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In place line numbering
Add permanent line numbers to a file without creating a temp file. The rm command deletes file10 while the nl command works on the open file descriptor of file10 which it outputs into a new file again named file10. The new file10 will now be numbered in the same directory with the same file name and content as before, but it will in fact be a new file, using (ls -i) to show its inode number will prove this.

Go to the previous sibling directory in alphabetical order
Based on linkinpark342 suggestion. Sometimes you have to browse your way through a lot of sub-directories. This command cd to the previous sub-directory in alphabetical order. For example, if you have the directories "lectures/01-intro", "lectures/02-basic", "lectures/03-advanced" and so on, and your PWD is "02-basic", it jumps to "01-intro".

Convert seconds to [DD:][HH:]MM:SS
Converts any number of seconds into days, hours, minutes and seconds. sec2dhms() { declare -i SS="$1" D=$(( SS / 86400 )) H=$(( SS % 86400 / 3600 )) M=$(( SS % 3600 / 60 )) S=$(( SS % 60 )) [ "$D" -gt 0 ] && echo -n "${D}:" [ "$H" -gt 0 ] && printf "%02g:" "$H" printf "%02g:%02g\n" "$M" "$S" }

move up through directories faster (set in your /etc/profile or .bash_profile)
Usage: $ up N I did not like two things in the submitted commands and fixed it here: 1) If I do cd - afterwards, I want to go back to the directory I've been before 2) If I call up without argument, I expect to go up one level It is sad, that I need eval (at least in bash), but I think it's safe here. eval is required, because in bash brace expansion happens before variable substitution, see http://rosettacode.org/wiki/Repeat_a_string#Using_printf

Show the disk usage for files pointed by symbolic link in a directory
You also can sum the file usage of all files $ find /usr/lib -maxdepth 1 -type l -print0 | xargs -r0 du -Lch

Converts uppercase chars in a string to lowercase

Change the homepage of Firefox
This command modifies the preferences file of Firefox that is located in .mozilla/firefox/*.default/prefs.js. It edits the file with sed and the -i option. Then it searches the string "browser.startup.homepage", and the string next to it (second string). Finally, it replaces the second string with the new homepage, that is http://sliceoflinux.com in the example. It doesn't work if you haven't set any homepage.

list files recursively by size

Find the package that installed a command

most used unix commands


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