Twenty most commonly used Unix commands listed in order:

1.
ls list files in current directory
ls -l long list of files in current directory
ls -a list all files (including hidden) in current directory
2.
pwd print working directory
3.
cd dirname moves into the directory with name `dirname'
cd cd on its own takes you back to your home directory
cd .. moves up one level in directory hierarchy
(the .. always means the parent directory to the one you presently occupy)
4.
mkdir dirname makes a new directory with name `dirname'
rmdir dirname removes the (empty) directory with name `dirname'
rmdir -r dirname recursively removes directories and subdirectories
5.
cp file1 file2 makes a copy of file1 and calls it file2
mv file1 file2 moves (deletes) file1 to (creates new file) file2
rm file1 removes (deletes) file1
rm -i file1 asks for confirmation that you want to delete file1
6.
history lists your last 10 (variable) commands and numbers them
!! repeats your last unix command
!23 repeats the command numbered 23
!f90 repeats the last command beginning with e.g. 'f90'
!!addtext appends 'addtext' to previous command line
^string1^string2 substitutes 'string2' for 'string1' in previous command
7.
command > file1 redirects the output of 'command' to 'file1'
instead of to standard output (screen)
command >> file1 appends the output of 'command' to 'file1'
instead of to standard output (screen)
command < file1 takes input for 'command' from file1
command1 | command2 pipe standard output of command1 to
standard input of command2
8.
. working directory
.. parent directory to working directory
~ home directory
/ root directory
* wildcard standing for any string of characters
? wildcard standing for any single character
9.
more file1 displays the file 'file1' a page at a time
10.
man utilityname manual pages for the utility 'utilityname'
11.
ctrl C interrupts whatever is currently running.
(It can get you out of trouble at embarrassing moments)
12.
chmod changes file and directory permissions
(ls -l will show xdrxdrxdrx which means all permissions granted;
a dash in any position means a permission denied)
(first x indicates a directory, - a file)
(next 3 groups of rwx correspond to user(u), group(g), others(o))
chmod u+r file1 + grants, -removes permission
to read(r), write(w) or execute(x)
for user(u), group(g), others(o) or all(a)
(for directories, ...)
13.
lp -Pprintername file1 prints file1 to printer `printername'
lpq -Pprinternmame enquiry, print queue
lprm -Pprintername jobnumber removes from print queue
14.
cal month year displays days/dates of month/year
(try cal 1752 and look at September!)
15.
wcfile counts lines, words, characters
16.
who display the users who are logged-on
17.
who am i obvious
18.
cat files concatenate and display files
19.
grep pattern files search files for pattern
20.
sort files sort the specified files