Bash变量赋值奇怪的行为

Question

I am trying to write a bash script, while doing that am stuck here:

I do not understand why this works:

MSG=$(pwd)
echo $MSG

Output:

/home/harsh/source/git/trunk

BUT this does not:

MSG=$(java -version)
echo $MSG

Output:

BLANK

Please help!

Answer 1

Some commands might need 2>&1 at the end to get any output:

MSG=$(java -version 2>&1)

It sends any standard error(2) to wherever standard output(1) is redirected.

Answer 2

Error messages are typically written to the standard error stream stderr instead of the standard output stream stdout . If java -version generates an error instead of what you expected (printing the version), it will likely do so to stderr . It is also possible that the version information could also printed to stderr .

The command substitution $() takes the output from stdout of what's inside the $() and substitutes it in its place. In case of an error, this could be nothing. If you are typing this from a terminal, you should still see any output (eg error messages) from java 's stderr in the terminal.