Can I get an explanation on escaping shell variables?

j0h

4/30/24, 2:55 AM

I have a C program I am integrating into a shell script. There is a line that gets the time of birth for $HOME using system("stat $HOME | awk 'FNR == 8 {print}' | awk '{print $2}'"); and it works great in C. when I echo that line to bash with:

echo system(\"stat $HOME | awk 'FNR == 8 {print}' | awk '{print $2}'\");>> file.c

the second awk pipe doesn't happen. so instead of getting: 2021-04-11

I get: Birth: 2021-04-11 18:28:25.373220337 -0400

I don't understand the issue here. I checked the resulting C code, and its missing $2 which is why the 2nd awk function fails. ok, so I escaped the with \$2 but I don't understand why I needed to. I didn't escape the other shell variable $HOME and that works fine.

1 + 1

command-line

bash

scripts

steeldriver

4/30/24, 1:18 PM

Presumably it "works fine" when you don't escape `$HOME` because you're executing the resulting program in the same environment that you're generating it in - so it doesn't matter whether `$HOME` is expanded by the current shell or by the shell invoked by the `system` call. FYI you can probably save a lot of headaches by using a [quoted here-doc](https://stackoverflow.com/a/47436574/4440445) to construct your code, rather than an `echo` command.

Score:3

Ubuntu

Marco

4/30/24, 6:31 AM

All $ are interpreted as a variable by the shell (bash) inside ". They need to be escaped.

Additionally, the ( are interpreted by the shell, which needs a complete quoting of the string.

echo "system(\"stat \$HOME | awk 'FNR == 8 {print}' | awk '{print \$2}'\")" >> file.c

Besides this, you will get a problem as soon as $HOME has a space inside.

Reminder: There are better ways in bash and in C to get the needed information.

e.g. for bash:

date -d "$( stat -c %w "$HOME" )" +%F

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