问题
I need to assign the output of a command to a variable. The command I tried is:
grep UUID fstab | awk '/ext4/ {print $1}' | awk '{print substr($0,6)}'
I try this code to assign a variable:
UUID=$(grep UUID fstab | awk '/ext4/ {print $1}' | awk '{print substr($0,6)}')
However, it gives a syntax error. In addition I want it to work in a bash script.
The error is:
./upload.sh: line 12: syntax error near unexpected token ENE=$( grep UUID fstab | awk '/ext4/ {print $1}' | awk '{print substr($0,6)}'
)'
./upload.sh: line 12: ENE=$( grep UUID fstab | awk '/ext4/ {print $1}' | awk '{print substr($0,6)}'
)'
回答1:
well, using the '$()' subshell operator is a common way to get the output of a bash command. As it spans a subshell it is not that efficient.
I tried :
UUID=$(grep UUID /etc/fstab|awk '/ext4/ {print $1}'|awk '{print substr($0,6)}')
echo $UUID # writes e577b87e-2fec-893b-c237-6a14aeb5b390
it works perfectly :)
EDIT:
Of course you can shorten your command :
# First step : Only one awk
UUID=$(grep UUID /etc/fstab|awk '/ext4/ {print substr($1,6)}')
Once more time :
# Second step : awk has a powerful regular expression engine ^^
UUID=$(cat /etc/fstab|awk '/UUID.*ext4/ {print substr($1,6)}')
You can also use awk with a file argument ::
# Third step : awk use fstab directlty
UUID=$(awk '/UUID.*ext4/ {print substr($1,6)}' /etc/fstab)
回答2:
Just for trouble-shooting purposes, and something else to try to see if you can get this to work, you could also try to use "backticks", e.g,
cur_dir=`pwd`
would save the output of the pwd command in your variable cur_dir, though using $() approach is generally preferable.
To quote from a pages given to me on http://unix.stackexchange.com:
The second form
`COMMAND`(using backticks) is more or less obsolete for Bash, since it has some trouble with nesting ("inner" backticks need to be escaped) and escaping characters. Use$(COMMAND), it's also POSIX!
来源：https://stackoverflow.com/questions/11527899/assigning-output-of-a-command-to-a-variablebash