Should know commands in UNIX/LINUX

Hey do you know these commands make life easier

The following screen contains a relatively painless romp through the sometimes painful terrain of UNIX commands. My intent is to describe a simple (though comprehensive) presentation of the basic, UNIX command line. The commands — below — are generic; that is to say, they should all be valid and execute on most modern, UNIX operating systems. (In the future, I may expand this page to include networking.)

1. File system
2. Permissions and ownership
3. The find utility
4. Data archives
5. Miscellaneous

Navigating a file system, listing files and manipulating them

Display the absolute PATH of the current working directory.

$ pwd

Display the absolute path of an executable, named: “rm”.

$ which rm

Change directory to /home/doctor/brain_abnormalities — using the absolute PATH.

$ cd /home/doctor/brain_abnormalities

Change from the current working directory to its sub-directory, named: “music” — using the relative path.

$ cd music

Change the working directory to $HOME.

$ cd ~/

List all non-hidden files and directories in the current working directory.

$ ls

List ALL files and directories (hidden or otherwise) within the current working directory in long format — displaying their attributes, i.e. size, ownership, time last edited, permissions, etc.

$ ls -la

Recursively list all non-hidden files and directories residing under the music directory.

$ ls -Rl music

List in long format a single file, named: “politics.txt”

$ ls -la politics.txt

List the attributes: size, ownership, time when updated, permissions, etc. on the_damned directory.

$ ls -ld the_damned

Print, as a numerical count, all files in the_damned directory.

$ ls -la the_damned |wc -l

Print the contents of hell.txt to standard out.

$ cat hell.txt

Copy (as a backup) hell.txt to hell.txt.OLD.

$ cp hell.txt hell.txt.OLD

Recursively copy the putrid directory to stinks, while preserving (“-p”) the modification time, access time, file flags, file mode, user ID, and group ID attributes.

$ cp -Rp putrid stinks

Move ugly.txt to stupid.txt, i.e. rename a file. This command works without additional flags for a directory, unlike cp.

$ mv ugly.txt stupid.txt

Create a new file, named: “operation.txt”.

$ touch operation.txt

Add the word “brain” to the operation.txt, append “surgery” to the bottom of it then erase all of the contents in the file.

$ echo “brain” > operation.txt 
 
 $ echo “surgery” >> operation.txt 
 
 $ echo “” > operation.txt

Delete operation.txt.

$ rm operation.txt

Create evil directory.

$ mkdir evil

Delete the evil directory and its contents recursively — without prompting for a confirmation when deleting any file or directory in (or under) evil.

$ rm -rf evil

Print to the screen the details of the type of a file queried: text, binary, image, audio, etc.

$ file air_supply.mp3

Create a symbolic link (“short cut”) from an existing file called “real_file” in the home directory to a linked_file in the www sub-directory. This command works identically for directories.

$ ln -s $HOME/real_file $HOME/www/linked_file

Altering permissions and ownership

“chmod” is employed to alter the read, write and execute permissions on files and directories. The breakdown of the absolute (numerical) mode for the permissions of files and directories is as follows: the first field signifies the owner, the second field the group and the third field all others, i.e. world.

(owner)(group)(other)

read = 4
write = 2
execute = 1

So — by placing 755 on a file means that the owner has read, write and execute permissions, while the group and other have only read and execute permissions on that file. Three examples follow:

The numerical bits, 644, signify that the owner of index.html will be able to write to and read it, all members of the owner’s group will be able to read it, and all other system users will be able to read it.

$ chmod 644 index.html

After the following command is executed, the owner will have read, write and execute permissions, the group will have read and execute permissions, and all others will have execute permissions over index.cgi.

$ chmod 751 index.cgi

Change the permissions on the www directory to user read, write and execute, group execute and other execute.

$ chmod 711 www

As the superuser, change both the user and group ownership on “trap.txt” to “rat” and “cheese” — respectively. (Verify that the cheese group exists in /etc/group. If not, add it with groupadd and associate the user to it with usermod.)

# chown -h rat:cheese trap.txt

As the superuser, change user and group ownership — recursively — on all files and sub-directories under the trap directory.

# chown -R rat:cheese trap

Change the group ownership to “graft” on a file, named: “politician.txt”. Verify that your username is associated with the “graft” group in /etc/group.

$ chgrp -h graft politician.txt

Change the group ownership to “graft” recursively on a directory, named: “judges”.

$ chgrp -R graft judges

Strip all permissions from a file, so as to have it not be executable, read from or written to. (Generally, a sysadmin will do so in the wake of a misbehaving script, and he will change the owner to root, preventing an under-priviledged user from altering the permissions on the script.)

# chmod 000 misbehaved.cgi 
 # chown -h root misbehaved.cgi

Locating files and directories with find

Locate and print all files in the www directory and its child directories which contain the .php extension. When using the find utility, always enclose wildcards expressions in quotes to prevent shell expansion.

$ find $HOME/www -iname “*.php”

Find and remove a file, -C with a non-escapable “-” character in its name.

$ find . -iname ‘-C’ -exec rm {} \;

Locate all files in the current directory with the extension of .html, then search each located file for “<p class=”e”>” and change every found instance of it to “<p>” with the sed (stream) editor. (N.B. On *BSD machines, sed requires an argument after “-i”. Usually, 2 double quotes, “ “, will work.)

$ find . -type f -iname “*.html” -exec sed -i \ ‘s/<p class=\”e\”>/<p>/g’ {} \;

Find all child directories with dangerous 777 permissions and alter them to reasonably sane 751 permissions.

$ find . -type d -perm 777 -exec chmod 751 {} \;

Find all files in the current and child directories with world write permissions and alter them so that only the user can write to them but group and other can read them.

$ find . -type f -perm 666 -exec chmod 644 {} \;

Find all child directories with the old group ID “felony” and change them to the new group ID “misdemeanor”.

$ find . -type d -group felony -exec chown -R misdemeanor {} \;

As the superuser, locate all files in the current directory and it sub-directories owned by the rat user, then delete the located files. (Be careful when using find with -exec and rm.)

# find . -type f -user rat -exec rm {} \;

Archiving data

Compress the frauds file with tar to create frauds.tar.

$ tar cvf frauds.tar frauds

Extract the contents of the frauds.tar file.

$ tar xvf frauds.tar

Extract the crimes.tar.gz file.

$ tar xzvf crimes.tar.gz

Extract the diseases.tgz file.

$ tar xzvf diseases.tgz

Compress music.tar with the highest level of gzip compression. After executing the command, the file created will be “music.tar.gz”.

$ gzip -9 music.tar

Decompress hack.gz.

$ gzip -d hack.gz

Extract the entire pr0n archive, which has the .tar.bz2 extension.

$ tar xjvf pr0n.tar.bz2

Create png_archive.tar from the contents of png_directory — located in the current directory.

$ tar cvf png_archive.tar ./png_directory

List (test) but do not extract any file from png_archive.tar.

$ tar tvf png_archive.tar

Extract only the dirty.png from png_archive.tar.

$ tar xvf png_archive.tar dirty.png

Unzip a file, named: “dresses.zip”.

$ unzip dresses.zip

Miscellaneous utilities

Search for and display a string of characters “Drone Assassination” in “cia.html” with grep. Below, “-i” = case insensitive.

$ grep -i “Drone Assassination” cia.html

Search for and display “Charles Manson” in every file residing under the “prison” directory.

$ grep -r “Charles Manson” prison \;

Determine which partitions are currently mounted and print their sizes in a human, understandable format.

$ df -h

Print the size of the contents of the “cracked” directory in a human, understandable format.

$ du -sh cracked

Change the password. The user will be prompted for his current password, a new password and the confirmation of the new password.

$ passwd

Become the superuser, i.e. root. A valid password is required, and the non-privileged executor must be associated with the wheel group.

$ su

Inspect the memory usage, percentage of cpu resources consumed, programs executed, etc. by the user “stinky” with top.

$ top -u stinky

Display all of the processes spawned by the user “pinhead”.

$ ps aux |grep pinhead

vi (visual editor) is the only text editor that a user needs to master. Written in the 1970s by ultra-guru, Berkeley-grad, co-inventor of the TCP/IP protocol, inventor of JAVA, etc. Bill Joy, vi is installed — by default — on virtually every UNIX operating system. For an introduction, see: http://www.linfo.org/vi.

$ vi joy.txt

Inspect the last screen of characters written to “log_file”. Use tail to monitor frequently updating files (e.g. logs) in real time.

$ tail -f log_file

Inspect the first screen of characters written to “crash.shtml” with more. Hit the space bar to navigate downwards into the text.

$ more crash.shtml