Intro to the linux CLI
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Posted by: alien on Aug. 29th, 2001 2:55 pm
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An Introduction to the Linux Command Line Interface
The Core 15 Commands
First I will list, describe, and explain the uses of what I consider to be the core 15 commands in Linux. With knowledge of these, you should be able to find your way around for the most part. The goal of this is *not* to give a thorough understanding of each command, but just its basic usage.
ls
Description: ls is the functional equivalent of dir on DOS systems. ls gives a basic file listing in the current directory.
Command Line:
To use ls, you may just type ls at any command prompt. The argument -a tells ls to list all files, including hidden ones. The argument -l tells ls to give a long listing, one with file sizes, ownership information, and dates/times.
Examples:
$ls -al
This will list all files in the current directory, including all hidden files, and will list their file sizes, ownership, and date/time information.
cd
Description: cd allows you to change the directory you are currently in. cd is a very simple command.
Command Line: cd takes no other arguments besides the name of the directory
you are changing to.
Examples: $cd /usr/local/
This will change the current directory to /usr/local
pwd
Description:
You know how to change directories, but can't figure out what directory you're in? pwd will give you the answer.
pwd outputs the current directory.
Command Line: pwd takes no arguments.
Examples:
pwd
The output will look something like:
/usr/local/
less
Description: Installed on most Linux systems, less is a "pager" - it pauses the output of text at every page and allows scrolling through a text file. less is the functional equivalent of more, but has features not found in more. Remember: less is more.
Command Line: There are no significant command line arguments for less
Usage
Once less has been run, and is pausing the output of text, you can scroll up and down using the arrow keys, use pageup and pagedown to scroll up and down by pages, and can press the spacebar to go down a page, and enter to go down one line.
Examples:less can be used in two ways. Firstly, it can directly read a text file, such as:
less document.txt
It can also have text piped to it, such as:
last | less
In this example, less allows "paging" of the output from last.
cp
Description: cp is a basic file copy utility. Just as in DOS, cp takes arguments that include the wildcards * and ?
Command Line: cp can be used in the following format: cp by default cp does not copy directories, but given the -R option, it will.
Examples: Copying multiple files:
#cp *.html /home/httpd/html/
this copies all .html files in the current directory to /home/httpd/html
$cp -R * ../backup/
copies all files and subdirectories to the directory backup which is on the same level as the current directory.
mv
Description:mv is a utility that functions both as a rename utility and a move utility. It can be used to move files and whole directory structures as well.
Command Line:mv, as cp, has few significant command line options besides the -b option, which makes a backup copy of the files being moved.
Examples: First, an example of moving files:
$mv misplaced* misc/
The example above moves all the files that match misplaced* (read about wildcards if you don't understand what this means) from the current directory into the subdirectory misc Another example, this time, renaming a file:
$mv bad-filename good-filename
rm
Description: All operating systems must have a delete feature, and rm does exactly that for Linux. Of course, you must remember, that in Linux, you have much more freedom to do things, and having such freedom, you must also be more careful how you go about deleting things. I advise you to be cautious when deleting things as root, as recovering erased data is very difficult, and often with the power rm gives you, it also assumes you know what you are doing in many cases, and will not confirm your actions before it does them.
Command Line: The most useful command line parameters are -r and -f - and combined, they can be deadly. -r deletes directories along with all subdirectories and files contained within that directory. -f "forces" it, which means it forces the files to be removed and does not ask for a confirmation. This can be helpful if you want to delete a directory tree of 100 files that you don't want to press 'y' for 100 times. These
two can be combined as -rf.
Examples: Removing a single file is simple:
$rm oldfile.c
A group of files can be removed either of these ways:
$rm badfiles* $rm badfile1 badfile2 badfile3
Removing a directory, without confirmation, can be done as follows:
$rm -rf tree/
mkdir
Description:mkdir makes a new directory - what could be simpler?
Command Line: Not much to speak of
Examples: To make a directory:
$mkdir mydirectory
grep
Description:grep is a powerful text search tool that can be used on files, piped streams of data, and input. grep can determine if a text string appears within the target and can return the line, the name of the file, or whatever else you want it to return.
Command Line: With all of grep's functionality, it is a given that there are many command line options. I find one option particularly useful in common applications: -l will tell grep to return the name of the file that the string was found in, instead of what it usually returns, which is the line the string was found in.
Examples: First of all, read the examples using grep given in the section about pipes and redirectors. A simple search of a textfile:
grep barath bigtextfile.txt
find
Description:find is yet another powerful utility with many options that is still simple to use for its basic features. find can give listing of files and can search within those file listings to find files.
Command Line: The most used option is -name which then allows you to provide a filename with wildcards that you are searching for.
Examples: To find all files on the system that begin with "temp", starting from the root directory "/" :
#find / -name temp*
ps
Description:ps allows you to see the "processes" - the programs - running on the system. ps uses a few strange terms and acronyms that I should explain. All processes on a Linux system have a job number associated with them. They can be attached to a tty, or terminal, which means they can interact with a user, or they can be detached and running in the background, even when you are logged off. You wouldn't want to interface with a web server daemon, but you would with an email program. Thus the daemon would be in the background but the email program would be in the foreground. Let's suppose you were running a ftp program, and were downloading a file.
Instead of waiting until it completed the download before using the terminal again, you could press Ctrl-Z (which suspends the process that you are using currently) and then would type bg to background it. This would allow it to run in the background. You would see this process when you do a ps - it would show up in the listing.
Command Line:ps has five common options - a, u, x, m, and f. Newer version of ps don't require the "-" before the options. a shows all processes on the system, u displays additional user information, and x displays an extended listing of processes, including ones that are detached.
m allows the process names to stretch one line longer for each "m" you use. f provides a tree layout of the processes,
so you can see which processes are owned by which other processes. When run as root, ps automatically does x
Examples: A simple example to list and classify all running processes, allow them each to extend two lines longer, and show them in a tree layout:
$ps -auxmmf
kill
Description:kill does what it means - it kills processes. There are three command reasons to use kill: 1. If a program has become idle or is no longer needed, a kill can be issued using the process's pid (you find this using ps). 2. If a daemon or other program needs to be restarted or reset, a kill -HUP can be issued. 3. If a program has gone haywire or otherwise will not go away with a regular kill, a kill -9 can be issued.
Command Line: As described above, the two common options are -HUP and -9. -HUP restarts most daemons and other programs. -9 forces a process to be killed and does not allow it to shutdown normally. Use -9 only when a process will not die with a normal kill
Examples: To kill a process, use kill. The pid number can be obtained by running ps. This will restart process id 237:
#kill -HUP 237
gzip
Description: Those of you familiar with DOS no doubt know of pkzip - a compression and archival program. Note that pkzip compresses individual files and lumps those compressed files into one big archive file. Linux handles this differently - gzip only handles compression, and tar handles archival.
Thus gzip can only compress individual files - and while it can
process many files at once, they will be compressed and stored individually.
Command Line: I urge you to use gzip's -h help feature to learn about the command line options as many of them are useful and cannot be done justice here. -d in particular tells it to decompress.
Examples: To decompress a gzipped file:
$gzip -d compressedfile.gz
tar
Description: As described with gzip, archival is separate, and tar does that. Since archival and compression together is so common, gzip usage has been integrated into tar
Command Line: To extract files with tar, use tar xvf filename.tar. To create a tar archive, use tar cvf. Note that a "z" can be added to the commands to tell tar to use gzip (de)compression.
Examples: To extract files form a tar archive, do:
$tar xvf bigarchive.tar
If a file is both .tar and .gz, which is either .tgz or .tar.gz, you can do the following to extract the files in one easy step:
$tar zxvf program-1.2.tar.gz
chmod
Description: Perhaps the most fearsome of the '15 core commands', chmod is a completely new dimension for a non-unix user. Since Linux (and unix in general) is a multi-user OS, files must be kept safe from other users. Thus Linux has very tight security in which all files and directories have owners, groups, and modes for access. The owner has control over the files/dirs, the group aids in the mode process, and the mode sets who has what access to the files. When you do an ls -l, it lists this information off to the left; there will be a strange looking bar of characters like:
-rw-r--r--
This means reading from left to right, that no special bit has been set, the owner can read to it, write to it, but not execute it, the group can read it, but not write to it, and not execute it, and the other users can read it, but not write to it or execute it. Think of it as groupings: first on the system.
Command Line:chmod takes options that are stuck together to tell it what to do. The first letter is a u, g, o, or a, which stand for user, group, other, or all - telling chmod whose permissions to change. Then comes a +, -, or = to add, remove, or set permissions accordingly. Lastly, there is r, w, and x, which I have described. After this compound comes the name of the file or directory.
Examples: To remove permissions for all others besides yourself:
$chmod go-rwx filename
To give everyone full permission:
$chmod a=rwx filename
To make a directory writeable but not readable:
$chmod go=w directory/
man
Description: You might think I'm sneaking in an extra command, but I'm not - man teaches you about other commands so it can't be counted. man retrieves and displays the manual pages for just about anything. Be warned: manual pages can often confuse you more than help you as they give quite a bit
of technical information.
Examples: $man grep
Supplementary Commands
n this section, I am going to outline 50 other commands that are useful
in many instances. I have tried to categorize these commands and alphabetize them within these categories, and provide brief descriptions of each.
File Handling
cat - outputs the contents of a file or inputs into a file from the commandline
file - finds out what type of file something is by looking it up from/etc/magic
fmt - simply formats and word wraps a text file
head - displays the beginning of a file
tail - displays the end of a file
wc - counts the number of words, lines, or characters in a file or stream
Process Control
at - schedules certain commands to be executed at specific times
bg - places a process that has been suspended to run in the background
crontab - schedules certain commands to be executed at specific times on certain days or at certain times
fg - places a process that has been suspended or backgrounded in the foreground
free - displays how much memory has been used / is available
killall - kills all processes by a given name instead of by a PID number
top - displays the processes that are using the most resources
Network Functions
finger - finds out information about a specific user
frm - displays a quick listing of current mail
lpr - prints a document or stream
ping - tests the network response time of a machine
talk - chats directly with another user
traceroute - finds the route data travels to arrive at a remote host
Filesystem Management
chgrp - changes the group ownership of a file/dir
chown - changes the user/group ownership of a file/dir
dd - does direct data transfers, usually from diskimages to disks or vice-versa
df - displays the space used up on all mounted filesystems
du - displays the space used by directories and recursively displays the disk space taken by sub-directories as well
fsck - checks a filesystem for errors and fixes them
ln - creates a symbolic or hard file or directory link
locate - locates a file using a pre built file database
rmdir - removes a directory
whereis - finds an executable, source file, or man page in common paths
which - displays the full path of a command that is run through the path
User Management
ac - displays user accounting information such as login time
chsh - changes a user's shell
last - displays the last users to login to the system
passwd - changes a user's password
adduser - adds a user
su - substitutes another user's id for the current user id
w - displays a list of the other users currently logged in and what they are doing
write - writes a message to the console of another user
Programming / Management
g++ - default c++ compiler
gcc - default c compiler
ldconfig - updates the list of loadable libraries that are in the paths specified in /etc/ld.so.conf
make - makes a program using the commands specified in the Makefile in the current directory or with another specified file
Misc
clear - clears the screen
dpkg - debian package application, allows installation, manipulation, and deletion of .deb packages from a system
export - exports a shell variable so that programs that are run can access it
rpm - redhat package manager, allows installation, manipulation, and deletion of .rpm packages from a system
uname - displays system information
uptime - displays system uptime and load
xargs - passes a stream as arguments to a specified command
