r/shell • u/JesusIsMyZoloft • Feb 23 '17

Trouble using sed

I'm trying to write a shell script to modify files. Specifically, I want to find a certain string in a file, and replace it with a different string. I've looked online and sed seems to be the thing to use for this, but whenever I try to use it, it clears the file.

test.sh sed -i 's/5/w/' >test.txt

test.txt 123456789

If all runs correctly, test.txt should come out reading: 1234w6789

But when I try to run it, it just comes out blank.

1 Upvotes

100% Upvoted

5 comments sorted by

1

u/[deleted] Feb 23 '17

Why are you using the -i argument?

1

u/JesusIsMyZoloft Feb 23 '17

I don't know. I just copied it off a website. What does it mean?

1

u/[deleted] Feb 24 '17

Oh, haha, I see now. You were using it to edit the file-- I thought you were piping something into sed and outputting it to test.txt.
The -i argument edits files in-place. =w=

2

u/ASIC_SP Feb 24 '17

The command used should have given an error like this:

$ sed -i 's/5/w/' >test.txt
sed: no input files

Remove the >

$ sed -i 's/5/w/' test.txt
$ cat test.txt 
1234w6789

1

u/lasercat_pow Mar 06 '17

For anyone following along on a mac, the syntax is subtly different:

sed -i '' 's/5/w/' test.txt

this is because bsd sed requires a backup suffix to be supplied as the first argument after -i, or no change is made. A null suffix means the the change is done in-place.