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I have recently started to shift my shell scripting from utilizing backticks to parens to execute a command in situ and use the results in something else. Eg:

for line in `cat file`
    echo "$line"

Now I use parens, substituting thusly:

for line in $(cat file)

What is the actual difference between the two methods, and why is paren substitution considered better than backticks?

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(pedantry follows) Note that in bash, you can replace $(cat file) with $(< file) that is performed entirely in the shell (saving precious microseconds otherwise wasted spawning a cat process). – glenn jackman Feb 21 '12 at 18:45
(pedantry continued) Never, ever, ever use for to parse something line-by-line. – grawity Feb 21 '12 at 18:51
@grawity - this is a contrivved example entirely to showcase the difference in the two approaches – warren Feb 21 '12 at 18:52
Also, BashFAQ/082 regarding this question. – grawity Feb 21 '12 at 18:54
up vote 12 down vote accepted

There is no functional difference, however $() makes nesting a bit nicer and easier to follow. Consider this silly example:

$ echo `echo \`echo \\\`echo foo\\\`\``


$ echo $(echo $(echo $(echo foo)))

Now consider doing it with a complex series of commands that do something useful ;).

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