Unix Primer - Basic Commands In the Unix Shell

• How To Find A File In Unix
• Search For A File In Unix

If you have no experience with the Unix command shell, it will bebest to work through this primer. The last section summarizes thebasic file manipulation commands.

/dev/zero: This file is used to create a file with no data but with required size(A file with all zero’s). In other words this will create a data file with all zeros in the file which will give the size to a file. Create a file with /dev/zero file. Dd if=/dev/zero of=/opt/abc.txt bs=4096 count=1000. 5 Ways to Empty or Delete a Large File Content in Linux. When you redirect the out of. We need to compress the contents of the file and truncate or “zero. It can be used to find files and directories and perform subsequent operations on them. It supports searching by file, folder, name, creation date, modification date, owner and permissions. By using the - exec other UNIX commands can be executed on files or folders found. Will give you a long format listing (needed to actually see the file size) and round file sizes up to the nearest MiB. If you want MB (10^6 bytes) rather than MiB (2^20 bytes) units, use --block-size=MB instead.

Unix Shell

The shell is a command programming language that provides an interface to the UNIX operating system. The remainder of thistutorial presents basic commands to use within the UNIX shell.

Directories

The shell should start you in your home directory. This is yourindividual space on the UNIX system for your files. You can find outthe name of your current working directory by typing:

(The '%' designates your command line prompt, and you should typethe letters 'p', 'w', 'd', and then 'enter' - always conclude eachcommand by pressing the 'enter' key. The response that follows on thenext line will be the name of your home directory, where the namefollowing the last slash should be your username.) Thedirectory structure can be conceptualized as an inverted tree.

No matter where in the directory structure you are, you can alwaysget back to your home directory by typing:

(without specifying a directory name).

From your home directory, create a new subdirectory named 'primer'for working through this tutorial:



You can remove an empty subdirectory with the following command(but don't do this right now):

(Note: if you do remove 'primer', please create it again.)

Now change to the 'primer' subdirectory, making it your currentworking directory:

Files

Files live within directories. You can see a list of the files inyour 'primer' directory (which should be your current working directory)by typing:

Since you just created the directory, nothing will be listedbecause the directory is empty. Create your first file using the picotext editor:

The pico editor fills the entire console window. You can typetext and move the cursor around with the arrow keys; the bottom of thescreen presents the commands available. Type the sentence: 'My firstfile.' Then press '^O' (hold the left 'control' key while pressing'O') and 'enter' to save the file, then '^X' (hold the left 'control'key while pressing 'X') to exit pico. Now when you list your files,you will see file 'first' listed:

You can view a text file with the following command:

('cat' is short for concatenate - you can use this to displaymultiple files together on the screen.) If you have a file that islonger than your 24-line console window, use instead 'more' to list onepage at a time or 'less' to scroll the file down and up with the arrowkeys. Don't use these programs to try to display binary (non-text)files on your console - the attempt to print the non-printable controlcharacters might alter your console settings and render the consoleunusable.

Copy file 'first' using the following command:

By doing this you have created a new file named '2nd' which is aduplicate of file 'first'. The file listing reveals:

Now rename the file '2nd' to 'second':

Listing the files still shows two files because you haven'tcreated a new file, just changed an existing file's name:

If you 'cat' the second file, you'll see the same sentence as inyour first file:

'mv' will allow you to move files, not just rename them. Performthe following commands:

This creates a new subdirectory named 'sub', moves 'second' into'sub', then lists the contents of both directories. You can list evenmore information about files by using the '-l' option with 'ls':

(where 'username' will be your username and 'group' will beyour group name). Among other things, this lists the creation date andtime, file access permissions, and file size in bytes. The letter 'd'(the first character on the line) indicates the directory names.

Next perform the following commands:

This changes your current working directory to the 'sub'subdirectory under 'primer', lists the files there, then changes youback up a level. The '.' always refers to the parent directory of thegiven subdirectory.

Finally, clean up the duplicate files by removing the 'second'file and the 'sub' subdirectory:

This shows that you can refer to a file in a different directoryusing the relative path name to the file (you can also use the absolutepath name to the file - something like '/Users/username/primer/sub/second',depending on your home directory). You can also include the '.'within the path name (for instance, you could have referred to the fileas './primer/sub/second').

Other Useful Commands

The current date and time are printed with the following command:

Remote login to another machine can be accomplished using the'ssh' command:

where 'myname' will be your username on the remote system(possibly identical to your username on this system) and 'host' is thename (or IP address) of the machine you are logging into. Note that'ssh' is secure unlike older programs such as 'telnet'.

Transfer files between machines using 'scp'. For example, to copyfile 'myfile' from the remote machine named 'host', the command wouldbe:

(The '.' refers to your current working directory, meaning thatthe destination for 'myfile' is your current directory.)

If you are running X11, you should be able to start someapplications from the command line, for example:

(The '&' character tells the console to run Matlab in thebackground - this way you immediately get a new prompt without firsthaving to quit Matlab.)



In OS X, you can open file using the command 'open'. This commandwill open the file with the application corresponding to the file type.For example

will open the file README.pdf using Preview. You can alsoopen applications using the -a flag

'I need to check whether a file is zero byte or not. If it is, I want it to exit from the script. Otherwise, I need to FTP the file. How do I do this?' How to find the file size in unix. Anybody can help. Command to find out total size of a specific file size (spread over the server) abhinov: Solaris: 3.

or open a file using TextEdit with the -e flag

Online Help

You can get online help from the 'man' pages ('man' is short for'manual'). The information is terse, but generally comprehensive, so itprovides a nice reminder if you have forgotten the syntax of aparticular command or need to know the full list of options.

Use the '-k' option to provide a list of commands that pertain toa particular topic. For instance, try:

(and make sure to include the quotation marks). One of thecommands listed should be the 'cp' file copy command. Display the manpage for 'cp' by typing:

Logging Out

It is very important to log out of your account whenever you aredone using it, especially if you are on a public machine.

To close a shell window in a graphical environment, you can type:

Logging out from a graphical environment will require clicking onthe appropriate icon. Logging out of a remote session can be done byeither using the 'exit' command or by typing:

Summary Of Basic Shell Commands

% pico myfile	text edit file 'myfile'
% ls	list files in current directory
% ls -l	long format listing
% cat myfile	view contents of text file 'myfile'
% more myfile	paged viewing of text file 'myfile'
% less myfile	scroll through text file 'myfile'
% cp srcfile destfile	copy file 'srcfile' to new file 'destfile'
% mv oldname newname	rename (or move) file 'oldname' to 'newname'
% rm myfile	remove file 'myfile'
% mkdir subdir	make new directory 'subdir'
% cd subdir	change current working directory to 'subdir'
% rmdir subdir	remove (empty) directory 'subdir'
% pwd	display current working directory
% date	display current date and time of day
% ssh -l myname host	remote shell login of username 'myname' to 'host'
% scp myname@host:myfile .	remote copy of file 'myfile' to current directory
% netscape &	start Netscape web browser (in background)
% man -k 'topic'	search manual pages for 'topic'
% man command	display man page for 'command'
% exit	exit a terminal window
% logout	logout of a console session

How To Find A File In Unix

I have an unarchiver that takes in an archive name, and a directory name, and dumps all files from that archive into that directory. No other command-line options. However, someone zipped a file in the archive I am looking to decompress, with 500-ish characters in the filename, and now that program fails when it hits that file (practically all file systems have a limit of 256). What alternative do I have, short of changing the source code and recompiling the unarchiver?

I must mount something as a directory, which would take the files that the unarchiver is writing, and dump them elsewhere-- possibly even as one big file. This something should not send fail messages, even if some write really did fail. Is this possible?

Search For A File In Unix

Browse other questions tagged unixdirectoryfilenames or ask your own question.