bash中导出环境变量时什么时候用括号？

Question

I've been trying to figure out what is the purpose of brackets in the bash environment variables. For example, in the below actual example of code, why are some of the definitions using a {} around the PATH , for example, export...=.../${PATH} . Note also that some of the definitions are different: some use {$ECLIPSE_DIR} with the $ within the brackets; some use ${PATH} with the $ outside of the brackets, and some omit brackets altogether. This code generally works, although sometimes errors like the one shown at the bottom are shown (they appear to be transient), and I'm not sure why such errors only show up sometimes and not others.

What are the common practices concerning ways to include bash environment variables, when should brackets be used, and what is the difference between putting the $ inside and outside of brackets? Also, why do some lines have an " export " before the variable name, and some do not? What is the difference here?

# ECLIPSE
ECLIPSE_DIR=$HOME/eclipse
PATH=${PATH}:{$ECLIPSE_DIR}

# ANT
ANT_HOME=/usr/bin/ant
PATH=${ANT_HOME}/bin:${PATH}
export ANT_HOME PATH

# GRADLE
export GRADLE_HOME=/usr/local/gradle
export PATH=$GRADLE_HOME/bin:$PATH</code>

-bash: export: `/usr/bin/ant/bin:/usr/local/bin:{/Users/me/eclipse}:/usr/bin/scala-2.9.0.1/bin:/usr/local/mysql/bin:/usr/local/bin:{/Users/me/eclipse}': not a valid identifier

Answer 1

The braces are usually used for clarity, but a practical use is breaking up text from variable names. Say I had the following:

$ word="ello"
$ echo "h$word"
hello
$ echo "y$wordw" # bash tries to find the var wordw, and replaces with a blank
y
$ echo "y${word}w"
yellow

Variable names are automatically separated by most punctuation (notably. or /).

echo "$word/$word.$word"
ello/ello.ello

Looking at that error you presented, {$ECLIPSE_DIR} gets the variable expanded and then surrounded with literal open and close braces. I think the solution should be changing it to ${ECLIPSE_DIR}

In response to the export question, export is used to make a variable accessible to the shell that called this script. Any variable set up in a script does not exist once the script is finished unless it is exported. Hence, if you want your PATH to change after running that script, export PATH will have to be called before the script is over.

Answer 2

Braces are used with bash variables to disambiguate between variables. For example, consider this:

VAR=this
echo $VAR_and_that
echo ${VAR}_and_that

The first echo prints nothing, since bash thinks you are trying to echo out the var this_and_that which of course doesn't exist. The second echo doesn't have this problem and outputs this_and_that , since bash knows to expand out the VAR variable due to the braces.