Question 176613 in Ubuntu Question posted in
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Can Ubuntu bash sed replace text instead of truncating file?

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.

Tags:

2

Answers


  1. 0

    That is because of the quotes that you use, the -n option ans 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 kep the lines that do not match, you just remove the -n.

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  2. 0

    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
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