bash sed truncates file instead of replacing text

bash sed truncates file instead of replacing text.

I want to update variables in bashrc source with sed but to my surprise sed truncates the file.

For testing i have a little file abc:

export PATH=$PATH:$HOME/bin:$JAVA_HOME/bin
export myip=1.2.3.4

and this test script:

#!/bin/bash
myip=1.2.3.4; newip=5.6.7.8
sed -ni 's/$myip/$newip/' abc 

running the script leaves abc empty.
Why is this so?

I am using Ubuntu 22.04 on an Intel I7.

That is because of the quotes that you use, the -n option and the absence of a p behind the substitute.

In bash, the single quote ' makes sure that the text is taken literally, without variable expansion. So, you would probably want to use double quotes ".

Because yo use -n, you will probably want to print the line that you want to keep as well with the p:

sed -ni "s/$myip/$newip/p" abc

Note that, if you want to keep the lines that do not match, you just remove the -n.

Note too that the . in your myip pattern matches any character. So, your pattern 1.2.3.4 will match the 112.3.4 part of the IP address 112.3.4.9, which will then become 5.6.7.8.9.

There are two errors in your script.

1. When you enclose a string into single quotes ', the substitution of the bash variables will not happen. You have to use double quotes ".

2. If you use the -n option, you will produce no output except with the command p that you don't use.

The correct script is:

#!/bin/bash
myip=1.2.3.4; newip=5.6.7.8
sed -i "s/$myip/$newip/" abc