Add a new line of text at the top of a file in bash shell

Question

I want to write a bash script that takes my file:

READ_ME.MD

two
three
four

and makes it

READ_ME.MD

one
two
three
four

There are a bunch of similar StackOverflow questions, but I tried their answers and haven't been successful.

These are the bash scripts that I have tried and failed with:

test.sh

sed '1s/^/one/' READ_ME.md > READ_ME.md

Result: Clears the contents of my file

test.sh

sed '1,1s/^/insert this /' READ_ME.md > READ_ME.md

Result: Clears the contents of my file

test.sh

sed -i '1s/^/one\n/' READ_ME.md

Result: sed: 1: "READ_ME.md": invalid command code R

Any help would be appreciated.

You could write to `READ_ME.md.tmp`, and then `mv READ_ME.md.tmp READ_ME.md`. Or use `cat READ_ME.md | sed ... > READ_ME.md`. The problem is you're truncating the file before it's read. — Robert Seaman, Apr 11 '18 at 15:15

anubhava · Accepted Answer · 2018-04-11T17:33:52.403

2

You can use this BSD sed command:

sed -i '' '1i\
one
' file

-i will save changes inline to file.

If you want to add a line at the top if same line is not already there then use BSD sed command:

line='one'

sed -i '' '1{/'"$line"'/!i\
'"$line"'
}' file

edited Apr 11 '18 at 17:33

answered Apr 11 '18 at 15:16

anubhava

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If I didn't want to save a backup would I just omit the `.bak` extension? – c_idle Apr 11 '18 at 15:19
@anubhave that doesn't modify the file. It just prints the result. – c_idle Apr 11 '18 at 15:21
No need to worry about if it's there. It doesn't work if I use `sed -i` using terminal on macOS. `sed: 1: "READ_ME.md": invalid command code R` – c_idle Apr 11 '18 at 17:22
You should have told OSX earlier :). On OSX use: `sed -i ''` to ignore ` backup files. – anubhava Apr 11 '18 at 17:31
You're right. Sorry about that! Is there docs for OSX bash? – c_idle Apr 11 '18 at 17:35
This is not BASH issue. BSD `sed` behaves differently and documentation of that is `man sed`. btw did you try edited answer? – anubhava Apr 11 '18 at 17:38
1

I did. Thank you! – c_idle Apr 11 '18 at 17:43

John Moon · Answer 2 · 2018-04-11T20:59:32.160

2

Your last example works for me with GNU sed. Based on the error message you added, I'd guess you're working on a Mac system? According to this blog post, a suffix argument may be required on Mac versions of sed:

sed -i ' ' '1s/^one\n/' READ_ME.md

edited Apr 11 '18 at 20:59

answered Apr 11 '18 at 15:22

John Moon

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`-i""` is **exactly** the same as `-i` -- there's no possible way `sed` can distinguish them, since they generate the same array of C strings after being parsed, and the actual OS-level interface whereby programs are invoked is the `execve` syscall, which accepts only such an array (and doesn't receive or otherwise have access to the original command line in full). Thus, it *absolutely must* be `-i ''`, with a space, to pass an empty string. – Charles Duffy Apr 11 '18 at 15:47
Good point, I hadn't considered that. Edited. Thanks, Charles! – John Moon Apr 11 '18 at 16:18
`-i ' '` -> with a space – kyodev Apr 11 '18 at 19:31

score 1 · Answer 3 · answered Apr 11 '18 at 15:26

1

If this is bash or zsh, you can use process substitution like so.

% cat x
one
two 
three

% cat <(echo "zero") x
zero
one
two 
three

Redirect this into a temp file and copy it back to the original

answered Apr 11 '18 at 15:26

Noufal Ibrahim

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score 0 · Answer 4 · answered Apr 11 '18 at 15:37

0

there is always ed

printf '%s\n' H 1i "one" . w | ed -s READ_ME.MD

answered Apr 11 '18 at 15:37

karakfa

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Add a new line of text at the top of a file in bash shell

4 Answers4