output awk command result to variable
我从控制台运行以下命令，它输出正确的结果：
1 | sudo -H -u hadoop bash -c"/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \$4}'" |
但是，如果我把它放在shell脚本中并将其分配给一个变量，"awk"将不再工作，它只输出"grep"的整个结果：
1 2 | replications=`sudo -H -u hadoop bash -c"/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \$4}'"` echo"Replications: $replications" |
结果：
在倒勾命令替换中，
Within the backquoted style of command substitution, backslash shall retain its literal meaning, except when followed by: '$', '`', 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.
更明确地说，从bash手册页：
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
$(command) form, all characters between the parentheses make up the command; none are treated specially.
所以最简单的方法是
1 | replications=$(sudo -H -u hadoop bash -c"/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \$4}'") |
但是
1 | replications=`sudo -H -u hadoop bash -c"/home/hadoop/hadoop-install/bin/hadoop dfsadmin -report | grep 'Under replicated blocks' | awk '{print \\\$4}'"` |
也可以工作。