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| <<Anchor(faq3)>> == How can I find the latest (newest, earliest, oldest) file in a directory? == The obvious way to do this is to use `ls` to sort the filenames, and take the first one. This works well in all the trivial cases, but breaks when the filenames are unfriendly; the `ls` approach [[ParsingLs|cannot be made robust]] unless someone adds a `-0` extension to `ls`. I don't know of any version of `ls` with such an extension. Therefore we need some way to compare the timestamps on the files. The most common requirement is to sort them based on ''modification time'' (mtime); Bash and ksh have `-nt` and `-ot` operators which can do this: {{{ # Bash/ksh latest= for file in "$dir"/*; do [[ $file -nt $latest ]] && latest=$file done }}} To find the oldest: {{{ # Bash/ksh oldest= for file in "$dir"/*; do [[ -z $oldest || $file -ot $oldest ]] && oldest=$file done }}} Neither shell has an analogous operator which compares by atime or ctime, so one would need external utilities for that; however, it is nearly impossible to ''handle'' the output of said utilities properly in standard Bourne or POSIX shells (or even Korn shell...). If the sorting criteria are different from "oldest or newest file by mtime", then GNU `find` and GNU `sort` may be used together to produce a sorted list of filenames + timestamps, delimited by NUL characters. This will of course operate recursively ([[UsingFind|GNU find's]] `-maxdepth` operator can prevent that); and Bash's `read` command can be used to extract the filenames from this stream: {{{ # Bash + GNU find + GNU sort IFS= read -r -d '' latest \ < <(find "$dir" -type f -printf '%T@ %p\0' | sort -znr) latest=${latest#* } # remove timestamp + space }}} The `find` and `sort` options may be adjusted as needed, for example to sort by atime instead of mtime, or to sort in ascending order to get the oldest file. A group of multiple `read` commands may be used if more than the single latest file are needed. We read whole lines and use [[BashFAQ/073|parameter expansion]], instead of reading two fields with [[IFS]] set to a space, because whitespace in `IFS` would cause all leading and trailing spaces to be stripped from the filename. We don't want that. Of course, one disadvantage of this approach is that the entire list is sorted, which is an ''O(n log n)'' operation, whereas simply iterating through the list to find the minimum or maximum timestamp (assuming we want just one file) would be ''O(n)''. Here's an alternative which does that: {{{ # Bash + GNU find time=0 latest= while IFS= read -r -d '' line; do t=${line%% *} t=${t%.*} # truncate fractional seconds ((t > time)) && { latest=${line#* } time=$t; } done < <(find "$dir" -type f -printf '%T@ %p\0') }}} In practice, reading lines in Bash is much slower than doing so in most other utilities, and so the algorithmic improvement may be less important than the inefficiency of the shell (in other words, the iterative loop may be ''slower'' than calling `sort`). Benchmarking is left as an exercise for the reader. Readers who are asking this question in order to rotate their log files may wish to look into `logrotate(1)` instead, if their operating system provides it. '''This question number has been recycled. The previous version of this question was:''' == How can I insert a blank character after each character? == {{{ sed 's/./& /g' }}} Example: {{{ $ echo "testing" | sed 's/./& /g' t e s t i n g }}} For more examples of sed 1-liners, see [[http://www.student.northpark.edu/pemente/sed/sed1line.txt|sed 1-liners]] or [[http://sed.sourceforge.net/sedfaq.html|the sed FAQ]]. '''This question almost never came up in several years of IRC, so there's no point keeping it, especially at such an early (prominent) point in the FAQ. I'll leave this Q&A here for a while before deleting it.''' ---- CategoryShell |
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