The shell Flashcards

Question

List all files (even ones with names beginning with a period character, which are normally hidden) in the parent of the working directory in long format

Answer 1

command -options arguments

Answer 2

a program that lets you view text files. This is very handy since many of the files used to control and configure Linux are human readable.

Answer 3

There are many ways to represent information on a computer. All methods involve defining a relationship between the information and some numbers that will be used to represent it. Computers, after all, only understand numbers and all data is converted to numeric representation. Some of these representation systems are very complex (such as compressed multimedia files), while others are rather simple. One of the earliest and simplest is called ASCII text. ASCII (pronounced "As-Key") is short for American Standard Code for Information Interchange. This is a simple encoding scheme that was first used on Teletype machines to map keyboard characters to numbers. Text is a simple one-to-one mapping of characters to numbers. It is very compact. Fifty characters of text translates to fifty bytes of data. Throughout a Linux system, many files are stored in text format and there are many Linux tools that work with text files. Even the legacy operating systems recognize the importance of this format. The well-known NOTEPAD.EXE program is an editor for plain ASCII text files.

Answer 4

less text_file Once started, less will display the text file one page at a time. You may use the Page Up and Page Down keys to move through the text file. To exit less, type "q".

Answer 5

Page Up or b: Scroll back one page Page Down or space: Scroll forward one page G: Go to the end of the text file 1G: Go to the beginning of the text file /characters: Search forward in the text file for an occurrence of the specified characters n: Repeat the previous search h: Display a complete list less commands and options q: Quit

Answer 6

examines a file and tell you what kind of file it is While it may seem that most files cannot be viewed as text, you will be surprised how many can. This is especially true of the important configuration files. You will also notice during our adventure that many features of the operating system are controlled by shell scripts. In Linux, there are no secrets!

Answer 7

File Type / Description / Viewable as text? ASCII text/The name says it all/yes Bourne-Again shell script text/A bash script/yes ELF 32-bit LSB core file/A core dump file (a program will create this when it crashes)/no ELF 32-bit LSB executable/An executable binary program/no ELF 32-bit LSB shared object/A shared library/no GNU tar archive/A tape archive file. A common way of storing groups of files./no, use tar tvf to view listing. gzip compressed data/An archive compressed with gzip/no HTML document text/A web page/yes JPEG image data/A compressed JPEG image/no PostScript document text/A PostScript file/yes RPM/A Red Hat Package Manager archive/no, use rpm -q to examine contents. Zip archive data/An archive compressed with zip/no

Answer 8

The root directory where the file system begins. In most cases the root directory only contains subdirectories.

Answer 9

This is where the Linux kernel and boot loader files are kept. The kernel is a file called vmlinuz

Answer 10

The /etc directory contains the configuration files for the system. All of the files in /etc should be text files. Points of interest: /etc/passwd The passwd file contains the essential information for each user. It is here that users are defined. /etc/fstab The fstab file contains a table of devices that get mounted when your system boots. This file defines your disk drives. /etc/hosts This file lists the network host names and IP addresses that are intrinsically known to the system. /etc/init.d This directory contains the scripts that start various system services typically at boot time.

Answer 11

These two directories contain most of the programs for the system. The /bin directory has the essential programs that the system requires to operate, while /usr/bin contains applications for the system's users.

Answer 12

The sbin directories contain programs for system administration, mostly for use by the superuser.

Answer 13

The /usr directory contains a variety of things that support user applications. Some highlights: /usr/share/X11 Support files for the X Window system /usr/share/dict Dictionaries for the spelling checker. Bet you didn't know that Linux had a spelling checker. See look and aspell. /usr/share/doc Various documentation files in a variety of formats. /usr/share/man The man pages are kept here. /usr/src Source code files. If you installed the kernel source code package, you will find the entire Linux kernel source code here.

Answer 14

/usr/local and its subdirectories are used for the installation of software and other files for use on the local machine. What this really means is that software that is not part of the official distribution (which usually goes in /usr/bin) goes here. When you find interesting programs to install on your system, they should be installed in one of the /usr/local directories. Most often, the directory of choice is /usr/local/bin.

Answer 15

The /var directory contains files that change as the system is running. This includes: /var/log Directory that contains log files. These are updated as the system runs. You should view the files in this directory from time to time, to monitor the health of your system. /var/spool This directory is used to hold files that are queued for some process, such as mail messages and print jobs. When a user's mail first arrives on the local system (assuming you have local mail), the messages are first stored in /var/spool/mail

Answer 16

The shared libraries (similar to DLLs in that other operating system) are kept here.

Answer 17

/home is where users keep their personal work. In general, this is the only place users are allowed to write files. This keeps things nice and clean :-)

Answer 18

This is the superuser's home directory.

Answer 19

/tmp is a directory in which programs can write their temporary files.

Answer 20

The /dev directory is a special directory, since it does not really contain files in the usual sense. Rather, it contains devices that are available to the system. In Linux (like Unix), devices are treated like files. You can read and write devices as though they were files. For example /dev/fd0 is the first floppy disk drive, /dev/sda (/dev/hda on older systems) is the first hard drive. All the devices that the kernel understands are represented here.

Answer 21

The /proc directory is also special. This directory does not contain files. In fact, this directory does not really exist at all. It is entirely virtual. The /proc directory contains little peep holes into the kernel itself. There are a group of numbered entries in this directory that correspond to all the processes running on the system. In addition, there are a number of named entries that permit access to the current configuration of the system. Many of these entries can be viewed. Try viewing /proc/cpuinfo. This entry will tell you what the kernel thinks of your CPU.

Answer 22

Finally, we come to /media, a normal directory which is used in a special way. The /media directory is used for mount points. As we learned in the second lesson, the different physical storage devices (like hard disk drives) are attached to the file system tree in various places. This process of attaching a device to the tree is called mounting. For a device to be available, it must first be mounted. When your system boots, it reads a list of mounting instructions in the file /etc/fstab, which describes which device is mounted at which mount point in the directory tree. This takes care of the hard drives, but you may also have devices that are considered temporary, such as CD-ROMs, thumb drives, and floppy disks. Since these are removable, they do not stay mounted all the time. The /media directory is used by the automatic device mounting mechanisms found in modern desktop oriented Linux distributions. On systems that require manual mounting of removable devices, the /mnt directory provides a convenient place for mounting these temporary devices. You will often see the directories /mnt/floppy and /mnt/cdrom. To see what devices and mount points are used, type mount.

Answer 23

System.map, module-info, and vmlinuz for example Symbolic links are a special type of file that points to another file. With symbolic links, it is possible for a single file to have multiple names. Here's how it works: Whenever the system is given a file name that is a symbolic link, it transparently maps it to the file it is pointing to Just what is this good for? This is a very handy feature. Let's consider the directory listing above (which is the /boot directory of an old Red Hat 5.2 system). This system has had multiple versions of the Linux kernel installed. We can see this from the files vmlinuz-2.0.36-0.7 and vmlinuz-2.0.36-3. These file names suggest that both version 2.0.36-0.7 and 2.0.36-3 are installed. Because the file names contain the version it is easy to see the differences in the directory listing. However, this would be confusing to programs that rely on a fixed name for the kernel file. These programs might expect the kernel to simply be called "vmlinuz". Here is where the beauty of the symbolic link comes in. By creating a symbolic link called vmlinuz that points to vmlinuz-2.0.36-3, we have solved the problem. Example of a symbolic link in the output of a "ls -l" command: lrwxrwxrwx 25 Jul 3 16:42 System.map -> /boot/System.map-2.0.36-3

Answer 24

ln command

Answer 25

cp, mv, rm, mkdir These four commands are among the most frequently used Linux commands. They are the basic commands for manipulating both files and directories. Now, to be frank, some of the tasks performed by these commands are more easily done with a graphical file manager. With a file manager, you can drag and drop a file from one directory to another, cut and paste files, delete files, etc. So why use these old command line programs? The answer is power and flexibility. While it is easy to perform simple file manipulations with a graphical file manager, complicated tasks can be easier with the command line programs.

Answer 26

[characters] The set of characters may also be expressed as a POSIX character class such as one of the following: POSIX Character Classes [:alnum:] Alphanumeric characters [:alpha:] Alphabetic characters [:digit:] Numerals [:upper:] Uppercase alphabetic characters [:lower:] Lowercase alphabetic characters

Answer 27

[!characters]

Answer 28

Pattern Matches ------------- * All filenames g* All filenames that begin with the character "g" b*.txt All filenames that begin with the character "b" and end with the characters ".txt" Data??? Any filename that begins with the characters "Data" followed by exactly 3 more characters [abc]* Any filename that begins with "a" or "b" or "c" followed by any other characters [[:upper:]]* Any filename that begins with an uppercase letter. This is an example of a character class. BACKUP.[[:digit:]][[:digit:]] Another example of character classes. This pattern matches any filename that begins with the characters "BACKUP." followed by exactly two numerals. *[![:lower:]] Any filename that does not end with a lowercase letter.

Answer 29

any command that accepts filename arguments

Answer 30

A note on notation: ... signifies that an item can be repeated one or more times.

Answer 31

Command Results cp file1 file2 Copies the contents of file1 into file2. If file2 does not exist, it is created; otherwise, file2 is silently overwritten with the contents of file1. cp -i file1 file2 Like above however, since the "-i" (interactive) option is specified, if file2 exists, the user is prompted before it is overwritten with the contents of file1. cp file1 dir1 Copy the contents of file1 (into a file named file1) inside of directory dir1. cp -R dir1 dir2 Copy the contents of the directory dir1. If directory dir2 does not exist, it is created. Otherwise, it creates a directory named dir1 within directory dir2.

Answer 32

Command Results ----------- mv file1 file2 If file2 does not exist, then file1 is renamed file2. If file2 exists, its contents are silently replaced with the contents of file1. mv -i file1 file2 Like above however, since the "-i" (interactive) option is specified, if file2 exists, the user is prompted before it is overwritten with the contents of file1. mv file1 file2 file3 dir1 The files file1, file2, file3 are moved to directory dir1. If dir1 does not exist, mv will exit with an error. mv dir1 dir2 If dir2 does not exist, then dir1 is renamed dir2. If dir2 exists, the directory dir1 is moved within directory dir2.

Answer 33

Command Results ---------- rm file1 file2 Delete file1 and file2. rm -i file1 file2 Like above however, since the "-i" (interactive) option is specified, the user is prompted before each file is deleted. rm -r dir1 dir2 Directories dir1 and dir2 are deleted along with all of their contents.

Answer 34

Be careful with rm! Linux does not have an undelete command. Once you delete something with rm, it's gone. You can inflict terrific damage on your system with rm if you are not careful, particularly with wildcards. Before you use rm with wildcards, try this helpful trick: construct your command using ls instead. By doing this, you can see the effect of your wildcards before you delete files. After you have tested your command with ls, recall the command with the up-arrow key and then substitute rm for ls in the command.

Answer 35

Command Results -------- cp *.txt text_files Copy all files in the current working directory with names ending with the characters ".txt" to an existing directory named text_files. mv my_dir ../*.bak my_new_dir Move the subdirectory my_dir and all the files ending in ".bak" in the current working directory's parent directory to an existing directory named my_new_dir. rm *~ Delete all files in the current working directory that end with the character "~". Some applications create backup files using this naming scheme. Using this command will clean them out of a directory.

Answer 36

type The type command is a shell builtin that displays the kind of command the shell will execute, given a particular command name. It works like this: type where “command” is the name of the command you want to examine. For example: "type type" will return "type is a shell builtin"; "type cp" will return "cp is /bin/cp"; "type ls" returns "ls is aliased to `ls --color=tty'" Notice that the one for ls (taken from a Fedora system) and how the ls command is actually an alias for the ls command with the “-- color=tty” option added. Now we know why the output from ls is displayed in color! In Mac, it returns "ls -GFh"

Answer 37

which Sometimes there is more than one version of an executable program installed on a system. While this is not very common on desktop systems, it's not unusual on large servers. To determine the exact location of a given executable, the which command is used: $ which ls /bin/ls which only works for executable programs, not builtins nor aliases that are substitutes for actual executable programs.

Answer 38

help bash has a built-in help facility available for each of the shell builtins. To use it, type “help” followed by the name of the shell builtin. Optionally, you may add the -m option (seems to be -p in Mac) to change the format of the output.

Answer 39

man Most executable programs intended for command line use provide a formal piece of documentation called a manual or man page. A special paging program called man is used to view them. It is used like this: man program where “program” is the name of the command to view. Man pages vary somewhat in format but generally contain a title, a synopsis of the command's syntax, a description of the command's purpose, and a listing and description of each of the command's options. Man pages, however, do not usually include examples, and are intended as a reference, not a tutorial On most Linux systems, man uses less to display the manual page, so all of the familiar less commands work while displaying the page

Answer 40

Commands can be one of 4 different kinds: An executable program like all those files we saw in /usr/bin. Within this category, programs can be compiled binaries such as programs written in C and C++, or programs written in scripting languages such as the shell, Perl, Python, Ruby, etc. A command built into the shell itself. bash provides a number of commands internally called shell builtins. The cd command, for example, is a shell builtin. A shell function. These are miniature shell scripts incorporated into the environment. We will cover configuring the environment and writing shell functions in later lessons, but for now, just be aware that they exist. An alias. Commands that you can define yourselves, built from other commands. This will be covered in a later lesson.

Answer 41

A note on notation: When square brackets appear in the description of a command's syntax, they indicate optional items. A vertical bar character indicates mutually exclusive items So, "cd [-L|-P] [dir]" means: the command cd may be followed optionally by either a “-L” or a “-P” and further, optionally followed by the argument “dir”.

Answer 42

Many executable programs support a “--help” option that displays a description of the command's supported syntax and options.

Answer 43

Many software packages installed on your system have documentation files residing in the /usr/share/doc directory. Most of these are stored in plain text format and can be viewed with less. Some of the files are in HTML format and can be viewed with your web browser. You may encounter some files ending with a “.gz” extension. This indicates that they have been compressed with the gzip compression program. The gzip package includes a special version of less called zless that will display the contents of gzip-compressed text files.

Answer 44

As we have seen, many commands such as ls print their output on the display. This does not have to be the case, however. By using some special notations we can redirect the output of many commands to files, devices, and even to the input of other commands. Most command line programs that display their results do so by sending their results to a facility called standard output. By default, standard output directs its contents to the display.

Answer 45

> Example: ls > file_list.txt In this example, the ls command is executed and the results are written in a file named file_list.txt. Since the output of ls was redirected to the file, no results appear on the display. Each time the command above is repeated, file_list.txt is overwritten from the beginning with the output of the command ls.

Answer 46

>> If you want the new results to be appended to the file instead, use ">>" like this: ls >> file_list.txt When the results are appended, the new results are added to the end of the file, thus making the file longer each time the command is repeated. If the file does not exist when you attempt to append the redirected output, the file will be created.

Answer 47

Many commands can accept input from a facility called standard input. By default, standard input gets its contents from the keyboard, but like standard output, it can be redirected.

Answer 48

< Example: sort < file_list.txt we used the sort command to process the contents of file_list.txt. The results are output on the display since the standard output was not redirected. We could redirect standard output to another file like this: sort < file_list.txt > sorted_file_list.txt As you can see, a command can have both its input and output redirected. Be aware that the order of the redirection does not matter. The only requirement is that the redirection operators (the "") must appear after the other options and arguments in the command.

Answer 49

The most useful and powerful thing you can do with I/O redirection is to connect multiple commands together with what are called pipelines. With pipelines, the standard output of one command is fed into the standard input of another.

Answer 50

By using this "| less" trick, you can make any command have scrolling output. I use this technique all the time.

Answer 51

ls -l | less

Answer 52

Command: What it does ------- ls -lt | head Displays the 10 newest files in the current directory. du | sort -nr Displays a list of directories and how much space they consume, sorted from the largest to the smallest. find . -type f -print | wc -l Displays the total number of files in the current working directory and all of its subdirectories.

Answer 53

One kind of program frequently used in pipelines is called filters. Filters take standard input and perform an operation upon it and send the results to standard output. In this way, they can be combined to process information in powerful ways.

Answer 54

Program: What it does ---------- sort Sorts standard input then outputs the sorted result on standard output. uniq Given a sorted stream of data from standard input, it removes duplicate lines of data (i.e., it makes sure that every line is unique). grep Examines each line of data it receives from standard input and outputs every line that contains a specified pattern of characters. fmt Reads text from standard input, then outputs formatted text on standard output. pr Takes text input from standard input and splits the data into pages with page breaks, headers and footers in preparation for printing. head Outputs the first few lines of its input. Useful for getting the header of a file. tail Outputs the last few lines of its input. Useful for things like getting the most recent entries from a log file. tr Translates characters. Can be used to perform tasks such as upper/lowercase conversions or changing line termination characters from one type to another (for example, converting DOS text files into Unix style text files). sed Stream editor. Can perform more sophisticated text translations than tr. awk An entire programming language designed for constructing filters. Extremely powerful.

Answer 55

Linux provides a program called lpr that accepts standard input and sends it to the printer. It is often used with pipes and filters. Here are a couple of examples: cat poorly_formatted_report.txt | fmt | pr | lpr cat unsorted_list_with_dupes.txt | sort | uniq | pr | lpr In the first example, we use cat to read the file and output it to standard output, which is piped into the standard input of fmt. fmt formats the text into neat paragraphs and outputs it to standard output, which is piped into the standard input of pr. pr splits the text neatly into pages and outputs it to standard output, which is piped into the standard input of lpr. lpr takes its standard input and sends it to the printer. The second example starts with an unsorted list of data with duplicate entries. First, cat sends the list into sort which sorts it and feeds it into uniq which removes any duplicates. Next pr and lpr are used to paginate and print the list.

Answer 56

Often you will see software distributed as a gzipped tar file. This is a traditional Unix style tape archive file (created with tar) that has been compressed with gzip. You can recognize these files by their traditional file extensions, ".tar.gz" or ".tgz". You can use the following command to view the directory of such a file on a Linux system: tar tzvf name_of_file.tar.gz | less

Answer 57

Each time you type a command line and press the enter key, bash performs several processes upon the text before it carries out your command. We have seen a couple of cases of how a simple character sequence, for example “*”, can have a lot of meaning to the shell. The process that makes this happen is called expansion. With expansion, you type something and it is expanded into something else before the shell acts upon it.

Answer 58

Result: Desktop Documents ls-output.txt Music Pictures Public Templates Videos So what just happened? Why didn't echo print “*”? As you recall from our work with wildcards, the “*” character means match any characters in a filename, but what we didn't see in our original discussion was how the shell does that. The simple answer is that the shell expands the “*” into something else (in this instance, the names of the files in the current working directory) before the echo command is executed. When the enter key is pressed, the shell automatically expands any qualifying characters on the command line before the command is carried out, so the echo command never saw the “*”, only its expanded result. Knowing this, we can see that echo behaved as expected.

Answer 59

The mechanism by which wildcards work is called pathname expansion. If we try some of the techniques that we employed in our earlier lessons, we will see that they are really expansions. Ex: echo [[:upper:]]* Result: Desktop Documents Music Pictures Public Templates Videos Another Ex: echo /usr/*/share Result: /usr/kerberos/share /usr/local/share

Answer 60

As you may recall from our introduction to the cd command, the tilde character (“~”) has a special meaning. When used at the beginning of a word, it expands into the name of the home directory of the named user, or if no user is named, the home directory of the current user echo ~ /home/me echo ~foo /home/foo

Answer 61

echo $((2 + 2)) Arithmetic expansion only supports integers (whole numbers, no decimals), but can perform quite a number of different operations.

Answer 62

$((expression)) where expression is an arithmetic expression consisting of values and arithmetic operators. Spaces are not significant in arithmetic expressions and expressions may be nested. For example, to multiply five squared by three: echo $(($((5**2)) * 3))

Answer 63

echo Five divided by two equals $((5/2)) Result: Five divided by two equals 2 (Notice the effect of integer division)

Answer 64

Perhaps the strangest expansion is called brace expansion. With it, you can create multiple text strings from a pattern containing braces. Here's an example: echo Front-{A,B,C}-Back Result: Front-A-Back Front-B-Back Front-C-Back Previous | Contents | Next Expansion Each time you type a command line and press the enter key, bash performs several processes upon the text before it carries out your command. We have seen a couple of cases of how a simple character sequence, for example “*”, can have a lot of meaning to the shell. The process that makes this happen is called expansion. With expansion, you type something and it is expanded into something else before the shell acts upon it. To demonstrate what we mean by this, let's take a look at the echo command. echo is a shell builtin that performs a very simple task. It prints out its text arguments on standard output: [me@linuxbox me]$ echo this is a test this is a test That's pretty straightforward. Any argument passed to echo gets displayed. Let's try another example: [me@linuxbox me]$ echo * Desktop Documents ls-output.txt Music Pictures Public Templates Videos So what just happened? Why didn't echo print “*”? As you recall from our work with wildcards, the “*” character means match any characters in a filename, but what we didn't see in our original discussion was how the shell does that. The simple answer is that the shell expands the “*” into something else (in this instance, the names of the files in the current working directory) before the echo command is executed. When the enter key is pressed, the shell automatically expands any qualifying characters on the command line before the command is carried out, so the echo command never saw the “*”, only its expanded result. Knowing this, we can see that echo behaved as expected. Pathname Expansion The mechanism by which wildcards work is called pathname expansion. If we try some of the techniques that we employed in our earlier lessons, we will see that they are really expansions. Given a home directory that looks like this: [me@linuxbox me]$ ls ``` Desktop ls-output.txt Documents Music Pictures Public Templates Videos we could carry out the following expansions: ``` [me@linuxbox me]$ echo D* Desktop Documents and: [me@linuxbox me]$ echo *s Documents Pictures Templates Videos or even: [me@linuxbox me]$ echo [[:upper:]]* Desktop Documents Music Pictures Public Templates Videos and looking beyond our home directory: [me@linuxbox me]$ echo /usr/*/share /usr/kerberos/share /usr/local/share Tilde Expansion As you may recall from our introduction to the cd command, the tilde character (“~”) has a special meaning. When used at the beginning of a word, it expands into the name of the home directory of the named user, or if no user is named, the home directory of the current user: [me@linuxbox me]$ echo ~ /home/me If user “foo” has an account, then: [me@linuxbox me]$ echo ~foo /home/foo Arithmetic Expansion The shell allows arithmetic to be performed by expansion. This allow us to use the shell prompt as a calculator: [me@linuxbox me]$ echo $((2 + 2)) 4 Arithmetic expansion uses the form: $((expression)) where expression is an arithmetic expression consisting of values and arithmetic operators. Arithmetic expansion only supports integers (whole numbers, no decimals), but can perform quite a number of different operations. Spaces are not significant in arithmetic expressions and expressions may be nested. For example, to multiply five squared by three: [me@linuxbox me]$ echo $(($((5**2)) * 3)) 75 Single parentheses may be used to group multiple subexpressions. With this technique, we can rewrite the example above and get the same result using a single expansion instead of two: [me@linuxbox me]$ echo $(((5**2) * 3)) 75 Here is an example using the division and remainder operators. Notice the effect of integer division: [me@linuxbox me]$ echo Five divided by two equals $((5/2)) Five divided by two equals 2 [me@linuxbox me]$ echo with $((5%2)) left over. with 1 left over. Brace Expansion Perhaps the strangest expansion is called brace expansion. With it, you can create multiple text strings from a pattern containing braces. Here's an example: [me@linuxbox me]$ echo Front-{A,B,C}-Back Front-A-Back Front-B-Back Front-C-Back Patterns to be brace expanded may contain a leading portion called a preamble and a trailing portion called a postscript. The brace expression itself may contain either a comma-separated list of strings, or a range of integers or single characters. The pattern may not contain embedded whitespace. Another example, using integer range: echo Number_{1..5} Result: Number_1 Number_2 Number_3 Number_4 Number_5 echo {Z..A} Result: Z Y X W V U T S R Q P O N M L K J I H G F E D C B A Nested brace expansions: echo a{A{1,2},B{3,4}}b Result: aA1b aA2b aB3b aB4b So what is this good for? The most common application is to make lists of files or directories to be created. For example, if you were a photographer and had a large collection of images you wanted to organize into years and months, the first thing you might do is create a series of directories named in numeric “Year-Month” format. This way, the directory names will sort in chronological order. You could type out a complete list of directories, but that's a lot of work and it's error-prone too. Instead, you could do this: [me@linuxbox me]$ mkdir Photos [me@linuxbox me]$ cd Photos [me@linuxbox Photos]$ mkdir {2007..2009}-0{1..9} {2007..2009}-{10..12} [me@linuxbox Photos]$ ls 2007-01 2007-07 2008-01 2008-07 2009-01 2009-07 2007-02 2007-08 2008-02 2008-08 2009-02 2009-08 2007-03 2007-09 2008-03 2008-09 2009-03 2009-09 2007-04 2007-10 2008-04 2008-10 2009-04 2009-10 2007-05 2007-11 2008-05 2008-11 2009-05 2009-11 2007-06 2007-12 2008-06 2008-12 2009-06 2009-12

Answer 65

On its own the ps command shows the running processes by the user running it within a terminal window. To invoke ps simply type the following: ps The output will show rows of data containing the following information: ``` PID TTY Time Command The PID is the process ID which identifies the running process. The TTY is the terminal type. ``` On its own the ps command is quite limited. You probably want to see all the running processes.

Answer 66

We're only going to touch briefly on parameter expansion in this lesson, but we'll be covering it more later. It's a feature that is more useful in shell scripts than directly on the command line. Many of its capabilities have to do with the system's ability to store small chunks of data and to give each chunk a name. Many such chunks, more properly called variables, are available for your examination. For example, the variable named “USER” contains your user name. To invoke parameter expansion and reveal the contents of USER you would do this: echo $USER me To see a list of available variables, try this: printenv | less

Answer 67

Command substitution allows us to use the output of a command as an expansion: echo $(ls) Desktop Documents ls-output.txt Music Pictures Public Templates Videos Some older shell programs used back-ticks instead of $()

Answer 68

Great example of command substitution Here we passed the results of which cp as an argument to the ls command, thereby getting the listing of of the cp program without having to know its full pathname. We are not limited to just simple commands. You can even use entire pipelines in command substitutions.

Answer 69

echo The total is $100.00 Result: The total is 00.00 this is because $1 is interpreted as an undefined variable, thus producing an empty string. We can use quotes to prevent this from happening. If you place text inside double quotes, all the special characters used by the shell lose their special meaning and are treated as ordinary characters. The exceptions are “$”, “\” (backslash), and “`” (back- quote). This means that word-splitting, pathname expansion, tilde expansion, and brace expansion are suppressed, but parameter expansion, arithmetic expansion, and command substitution are still carried out. Example with space in file name without quotes: ls -l two words.txt ls: cannot access two: No such file or directory ls: cannot access words.txt: No such file or directory Example with space in file name WITH quotes: ls -l "two words.txt" -rw-rw-r-- 1 me me 18 2008-02-20 13:03 two words.txt Would be smart to do this: mv "two words.txt" two_words.txt

Answer 70

Once the double quotes are added, our command line contains a command followed by a single argument. The fact that newlines are considered delimiters by the word-splitting mechanism causes an interesting, albeit subtle, effect on command substitution. In the first instance, the unquoted command substitution resulted in a command line containing thirty-eight arguments. In the second, a command line with one argument that includes the embedded spaces and newlines.

Answer 71

Single quotes suppress all expansions. Parameter expansion, arithmetic expansion, and command substitution still take place within double quotes. As you can see, with each succeeding level of quoting, more and more of the expansions are suppressed: [me@linuxbox me]$ echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER text /home/me/ls-output.txt a b foo 4 me [me@linuxbox me]$ echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER" text ~/*.txt {a,b} foo 4 me [me@linuxbox me]$ echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER' text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER

Answer 72

Sometimes you only want to quote a single character. To do this, you can precede a character with a backslash, which in this context is called the escape character. Often this is done inside double quotes to selectively prevent an expansion: [me@linuxbox me]$ echo "The balance for user $USER is: \$5.00" The balance for user me is: $5.00 It is also common to use escaping to eliminate the special meaning of a character in a filename. For example, it is possible to use characters in filenames that normally have special meaning to the shell. These would include “$”, “!”, “&”, “ “, and others. To include a special character in a filename you can to this: [me@linuxbox me]$ mv bad\&filename good_filename

Answer 73

ls -l \ - -reverse \ - -human-readable \ - -full-time Using the backslash in this way allows us to embed newlines in our command. Note that for this trick to work, the newline must be typed immediately after the backslash. If you put a space after the backslash, the space will be ignored, not the newline.

Answer 74

Escape Character/Name/Possible Uses \n newline Adding blank lines to text \t tab Inserting horizontal tabs to text \a alert Makes your terminal beep \\ backslash Inserts a backslash \f formfeed Sending this to your printer ejects the page

Answer 75

The Unix-like operating systems, such as Linux differ from other computing systems in that they are not only multitasking but also multi-user. What exactly does this mean? It means that more than one user can be operating the computer at the same time. While your computer only has one keyboard and monitor, it can still be used by more than one user. For example, if your computer is attached to a network, or the Internet, remote users can log in via ssh (secure shell) and operate the computer. In fact, remote users can execute graphical applications and have the output displayed on a remote computer. The X Window system supports this. The multi-user capability of Unix-like systems is a feature that is deeply ingrained into the design of the operating system. If you remember the environment in which Unix was created, this makes perfect sense. Years ago before computers were "personal," they were large, expensive, and centralized. A typical university computer system consisted of a large mainframe computer located in some building on campus and terminals were located throughout the campus, each connected to the large central computer. The computer would support many users at the same time. In order to make this practical, a method had to be devised to protect the users from each other. After all, you could not allow the actions of one user to crash the computer, nor could you allow one user to interfere with the files belonging to another user.

Answer 76

chmod The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify the permissions. In this lesson we will focus on one of these, called the octal notation method. It is easy to think of the permission settings as a series of bits (which is how the computer thinks about them). Here's how it works: rwx rwx rwx = 111 111 111 rw- rw- rw- = 110 110 110 rwx --- --- = 111 000 000 and so on... ``` rwx = 111 in binary = 7 rw- = 110 in binary = 6 r-x = 101 in binary = 5 r-- = 100 in binary = 4 ```

Answer 77

su or sudo difference: Comparing the both, sudo lets one use the user account password to run system command. On the other hand, su forces one to share the root passwords to other users. Also, sudo doesn’t activate the root shell and runs a single command.

Answer 78

chgrp In the example below, we changed the group ownership of some_file from its previous group to "new_group". You must be the owner of the file or directory to perform a chgrp. chgrp new_group some_file

Answer 79

On a Linux system, each file and directory is assigned access rights for the owner of the file, the members of a group of related users, and everybody else. Rights can be assigned to read a file, to write a file, and to execute a file (i.e., run the file as a program). To see the permission settings for a file, we can use the ls command. As an example, we will look at the bash program which is located in the /bin directory: [me@linuxbox me]$ ls -l /bin/bash -rwxr-xr-x 1 root root 316848 Feb 27 2000 /bin/bash Here we can see: - The file "/bin/bash" is owned by user "root" - The superuser has the right to read, write, and execute this file - The file is owned by the group "root" - Members of the group "root" can also read and execute this file - Everybody else can read and execute this file

Answer 80

- or d = file or directory first rwx: read/write/execute permissions for file owner second rwx: read/write/execute permissions for the group owner of the file third rwx: read/write/execute permissions for all other users

Answer 81

The ones beginning with "7" are used with programs (since they enable execution) and the rest are for other kinds of files. Value: Meaning ---------- 777 (rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting. 755 (rwxr-xr-x) The file's owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users. 700 (rwx------) The file's owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others. 666 (rw-rw-rw-) All users may read and write the file. 644 (rw-r--r--) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change. 600 (rw-------) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.

Answer 82

The chmod command can also be used to control the access permissions for directories. Again, we can use the octal notation to set permissions, but the meaning of the r, w, and x attributes is different: r - Allows the contents of the directory to be listed if the x attribute is also set. w - Allows files within the directory to be created, deleted, or renamed if the x attribute is also set. x - Allows a directory to be entered (i.e. cd dir). Here are some useful settings for directories: Value: Meaning ----------- 777 (rwxrwxrwx) No restrictions on permissions. Anybody may list files, create new files in the directory and delete files in the directory. Generally not a good setting. 755 (rwxr-xr-x) The directory owner has full access. All others may list the directory, but cannot create files nor delete them. This setting is common for directories that you wish to share with other users. 700 (rwx------) The directory owner has full access. Nobody else has any rights. This setting is useful for directories that only the owner may use and must be kept private from others.

Answer 83

With sudo, one or more users are granted superuser privileges on an as needed basis. To execute a command as the superuser, the desired command is simply preceded with the sudo command. After the command is entered, the user is prompted for the user's password rather than the superuser's

Answer 84

Notice that in order to change the owner of a file, you must be the superuser. To do this, our example employed the su command, then we executed chown, and finally we typed exit to return to our previous session. chown works the same way on directories as it does on files.

Answer 85

In the previous lesson, we looked at some of the implications of Linux being a multi-user operating system. In this lesson, we will examine the multitasking nature of Linux, and how this is manipulated with the command line interface. As with any multitasking operating system, Linux executes multiple, simultaneous processes. Well, they appear simultaneous, anyway. Actually, a single processor computer can only execute one process at a time but the Linux kernel manages to give each process its turn at the processor and each appears to be running at the same time. There are several commands that can be used to control processes. They are: ps - list the processes running on the system kill - send a signal to one or more processes (usually to "kill" a process) jobs - an alternate way of listing your own processes bg - put a process in the background fg - put a process in the foreground

Answer 86

kill While the kill command is used to "kill" processes, its real purpose is to send signals to processes. Most of the time the signal is intended to tell the process to go away, but there is more to it than that. Programs (if they are properly written) listen for signals from the operating system and respond to them, most often to allow some graceful method of terminating. For example, a text editor might listen for any signal that indicates that the user is logging off, or that the computer is shutting down. When it receives this signal, it saves the work in progress before it exits. The kill command can send a variety of signals to processes.

Answer 87

Suppose that you have a program that becomes unresponsive; how do you get rid of it? You use the kill command, of course. Let's try this out on xload. First, you need to identify the process you want to kill. You can use either jobs or ps, to do this. If you use jobs you will get back a job number. With ps, you are given a process id (PID).

Answer 88

gives you a list of the signals it supports. Most are rather obscure, but several are useful to know: Signal # / Name / Description 1 SIGHUP Hang up signal. Programs can listen for this signal and act upon it. This signal is sent to processes running in a terminal when you close the terminal. 2 SIGINT Interrupt signal. This signal is given to processes to interrupt them. Programs can process this signal and act upon it. You can also issue this signal directly by typing Ctrl-c in the terminal window where the program is running. 15 SIGTERM Termination signal. This signal is given to processes to terminate them. Again, programs can process this signal and act upon it. This is the default signal sent by the kill command if no signal is specified. 9 SIGKILL Kill signal. This signal causes the immediate termination of the process by the Linux kernel. Programs cannot listen for this signal.

Answer 89

Use the ps command to get the process id (PID) of the process you want to terminate. Issue a kill command for that PID. If the process refuses to terminate (i.e., it is ignoring the signal), send increasingly harsh signals until it does terminate. ``` [me@linuxbox me]$ ps x | grep bad_program PID TTY STAT TIME COMMAND 2931 pts/5 SN 0:00 bad_program [me@linuxbox me]$ kill -SIGTERM 2931 [me@linuxbox me]$ kill -SIGKILL 2931 ``` In the example above I used the ps command with the x option to list all of my processes (even those not launched from the current terminal). In addition, I piped the output of the ps command into grep to list only list the program I was interested in. Next, I used kill to issue a SIGTERM signal to the troublesome program. In actual practice, it is more common to do it in the following way since the default signal sent by kill is SIGTERM and kill can also use the signal number instead of the signal name: [me@linuxbox me]$ kill 2931 Then, if the process does not terminate, force it with the SIGKILL signal: [me@linuxbox me]$ kill -9 2931

Answer 90

With the thousands of commands available for the command line user, how can you remember them all? The answer is, you don't. The real power of the computer is its ability to do the work for you. To get it to do that, we use the power of the shell to automate things. We write shell scripts. In the simplest terms, a shell script is a file containing a series of commands. The shell reads this file and carries out the commands as though they have been entered directly on the command line. The shell is somewhat unique, in that it is both a powerful command line interface to the system and a scripting language interpreter. As we will see, most of the things that can be done on the command line can be done in scripts, and most of the things that can be done in scripts can be done on the command line. We have covered many shell features, but we have focused on those features most often used directly on the command line. The shell also provides a set of features usually (but not always) used when writing programs.