As I understand, the ls command calls getdents, which returns up to x number of directory entries. Are there any other system calls involved? If I run ls -l, are there any more system calls? I am trying to determine if ls -l is more expensive and hence slower than ls.
2 Answers
/bin/ls usually sorts the output. I'm not sure if your "efficient" question is just over system calls or the entire work that is done, but /bin/ls -f would probably do the least work. It only returns the filenames in directory order. No sorting, no additional inode lookups to get metadata (as ls -l would do).
Also, if your default ls is colorizing, it may be doing the equivalent of ls -l anyway so that it can tell how to color the output.
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ls -lperforms astatfor every single file. It need inodes for each file in order to get informations such as size, permission or number of hard links.– pqnetAug 6, 2014 at 23:34
ls -l is definitely more expensive, since it has to query the file system for metadata such as owner, group, permissions, access time, etc. Vanilla /bin/ls only has to look up the names of the entries in the directory being listed.
Note that ls may be aliased on your system to something less vanilla than /bin/ls. Run type ls to see if that's the case.
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Vanilla ls gets the inode numbers and then has to look up the directory names and filenames from each inode. Is this correct? ls -l has to reach out to the same inodes as above but also has to read metadata from each inode in addition to the filename or directory name. Is this correct? Aug 6, 2014 at 20:10
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Yes, but depending on the type of file system you have, how it was configured, and its usage patterns, it may be expensive to look up metadata. The metadata may not be stored with the inodes -- it may be stored on a different partition, or even on different hardware entirely. (If you're using a standard desktop, though, then that's probably not the case.) This is why the man page for
findspends some time talking about efficient vs. inefficient searches; searches that just look at file names are far more efficient.– dg99Aug 6, 2014 at 20:14 -
1@user100503: I'm pretty sure that the directory contains the names with the inode numbers; you don't go to the inode to get the name (if you have several hardlinks to a file, which of the file names would be in the inode?)– celtschkAug 6, 2014 at 20:33
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@celtschk Yes, I think my answer was overly wordy. I have edited it to remove some of the confusion.– dg99Aug 6, 2014 at 21:09
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2@user100503, no it's not. The data in the directory can be thought of as a list of names and their associated inodes. That is the main purpose of the directory. The name does not exist inside the inode. As celtschk mentions, multiply linked inodes do not have a single name. Aug 6, 2014 at 21:23
strace lsandstrace ls -lto see the system calls for each command.lsis not intended for automation, there are always more efficient and safer ways to illuminate the filesystem for programming purposes.