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Some Essential Unix Commands
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GETTING AROUND DIRECTORIES & FILES
Like DOS, Unix organizes information by putting *files* in
*directories*. Breaking information down into directories
helps to make a large body of information more manageable.
A directory is like a folder, which can contain either
information (a file), or more folders (called
subdirectories). Each directory can contain any number of
files or subdirectories, and every file and every directory
has a name, made up of letters and numbers. The full name of
a file includes its "path" -- in other words, the complete
directory structure, with the directories and subdirectories
separated by slashes (forward slashes, "/", rather than the
backslash, "\", of DOS). A file called "file1" in the
directory /home4/jqpublic would have the full path
/home4/jqpublic/file1.
The "working directory" is the directory you are in at the
moment. When you are in a working directory such as
/home4/jqublic, most commands by default apply only to that
directory. To work with other files, it's often easiest to
change into another directory (see "cd," below).
pwd -- Print Working Directory.
- At any time you can find out the current working directory by
typing "pwd" on its own.
ls -- Display a list of files and directories.
- Typing just "ls" will give you a list of all the files and
subdirectories in the current directory. (Technically it
will show you only the "visible" files: those whose names
begin with a character other than a period.)
- You can also get a list of files in another directory by
specifying the directory after ls:
/usr/local/gopher/Courses
- A few flags might be useful:
- ls -a -- Show all files, including the invisible ones
(those whose names begin with a period).
- ls -l -- Use the Long format, including date, file owners,
&c.
cd -- Change Directory.
- You can change the current directory to any directory on the
system.
- "cd" alone will change back to your home directory.
cp -- Copy a file.
You can copy a file to another name or to another directory
without deleting the original file. The "source" file comes
first, followed by the "target" file: in other words,
cp file1 file2
will take file1 and create a new file called file2 that
contains the same information as file1, and is in the same
directory.
cp file1 Texts
will copy file1 into the subdirectory called Texts, keeping
the same name: in other words, it will create a new file
called Texts/file1.
You can also copy into another directory with a different
name:
cp file1 Texts/file2
will create a new file called Texts/file2 that contains the
same information as file1.
mv -- Move or rename a file.
mv behaves much like cp, except that it *deletes the original
file*. Like cp, you specify the source first, the target
second.
mv file1 file2
will copy file1 into file2 and *delete file1* -- the
practical effect is that you've renamed file1 into file2.
mv file1 Texts
will copy file1 into Texts/file1 and then delete the original
file1.
mv file1 Texts/file2
will simultaneously move the file into the directory and
rename it to file2.
rm -- Remove a file.
- "rm file1" will remove the file called file1. You can also
specify several files at once: "rm file1 file2 file3" will
remove all three.
- You can also use wildcards, the most common of which is * --
the asterisk means "everything beginning with." You can type
"rm file*" to remove every file beginning with the letters
file. "rm *" would delete *everything* in a directory.
- By default, rm will ask if you're sure about each deletion.
The -f flag overrides the question and deletes all the files
without asking whether you're sure. Use it with great
caution; there's no way to get the files back.
- rm works only with files, not directories. To remove a
directory, use rmdir (below).
rmdir -- Remove a Directory.
-
Works like rm, except it removes directories instead of
files. Directories must be empty before they can be removed.
mkdir -- Make a Directory.
- Creates a new directory, a subdirectory of the current
directory. If you're in /home1/jqpublic and type
mkdir Texts
you'll create a directory called /home1/jqpublic/Texts.
FILE PERMISSIONS
Unix is a multi-user operating system, meaning many people
are on the same system at once. To ensure security, some
files are restricted: only certain people can read or write
them. This is known as file permissions.
When you use "ls -l" ("ls" for list files, "-l" for "Long
format"), the far left will include a pattern of letters
like this:
-rwxr-xr--
The ten positions here indicate the file permissions. The
first (leftmost) position can be either "d" or "-" -- "d"
means the entry is a directory, while "-" means it's a file.
The remaining positions come in three groups of three. Each
group consists of three positions: "r" or "-"; "w" or "-";
"x" or "-." "r" means "Read permission"; "w" means "Write
permission"; "x" means "execute permission." The most
important are Read and Write permissions: Read permission is
what it sounds like: it means the User, Group, or Other can
look at the contents of the file. Write permission means the
User, Group, or Other can change or delete the file. An "r"
means read permission is enabled, while a "-" means it isn't;
"w" means write permission is enabled, and "-" means it
isn't.
Each group of three, then, shows read, write, and execute
permission. The three groups are: User, Group, and Other.
The User is the person who created (or owns) the file; the
Group is a collection of people (defined on the system) who
have similar permissions; Other is anyone else on the system.
The example above, therefore --
-rwxr-xr--
-- can be broken down this way: The first position is a "-"
instead of a "d," so it's a file, not a directory. The
User's permission -- the next three characters -- is "rwx,"
meaning the person who created the file can read, write, and
execute the file. The Group permission is "r-x," meaning the
Group can read and execute, but not write. The Other
permission is "r--," meaning Others can read but not write or
execute.
You'll want to set permissions carefully. By default, most
files you create can be read and written by you alone. If
you want others to have access to the files, you'll have to
set the file permissions to allow other people -- either
those in your group or everyone -- to read or write the
files. Be careful not to allow others to tamper with your
files. Your personal mail, for instance, should give read
and write permission only to you: -rw-------. If you want
someone else to be able to see the contents of your file,
give read permission to either the group or to other, but
keep write permission only for yourself. If you give write
permission to other, anyone on the system can change or
delete your files.
To change file permissions, use the "chmod" command.
chmod -- Change File Mode.
LOOKING AT FILES
You can always use a text editor (such as pico, emacs, or vi) to
look at a file. Some other methods are sometimes useful:
- cat -- Display an entire file.
"cat" -- it stands for "concatenate" for obscure reasons --
will display an entire file on the screen. "cat file1" will
display the entire thing. You can also display several
files, as with "cat file1 file2" or "cat *".
- more -- Display an entire file screen by screen.
"more file1" will display an entire file, one screen at a
time. You can then press the space bar to go forward a
screen, or "q" to quit.
- head -- Display the top of a file.
"head file1" will display the top ten lines of file1. You
can also choose exactly how many lines to display by
specifying the number after a hyphen:
head -20 file1
will display the top twenty lines. "head" is useful for
taking a quick look at a long text file.
- tail -- Display the bottom of a file.
Works exactly like head, above, but shows the bottom of a
file.
OTHER FILE OPERATIONS
grep -- Search for a String.
grep -- it stands for "Get Regular Expression and Print" --
searches through a file or group of files for a string. To
find every occurrence of the letters "light" in the file
parlost.01, type
grep light parlost.01
You can also search every file in a directory:
grep light *
will go through every file in the current directory and look
for the letters "light."
To search for more than one word, put the string in quotation
marks:
grep "he said" *
Some useful flags:
- grep -i -- Ignore case -- "grep -i light *" will search
for "light," "Light," "LIGHT," whatever. By
default, grep is case-sensitive.
- grep -n -- Display the line number next to each match.
- grep -c -- Count the number of occurrences without
displaying each one.
- grep -v -- The opposite of normal grep: display every
line that does *not* contain the string.
- grep -w -- Search for whole words only. If you type
"grep light," you'll see every ocurrence of
"enlightened," "lightning," and so on. The
-w flag limits the search to the whole word
"light" only.
sort -- Sort a file.
"sort" will put the contents of a file into alphabetical
order. For instance, you can type
sort file1
and get a list of all the lines in file1 sorted in
alphabetical order. The output will appear on the screen.
A few flags are useful:
- sort -b -- Ignore leading tabs and spaces.
- sort -f -- Sort upper- and lowercase together.
- sort -r -- Sort in reverse order.
- sort -u -- Skip duplicates. If two lines are identical,
"sort -u" will display only one of them.
cut -- Look at part of each line.
"cut" lets you select just part of the information from each
line of a file. If, for instance, you have a file called
"file1" with data in this format:
0001 This is the first line
0002 This is the second
and so on, you can look at just the numbers by typing
cut -c1-4 file1
The "-c" flag means "columns"; it will display the first four
columns of each line of the file. You can also look at
everything but the line numbers:
cut -c6-100 file1
will display the sixth through one hundredth column (if the
line is less than a hundred characters -- and most will be --
you'll see up to the end of the line).
You can also use cut to look at fields instead of columns:
for instance, if a file looks like this:
curran:Stuart Curran
jlynch:Jack Lynch
afilreis:Al Filreis
loh:Lucy Oh
you can use cut to find the full name of each person, even
though it's not always in the same place on each line. Type
cut -f2 -d: file1
"-f2" means "the second field"; "-d:" means the delimiter
(the character that separates the fields) is a colon. To use
a space as a delimiter, put it in quotations:
cut -f2 -d" " file1
sed -- Stream Editor.
"sed" is a sophisticated tool that allows you to perform
large-scale search-and-replace operations on a file. There
are many possibilities; the most important are "s"
("Substitute") and "d" ("Delete").
sed "s/colour/color/" filename
will take the entire file called "filename" and replace every
occurrence of the pattern "colour" with "color", then display
the output to the screen. You can also use "s" to remove a
pattern of letters:
sed "s/quite//" filename
will replace "quite" with "" -- in other words, remove it.
You can also use sed to delete entire lines:
sed "/light/d" parlost.01
will delete every entire line of the file parlost.01 that
contains the word "light."
REDIRECTION & PIPING
By default, Unix sends the output of all these commands to the
screen. But this isn't necessary. Unix lets you take the output
of one command and either save it in a file, or turn it into the
input of another command, allowing you to chain several commands
together. You can do two things with output: (1) redirect it
into a file; and (2) pipe it into another command.
REDIRECTION
The ">" (greater than) symbol lets you take the output of a
command and save it to a file. For instance, "ls" alone will
display the contents of the directory on the screen. Typing
ls > dirfile
will save that output to a file called "dirfile" (it won't appear
on the screen).
Programs such as cut, sort, grep, and sed can
produce very long outputs; you might want to save them to a file. For
instance, to translate "colour" to "color" you can use sed, but the output
will scroll past you too quickly to see. Use redirection to save it to a
file:
sed "s/colour/color/" file1 > file1.out
will produce a new file called "file1.out" that has changed the
spelling of "colour" to "color" in every line.
You can use redirection to join two files together with "cat":
cat file1 file2 > file3
will take file1 and file2 and combine them into a new file called
file3.
WARNING: "ls > dirfile" will overwrite "dirfile" if it already
exists. If you want to add the contents of the directory to the
end of a pre-existing dirfile, type
ls >> dirfile
-- the ">>" means "add to the end."
PIPING
The "|" symbol lets you take the output of a command and send it
to the input of another command. For instance, you take the
output of the "ls" command and sort it in reverse order:
ls | sort -r
To see just the first five matches of the word "light" in a group
of files, type
grep "light" * | head -5
If the search for the word "light" produces too many matches, you
can see them a screen at a time by typing
grep "light" * | more
There's no limit on the size of the chain you can put together.
To see all the files in a directory created in January, sort them
by name, and save them to a file, you can type
ls -l | grep " Jan " | cut -c55-100 | sort > janfiles
GETTING HELP
man -- Display the manual pages for a command.
"man" is the Unix "help" -- type, for instance, "man grep"
and you'll get the complete instruction manual for the grep
command.
apropos -- Search for commands that relate to a particular
topic.
"apropos directory" will display every command that includes
the word "directory" in its brief description. You can then
use "man" to get a fuller description of the command.
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