-file.txt)
EOF

Results of cat my-file.txt:

Hello, there!

Lonely string

This works because the read of my-file.txt happens first and in a subshell. I use this trick all the time to append important rules to config files in Docker containers rather than copy over entire config files.

Answer 9

Process Substitution

I'm surprised no one mentioned this.

cat <(echo "before") text.txt > newfile.txt

which is arguably more natural than the accepted answer (printing something and piping it into a substitution command is lexicographically counter-intuitive).

...and hijacking what ryan said above, with sponge you don't need a temporary file:

sudo apt-get install moreutils
<<(echo "to be prepended") < text.txt | sponge text.txt

EDIT: Looks like this doesn't work in Bourne Shell /bin/sh

---

Here String (zsh only)

Using a here-string - <<<, you can do:

<<< "to be prepended" < text.txt | sponge text.txt

Answer 10

As tested in Bash (in Ubuntu), if starting with a test file via;

echo "Original Line" > test_file.txt

you can execute;

echo "$(echo "New Line"; cat test_file.txt)" > test_file.txt

or, if the version of bash is too old for $(), you can use backticks;

echo "`echo "New Line"; cat test_file.txt`" > test_file.txt

and receive the following contents of "test_file.txt";

New Line
Original Line

No intermediary file, just bash/echo.

Answer 11

I'd recommend defining a function and then importing and using that where needed.

prepend_to_file() { 
    file=$1
    text=$2

    if ! [[ -f $file ]] then
        touch $file
    fi

    echo "$text" | cat - $file > $file.new

    mv -f $file.new $file
}

Then use it like so:

prepend_to_file test.txt "This is first"
prepend_to_file test.txt "This is second"

Your file contents will then be:

This is second
This is first

I'm about to use this approach for implementing a change log updater.

Answer 12

printf '%s\n%s\n' "to be prepended" "$(cat text.txt)" >text.txt

Answer 13

Prefer Adam's answer

We can make it easier to use sponge. Now we don't need to create a temporary file and rename it by

echo -e "to be prepended \n another line" | cat - text.txt | sponge text.txt

Answer 14

Another fairly straight forward solution is:

    $ echo -e "string\n" $(cat file)

Answer 15

# create a file with content..
echo foo > /tmp/foo
# prepend a line containing "jim" to the file
sed -i "1s/^/jim\n/" /tmp/foo
# verify the content of the file has the new line prepened to it
cat /tmp/foo

Answer 16

If you like vi/vim, this may be more your style.

printf '0i\n%s\n.\nwq\n' prepend-text | ed file

Answer 17

Another way using sed:

sed -i.old '1 {i to be prepended
}' inFile

If the line to be prepended is multiline:

sed -i.old '1 {i\ 
to be prepended\
multiline
}' inFile

Answer 18

This will work to form the output. The - means standard input, which is provide via the pipe from echo.

echo -e "to be prepended \n another line" | cat - text.txt

To rewrite the file a temporary file is required as cannot pipe back into the input file.

echo "to be prepended" | cat - text.txt > text.txt.tmp
mv text.txt.tmp text.txt

Answer 19

This is one possibility:

(echo "to be prepended"; cat text.txt) > newfile.txt

you'll probably not easily get around an intermediate file.

Alternatives (can be cumbersome with shell escaping):

sed -i '0,/^/s//to be prepended/' text.txt

Answer 20

Probably nothing built-in, but you could write your own pretty easily, like this:

#!/bin/bash
echo -n "$1" > /tmp/tmpfile.$$
cat "$2" >> /tmp/tmpfile.$$
mv /tmp/tmpfile.$$ "$2"

Something like that at least...