如何从 bash 中的文件打印环境变量（不使用源）？

Question

Many shell scripts, including the bash profile, are simply lists of environment variable settings. One such script on Debian is /etc/os-release which looks like this:

PRETTY_NAME="Debian GNU/Linux 10 (buster)"
NAME="Debian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"

The VERSION_CODENAME is particularly useful for adding to apt sources ( /etc/apt/sources.list ) for, say,Steam to work on ChromeOS . Note that most instructions hard-code this value which can cause compatibility problems.

So my question then is how to echo an env var such as VERSION_CODENAME from a file such as /etc/os-release without using source ? That's key because I don't want to clutter up my environment variables with these for a one-time use.

Here's what I know I can do now but it leaves the variables in my current environment which is undesirable:

source /etc/os-release && echo "deb http://httpredir.debian.org/debian/ $VERSION_CODENAME main contrib non-free | sudo tee -a /etc/apt/sources.list"

I thought perhaps there is a way to start a new (temporary) bash process and load the variables into that environment. I haven't been able to figure that out without an actual shell script.

Answer 1

4 different answer here...

Sorry, but there is more than one way;). You may found a lot of other ways, but there are the most appropriate (quickness, efficience, footprint, readability...).

1. import through sed to populate associative arrray :

declare -A IMPORTED="($(sed < /etc/os-release 's/^\([^=]\+\)=/[\1]=/'))"

Then

echo ${IMPORTED[VERSION_CODENAME]}
buster

2. extract required field (by using sed again)

AltVersionCodename=$(sed </etc/os-release -ne 's/^VERSION_CODENAME=//p')
echo $AltVersionCodename
buster

3. parenthesis to drop down to subshell using his own environment

( . /etc/os-release ; echo $VERSION_CODENAME )
buster

echo $VERSION_CODENAME
 

Current environment don't know about $VERSION_CODENAME

4. reading variable file in pure bash, without forks

As we are working on a small file, we could use bash loop to read the file until required info is found:

while IFS== read varname value;do
    [ "$varname" = "VERSION_CODENAME" ] &&
        ImportedVersionCodename=$value && break
done </etc/os-release
echo $ImportedVersionCodename 
buster

Answer 2

I thought perhaps there is a way to start a new (temporary) bash process and load the variables into that environment.

That's what using parenthesis to create a subshell does.

(
  . /etc/os-release
  echo "deb http://httpredir.debian.org/debian/ $VERSION_CODENAME main contrib non-free" \
  | sudo tee -a /etc/apt/sources.list
)

...as soon as the ) is hit, your variables are removed, as the subshell they were loaded into exits.

Answer 3

Or using read and printf like this:

while read line; do
    name=${line%%=*}
    data=${line#*=}
    printf -v $name "${data//\"}"
done < vars

Answer 4

The next function only sources the line with the key (value in single quotes).

my_set() {
   configfile="$1"
   key="$2"
   print -v "$key" $(sed -n "s/^${key}=//p" "${configfile}")
}
my_set /etc/os-release VERSION_CODENAME
echo "deb http://httpredir.debian.org/debian/ $VERSION_CODENAME main.."

When you don't need the var in the environment, use

my_set2() {
   configfile="$1"
   key="$2"
   sed -n "s/^${key}=//p" "${configfile}"
}

echo "deb http://httpredir.debian.org/debian/ $(my_set2 /etc/os-release VERSION_CODENAME) main.."