Commands
Every time we have done anything useful, we have executed a command to do it. Here we will discuss what a command is, and what they will enable you to do.
Your First Command
We'll start off with a practical example: the cal command. Type the following at the UNIX prompt:
February 2000
S M Tu W Th F S
1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29
As you can see, the cal command simply prints out the current month's calendar.
Getting Into Arguments
Of course, what use is a calendar that only shows the current month? If only we could specify what month we wanted....
June 1977
S M Tu W Th F S
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30
As you can see, we passed along extra information to the command. Each piece of extra information is called an argument. Commands differ widely in terms of the number and type of arguments they can accept. The same command may behave differently with a different number of argumants, as well. For example....
1977
Jan Feb Mar
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 1 2 3 4 5 1 2 3 4 5
2 3 4 5 6 7 8 6 7 8 9 10 11 12 6 7 8 9 10 11 12
9 10 11 12 13 14 15 13 14 15 16 17 18 19 13 14 15 16 17 18 19
16 17 18 19 20 21 22 20 21 22 23 24 25 26 20 21 22 23 24 25 26
23 24 25 26 27 28 29 27 28 27 28 29 30 31
30 31
Apr May Jun
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 1 2 3 4 5 6 7 1 2 3 4
3 4 5 6 7 8 9 8 9 10 11 12 13 14 5 6 7 8 9 10 11
10 11 12 13 14 15 16 15 16 17 18 19 20 21 12 13 14 15 16 17 18
17 18 19 20 21 22 23 22 23 24 25 26 27 28 19 20 21 22 23 24 25
24 25 26 27 28 29 30 29 30 31 26 27 28 29 30
Jul Aug Sep
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 1 2 3 4 5 6 1 2 3
3 4 5 6 7 8 9 7 8 9 10 11 12 13 4 5 6 7 8 9 10
10 11 12 13 14 15 16 14 15 16 17 18 19 20 11 12 13 14 15 16 17
17 18 19 20 21 22 23 21 22 23 24 25 26 27 18 19 20 21 22 23 24
24 25 26 27 28 29 30 28 29 30 31 25 26 27 28 29 30
31
Oct Nov Dec
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 1 2 3 4 5 1 2 3
2 3 4 5 6 7 8 6 7 8 9 10 11 12 4 5 6 7 8 9 10
9 10 11 12 13 14 15 13 14 15 16 17 18 19 11 12 13 14 15 16 17
16 17 18 19 20 21 22 20 21 22 23 24 25 26 18 19 20 21 22 23 24
23 24 25 26 27 28 29 27 28 29 30 25 26 27 28 29 30 31
30 31
With only one argument, cal assumes it is a year, and prints out a 12-month calendar. This could be confusing; what if you wanted August's calendar, and tried....
8
Jan Feb Mar
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 3 4 5 6 7 1 2 3 4 1 2 3
8 9 10 11 12 13 14 5 6 7 8 9 10 11 4 5 6 7 8 9 10
15 16 17 18 19 20 21 12 13 14 15 16 17 18 11 12 13 14 15 16 17
22 23 24 25 26 27 28 19 20 21 22 23 24 25 18 19 20 21 22 23 24
29 30 31 26 27 28 29 25 26 27 28 29 30 31
Apr May Jun
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 3 4 5 6 7 1 2 3 4 5 1 2
8 9 10 11 12 13 14 6 7 8 9 10 11 12 3 4 5 6 7 8 9
15 16 17 18 19 20 21 13 14 15 16 17 18 19 10 11 12 13 14 15 16
22 23 24 25 26 27 28 20 21 22 23 24 25 26 17 18 19 20 21 22 23
29 30 27 28 29 30 31 24 25 26 27 28 29 30
Jul Aug Sep
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 3 4 5 6 7 1 2 3 4 1
8 9 10 11 12 13 14 5 6 7 8 9 10 11 2 3 4 5 6 7 8
15 16 17 18 19 20 21 12 13 14 15 16 17 18 9 10 11 12 13 14 15
22 23 24 25 26 27 28 19 20 21 22 23 24 25 16 17 18 19 20 21 22
29 30 31 26 27 28 29 30 31 23 24 25 26 27 28 29
30
Oct Nov Dec
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 3 4 5 6 1 2 3 1
7 8 9 10 11 12 13 4 5 6 7 8 9 10 2 3 4 5 6 7 8
14 15 16 17 18 19 20 11 12 13 14 15 16 17 9 10 11 12 13 14 15
21 22 23 24 25 26 27 18 19 20 21 22 23 24 16 17 18 19 20 21 22
28 29 30 31 25 26 27 28 29 30 23 24 25 26 27 28 29
30 31
Naturally, cal printed out the calendar for the year 8, not the month. Be sure to do a sanity check on your command before pressing enter. cal is harmless, but you wouldn't want to pass the wrong argument to a more serious command, such as rm (which deletes files).
In addition to arguments, many commands allow you to specify options. An option is simply a modifer - it changes the way a command works. Let's try this on another non-destructive command, ls.
Mail dead.letter matlabrc.m samuel.txt MailArchive matlab personal www
ls prints out a list of all of the files in the current directory. We'll talk about ls in more detail later, but for now, let's try using options with ls to get more information.
total 10 drwx------ 2 clwolfe staff 1024 Feb 21 14:32 Mail drwx------ 2 clwolfe staff 96 Jan 6 10:10 MailArchive -rw------- 1 clwolfe staff 3148 Feb 17 14:03 dead.letter drwx------ 2 clwolfe staff 1024 Nov 12 16:54 matlab -rw------- 1 clwolfe staff 4725 Aug 16 1999 matlabrc.m drwx------ 2 clwolfe staff 96 Nov 9 15:31 personal -rw------- 1 clwolfe staff 3305 Dec 10 12:19 samuel.txt drwx--x--x 8 clwolfe staff 1024 Feb 10 14:05 www
Here we passed the l option (l, as in L, not 1) to the ls command. Most options begin with a -. That's how the command can tell that it is an option, not an argument. Some commands have options that start with two hyphens.
Gluing Commands Together
You may have noticed, when we were playing with the cal commands, that sometimes the output of a command will fly by too fast for you to read. This is a common problem, and it has a simple solution. You might think that there is an option that slows down the output, or something like that. In fact, there is a much more general solution - the more command. The more command is very stupid - all it knows how to do is to take whatever is put into it, and then print it to the screen, one screenful at a time. So, we need a way of gluing commands together.
We can do this using the "pipe operator", which is provided by your shell. It looks like a little vertical line (|), and is typically found near the enter key. Here it is in action:
1999
Jan Feb Mar
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 1 2 3 4 5 6 1 2 3 4 5 6
3 4 5 6 7 8 9 7 8 9 10 11 12 13 7 8 9 10 11 12 13
10 11 12 13 14 15 16 14 15 16 17 18 19 20 14 15 16 17 18 19 20
17 18 19 20 21 22 23 21 22 23 24 25 26 27 21 22 23 24 25 26 27
24 25 26 27 28 29 30 28 28 29 30 31
31
Apr May Jun
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 3 1 1 2 3 4 5
4 5 6 7 8 9 10 2 3 4 5 6 7 8 6 7 8 9 10 11 12
11 12 13 14 15 16 17 9 10 11 12 13 14 15 13 14 15 16 17 18 19
18 19 20 21 22 23 24 16 17 18 19 20 21 22 20 21 22 23 24 25 26
25 26 27 28 29 30 23 24 25 26 27 28 29 27 28 29 30
30 31
Jul Aug Sep
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 3 1 2 3 4 5 6 7 1 2 3 4
4 5 6 7 8 9 10 8 9 10 11 12 13 14 5 6 7 8 9 10 11
11 12 13 14 15 16 17 15 16 17 18 19 20 21 12 13 14 15 16 17 18
18 19 20 21 22 23 24 22 23 24 25 26 27 28 19 20 21 22 23 24 25
25 26 27 28 29 30 31 29 30 31 26 27 28 29 30
Oct Nov Dec
S M Tu W Th F S S M Tu W Th F S S M Tu W Th F S
1 2 1 2 3 4 5 6 1 2 3 4
3 4 5 6 7 8 9 7 8 9 10 11 12 13 5 6 7 8 9 10 11
10 11 12 13 14 15 16 14 15 16 17 18 19 20 12 13 14 15 16 17 18
17 18 19 20 21 22 23 21 22 23 24 25 26 27 19 20 21 22 23 24 25
24 25 26 27 28 29 30 28 29 30 26 27 28 29 30 31
31
That isn't very interesting on a web page, but you should notice that in the telnet session, the output pauses when the screen is full. Press Space to see the next page.
more isn't the only command that works this way. In fact, all commands can be linked using the pipe operator.
Try thinking of a command as a box with pipes going in and out of it, like so:
In UNIX terminology, the pipe going in is called Standard Input (or STDIN), the pipe going out is called Standard Out (STDOUT), the gauge on top is called Standard Error (STDERR), and the lever on top is the command you use to start the box in motion. Not all commands use STDIN and STDERR, but every command uses STDOUT if it produces output. The pipe operator simply connects the commands together, like this:
All the pipe operator does is join the two commands together.
If you would like to direct output to a file rather than your screen (which is where STDOUT goes, by default), you can say so using another shell operator. Try this:
The greater-than symbol (>) tells your shell that you would like STDOUT to go to a file, rather than to the terminal or to another program. Likewise, the less-than sign (<) tells your shell to feed the contents of a file to a command as STDIN. This won't work with cal, since cal doesn't use STDIN. We'll use this trick later when working with other commands.
What Happens When you Press Enter
When you press enter, after typing a command line, your shell does the following:
- Look for any "shell meta-characters" - things like <,>, and |. Split the command line up using these as markers.
- Look for an environment variable, called $PATH, and look in the places it lists for commands that match the names of the command you typed. We'll talk about $PATH more later.
- If there are any >'s or <'s, open the named files for reading or writing, accordingly.
- Connect the plumbing together, and begin executing the commands.
If something goes wrong, the command will output error messages to STDERR. STDERR by default goes to your terminal, regardless of where STDOUT is going.
Command for Commands
The following are commands that are useful for finding out more about commands.
The man command
The man command is by far the most useful command in UNIX. man is an abbreviation for "manual", as in "user's manual". If you need help with a command, you first instinct should be to type
where command is the command that you need help with. For example, if you needed help with the cal command, you would type:
Reformatting page. Wait... done
User Commands cal(1)
NAME
cal - display a calendar
SYNOPSIS
cal [ [ month ] year ]
DESCRIPTION
The cal utility writes a Gregorian calendar to standard out-
put. If the year operand is specified, a calendar for that
year is written. If no operands are specified, a calendar
for the current month is written.
OPERANDS
The following operands are supported:
month Specify the month to be displayed, represented as a
decimal integer from 1 (January) to 12 (December).
The default is the current month.
year Specify the year for which the calendar is
displayed, represented as a decimal integer from 1
to 9999. The default is the current year.
ENVIRONMENT
See environ(5) for descriptions of the following environment
variables that affect the execution of cal: LC_TIME,
LC_MESSAGES, and NLSPATH.
EXIT STATUS
The following exit values are returned:
0 Successful completion.
>0 An error occurred.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
__________________________________
| ATTRIBUTE TYPE| ATTRIBUTE VALUE|
|__________________________________
| Availability | SUNWesu |
|_______________|_________________|
SEE ALSO
calendar(1), attributes(5), environ(5)
NOTES
An unusual calendar is printed for September 1752. That is
the month 11 days were skipped to make up for lack of leap
year adjustments. To see this calendar, type:
cal 9 1752
SunOS 5.6 Last change: 1 Feb 1995 1
User Commands cal(1)
The command cal 83 refers to the year 83, not 1983.
The year is always considered to start in January.
SunOS 5.6 Last change: 1 Feb 1995 2
Notice that the man page is automatically fed through more for you.
You may find that the man pages are not very helpful, or are confusingly written. You would be correct. Here is some advice about reading man pages:
- The Synopsis section is often the most useful part. It gives the basic form of the command, with optional options and arguments in brackets ([]).
- Many of the other sections are there for the system administrator (the person who keeps the UNIX machine at a more tolerable level of disarray). You usually won't be concerned with the Attributes or Exit Status sections.
- Read the examples, and try them out.
- Keep in mind that some of the really old commands (such as tar, for example) have so many modes and features that their man page is indecipherable.
Have a look at the See Also section above. Notice that the commands listed there have a number in parentheses following them. This refers to which section of the man system that the command appears in. Historically, man pages were divided into eight (or more) sections, as follows.
- User Commands
- System Calls
- Subroutines
- Devices
- File Formats
- Games
- Miscellaneous
- Sys. Administration
So if someone tells you to look at the hostname(1) page, they mean that you should look at the man page for the hostname user command. This is provided so that you could also look at the hostname(3) page, which is the C language system call (not, generally, what you want).
If you would like to see man pages from other sections, use this form of man:
which specifies to only display man pages from section 3, for example.
The apropos Command
Of course, what good is the man command if you don't know what the name of the command is? The apropos command searches through the man pages for a keyword that you specify.
For exmaple, if you wanted to find a command for working with calendars, you would use....
cal cal (1) - display a calendar calendar calendar (1) - reminder service difftime difftime (3c) - computes the difference between two calendar times mktime mktime (3c) - converts a tm structure to a calendar time
As you can see from the results, there are four results from the search.
The whereis Command
Often, you may know of a application or command that may or may not be installed on the machine. A good way of finding out is the whereis command. whereis looks for command files in the standard locations, based on the keywords you provide.
perl: /usr/bin/perl /usr/local/bin/perl5.00404 /usr/local/bin/perl
This example tells us that the popular program perl is installed in three locations.
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