cd – Change directory
pwd – Print name of current working directory
ls – List directory contents
file – Print a brief description of the file’s contents.
cd command, is the main command you use to navigate around the file system. While moving, the pwd command, will tell you where you are now in the current file system. ls command will list the files and directory contents in your current directory.
The Root Directory or “/” is the start of the file system directory, or parent of all directories.
An absolute pathname begins with the root directory and follows the tree branch by branch until the path to the desired directory or file is completed. For example,
$ cd /etc/network (changes your current directory to root --> etc --> network directory)
The shell uses special symbols to represent relative positions in the file system tree.
- “.” symbol refers to the working directory
- “..” symbol refers to the parent directory of the current working directory.
- “cd ..” means move up to parent directory
- “cd ./bin” means from current directory move down to a directory name “bin”
- “cd bin” and point 2 does the same thing.
In general if you do not specify a pathname to something, the working directory will be assumed.
Changes the working directory to your home directory
- cd -
Changes the working directory to the previous working directory.
- cd ~user_name
Changes the working directory to the home directory of user-name.
Facts about Filenames
- Filenames that begin with a period character are hidden. ls command will not list them unless you use ls -a.
- Filenames are case sensitive.
- There is no concept of “file extension”.
- You can, but try not to use punctuation characters in filenames. Limit the punctuation characters in the filename you create to “.” period, “-“ dash and “_” underscore.
- You can, but try not to use spaces between filename. Try to use “_” (underscore) to replace your spaces between filename.
The ls Command
List current directory. List filename only.
- ls /usr
List directory under /usr directory
- ls ~
List home directory
- ls -a
List all files, including hidden files.
- ls -l or ll
List current directory with long format. File permission rights, owner, group, size, date time last modified, filename, links.
The file Command
As there is no concept of “file extension”, you can use the “file” command to print out a brief description of the file’s contents.
$ file /bin/ls
ls: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=0xf31e99218b4d7034cf8257055686bca22f5a3c01, stripped
$file * (list all file type in the current directory)
The root directory. Where everything begins.
Contains the Linux kernel (vmlinuz), initial RAM disk image (initrd.img) and boot loader (grub).
Contain common binaries programs. Example, bash, cat, chmod, chgrup, chown, date, grep, kill, less, login, ls, mkdir, rm, rmdir, ping, sed, su, tar.
This directory contains system binaries. These are programs that perform system tasks that are generally
reserved for the superuser. Example, fsck, ifconfig, ifdown, ifup, mkfs, reboot, reload, route, runlevel, start, stop, shutdown.
Contains shared library files used by the core system programs. These are similar to DLLs in Windows.
The /usr directory tree is likely the largest one on a Linux system. It contains all the programs and support files used
by regular users.
- /usr/bin, contains the executable programs installed by your Linux distribution.
- /usr/sbin, contains more system administration programs
- /usr/lib, contains shared libraries for the programs in /usr/bin
The /opt directory is used to install “optional” software. This is mainly used to hold commercial software products
that may be installed on your system. Most of the software will be placed here after system setup.
The /etc directory contains all of the system-wide configuration files. It also contains a collection of shell
scripts which start each of the system services at boot time. Everything in this directory should be readable text.
- /etc/crontab, a file that defines when automated jobs will run
- /etc/fstab, a table of storage devices and their associated mount points.
- /etc/passwd, a list of the user accounts
- /etc/hostname, define of system name
- /etc/hosts, IP to name hosts file
- /etc/profile, bash system-wide profile for users
This is a special directory which contains device nodes. “Everything is a file” also applies to devices. Here is where
the kernel maintains a list of all the devices it understands.
On modern Linux systems the /media directory will contain the mount points for removable media such USB
drives, CD-ROMs, etc. that are mounted automatically at insertion.
On older Linux systems, the /mnt directory contains mount points for removable devices that have been mounted
The /proc directory is special. It's not a real file system in the sense of files stored on your hard drive. Rather, it is a
virtual file system maintained by the Linux kernel. The “files” it contains are peepholes into the kernel itself. The
files are readable and will give you a picture of how the kernel sees your computer.
Each formatted partition or device using a Linux file system, such as ext3, will have this directory. It is used in the case of a partial recovery from a file system corruption event. Unless something really bad has happened to your system, this directory will remain empty.
In normal configurations, each user is given a directory in /home. Ordinary users can only write files in their home
directories. This limitation protects the system from errant user activity.
This is the home directory for the root account.
The /tmp directory is intended for storage of temporary, transient files created by various programs. Some
configurations cause this directory to be emptied each time the system is rebooted.
The /var directory tree is where data that is likely to change is stored. Various databases, spool files, user mail, etc. are located here.
- /var/log, contains log files, records of various system activity. These are very important and should be monitored from time to time.