i ran the following command from console it output the correct result:0,
sudo -H -u hadoop bash -c "/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \$4}'"
however if i put it in shell script and assigned it to a variable the 'awk' won't work anymore, it just output the whole result from 'grep':
replications=`sudo -H -u hadoop bash -c "/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \$4}'"`
echo "Replications: $replications"
result: Replications: Under replicated blocks: 0 how can i make the awk work again to only output 4th column which is 0 instead of the whole string?
In backtick command substitution, \ followed by $ means just $. From the POSIX standard:
Within the backquoted style of command substitution, backslash shall retain its literal meaning, except when followed by: '$', '`', or '\' (dollar sign, backquote, backslash). (...)
With the
$(command)form, all characters following the open parenthesis to the matching closing parenthesis constitute the command. Any valid shell script can be used for command, except a script consisting solely of redirections which produces unspecified results.
And yet more explicitly from the bash manpage:
When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $, `, or \. The first backquote not preceded by a backslash terminates the command substitution. When using the
$(command)form, all characters between the parentheses make up the command; none are treated specially.
So the easiest way is to use
replications=$(sudo -H -u hadoop bash -c "/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \$4}'")
but
replications=`sudo -H -u hadoop bash -c "/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \\\$4}'"`
also works.
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