Linux Commands

C151 Multi-User Operating Systems

Expressions in CommandsCurrent directory: ..Parent directory: ....Home directory: ~~Root directory: //

PathAbsolute pathAbsolute path: complete sequence of

directories to a file starting from the root directory //:/home/yul/c151/cat.jpg

Relative pathRelative path: the sequence of directories to reach a file starting from the current directory:c151/cat.jpg

The absolute paths have the same meaning no matter what the working directory is. The relative paths depend on the current working directory.

An absolute path is the concatenation of the current working directory and the relative path:/home/yul/ + c151/cat.jpg

Linux File Systems//      root directory /bin/bin   executable for single user mode /dev/dev   devices that can be mounted by the system /etc/etc  local configuration, administrative things /home/home home directories for most users /lib/lib   shared libraries /mnt/mnt   temporary mount point for mountable devices /proc/proc  kernel and process information /root/root  home of the superuser (root) /sbin/sbin  system binaries (executables) /tmp/tmp   temporary files /usr/usr   lots of information used by the system

Linux UsersThere is one special user that has

administrative powers - the superuser. It's name is rootroot. Every user has a home directory that can be found under /home/username/home/username. They have an allocated disk space.

Every user’s home directory contains configuration (resource) files that define their environment. They all start with a dot. Example: .signature.

For example, the file ".cshrc" will always be executed whenever the user logs in or opens a terminal window.

Environment VariablesThe environment variables are global variables

that are defined within the shell and that can be used for many purposes.

Their names are in general in all uppercase.Display the value of an environment variable:

echo $PATH$PATH

lookup list for binaries to be executed.$HOME

user's home directory

Linux CommandsSmall utilities/programs under Linux that can

take options and arguments.ArgumentArgument: what the command applies to.

lsls /usr/usrOptionOption: how do we want the command to be

executed

lsls –l–lThe options and arguments can be combined:

ls ls -l-l /usr/include/usr/includeSome commands take more than one argument

cp requires a source file and a destination file.

More On CommandsSome useful commands: cd, ls, man, pwd, cat, find, grep, wc

Manipulating files and directories: mv, rm, cp, mkdir, rmdir

Linux Commandsls: list files

Example: ls ~cd: switch working directories

Example: cd \pwd: display current working directory

Example: pwdman: display manual of command

Example: man lscat: display the entire content of a file

Example: cat lab1.txt

Linux Commandsmkdir: create a directory

Example: mkdir C151rm:delete files

Example: rm *.txtrmdir: delete a directory

Example: rmdir C151

Linux CommandsViewing the Contents of a File

The more commandDisplays the entire contents of a file one page

at a timeSpacebar moves through file one page at a

timeEnter key moves through file one line at a time Only moves forward through a file, not

backwardExit: qExample 1: more lab1.txtExample 2: man ls | more

Linux CommandsViewing the Contents of a File

(continued)The less command

Nearly equivalent to the more command

Allows you to move forward and backward in the file

Usage: same as more

Linux CommandsViewing the Contents of a File

(continued)The head command

Displays just the first ten lines of a fileThe tail command

Displays just the last ten lines of a fileUsage: same as more and less

Linux CommandsThe find command: find files

The argument is a path. The name of the file must be specified with the option –name.

There more optionsFind file with name “hw1.txt” in current

directory (including sub directories)

find ./ -name hw1.txtFind file with name extension “.txt” in root

(including all sub directories)

find / -name “*.txt”

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Linux Commandsmv: rename or move files

Requires two parameters: name of original file and new name or location of file

Examples:Move file “a.cpp” from home directory to

current working directory:

mv ~/a.pp ./Rename file “a.cpp” in home directory to

“b.cpp”

mv ~/a.pp ~/b.cpp

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Linux Commandscp: copy files

Requires two parameters: path and name of the original file and path and name or the new file

Examples:Copy file “a.cpp” from home directory to

current working directory (without changing name):

cp ~/a.pp ./copy file “a.cpp” in home directory to “b.cpp”

cp ~/a.pp ~/b.cpp

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Text Editor: PicoProvides a series of commands at the

bottom of the screenBackspace, Delete, and other keys work

as expectedEasy to UseStart Pico

Pico file_nameExit Pico

[ctrl]+x

Linux ProcessesA process is any running program. Under

Linux a process has an identity (PID), a parent process (PPID), an owner (UID).

To view the active processes: ps Useful options: -A (all), -l (long list)

Stop the execution of a program launched from a terminal: Ctrl-c

Stop the execution of any processkill pid

The signal 9 is the most powerful. Kill a process with id 1258

Kill 1258 kill 9 1258

Launching Jobs on LinuxA jobjob is any executable run from the

terminal. The jobs are launched as foreground

tasks by default - they will block the terminal until they are done.

To launch a job in the background, use the symbol && at the end of the command line.

A job can be intentionally brought to the foreground with the command fgfg.

PipePipe (|) enables you to pass the output

of one command through the input of another command

ls -l | moreThe way this works is that when the

shell sees the pipe symbol, it creates a temporary file to store the result of the first command

Then this file becomes the input to second command

Input and Output RedirectionRedirecting the input: command < filenamecommand < filenameAny input will read from that file (must be file).

Redirecting the output. The simple > rewrites the output file, while the double one >> appends to the file (must be file).command > filenamecommand > filenamecommand >> filenamecommand >> filename

Combine input and ouput redirectionsExample:

wc < my_text_file.txt > output_file.txt

Compressing FilesThe most common utilities for compressing and

decompressing files on Linux are tartar, gzipgzip, and gunzipgunzip.

tartar allows us to compress several files (directories) into one archive. Options: cfcf for compressing, xfxf for decompressing.Example: tar –cf C151.tar C151

gzipgzip compresses one file at a time.Options: none to compress, -d-d to decompress. Example: gzip C151.tar

To archive more than one file, most people use a combination of tar and gzip in that order. Those archives have the extension .tar.gz.tar.gz or simply .tgz.tgz.

Decompressing FilesSuppose we have a compressed file

called C151.tar.gz, how could we decompress it?1. gzip –d C151.tar.gz (or gunzip

C151.tar.gz)We get C151.tar

2. tar –xf C151.tarWe get uncompressed files (folders)

File Ownership and PermissionsEvery file belongs to a particular user, generally

the creator of that file. This is the ownerowner of the file. Users may be organized in groupsgroups and a group can have special permissions to a file.

A file can be accessible for reading rr, writing ww, or executing xx. To view the content of a directory, the directory should be executable.

Usually the owner has read and write permission to a file, also execute permission if applicable. The root (super user)has read and write permissions for any file.

The command ls -l displays the permissions of a file for the owner, the group, and everyone else (all).

Change permissions: chmodchmod

File Ownership and PermissionsLinux file and folder attributes seen with

ls -lColumn of 10 characters on left

First character: file (-), directory (d), or link (l)2nd, 3rd, and 4th characters show permissions of

owner5th, 6th, and 7th characters show permissions of

group8th , 9th, and 10th characters show permissions of

all others

Managing Files and Directories with Shell Commands

File and Folder Permissions (continued)

•Permissions•R = read•W = write•X = execute•- =disabled

•Permission mode values•read=4•write=2•execute=1

File Ownership and Permissions

Change permissions with chmod Requires two parameters

Access mode numberFile or directory name to change

Examplechmod 644 reports

Reading AssignmentTextbook: Chapter 3, Chapter 4, and

Chapter 5