CAUTION! Do not use xargs for repetition.
If you don't understand why xargs is
BrokenAsDesigned, try the following code:
xargs <<EOF
xargs isn't broken
EOF
or, alternatively:
echo "xargs isn't broken" | xargs
Usual result is xargs stopping with diagnostic message:
xargs: unterminated quote
Always use xargs -0. This is the only way to avoid weird failures on untested corner cases. --BottomMind
Shell efficiency is often greatly impeded by
fork/exec overhead. Consider the following code:
while
read filename
do
ls -ld $filename
done
The
while loop makes it possible to invoke the same Shell command several times with the same general parameters, conveniently parameterized as shell variables. In this code, a
fork and
exec system call is performed for every file name in standard input. For loops with very simple commands, like
ls in this case, the
fork/exec overhead is the vast majority of the run time. It would be nice if
ls could handle multiple files in a single invocation:
ls a b c
is the same as
ls a; ls b; ls c
a property true of many Shell commands (
ls, cat, echo, etc.). But there is no syntactically convenient way to bunch up parameters coming through a pipeline. We could "bunch" parameters ourselves:
while
read filename1 filename2 filename3 filename4
do
ls -ld $filename1 $filename2 $filename3 $filename4
done
but it's difficult to handle the leftovers when the number of data items is not an even multiple of the number in the bunch (4 in this case).
Therefore:
Use xargs to "vectorize" loops. The following line replaces the loop above.
xargs ls -ld
Efficient shell scripts often resemble APL, they consist of vector operations.
CategoryUnixShellPattern