Chapter 4: Commands and Programs

4.1 Command basics

Issuing commands always takes the same form:

command… then options… then arguments

echo 'hello world'

echo is actually a C program. UNIX is written in C. bin is where most of the C programs are stored. The program echo is stored there. When you type echo into terminal you are just called the compiled program called "echo" which is stored in bin.

If UNIX didn't know where the executable C program called "echo" was located you could explicitly tell it:

bin/echo 'hello world'

to find the location of a program file use which

which echo

or whereis

whereis echo

for info about a program use info

info echo

4.2 Common options for most programs

Most unix programs respond to these options:

-v or --version to get the version of the program

--help to read about how to use the program

for example:

npm --version

4.3 Quitting a program

There are several options for quitting programs:

q, or x, or Control + q, or Control + x, or ESC, or force quit, or Control + c,...

With some of these options, if you close the window the process may keep running so to be safe, use:

Control + c

4.4 The PATH variable

In UNIX for MacOS most programs are located at /bin/. UNIX will look there for executable programs.

However, an executable program can be placed anywhere. But how will UNIX find an executable program if it is just placed ni any old folder?

UNIX finds program by looking through the directories assigned to the $PATH variable

echo $PATH

returns something like (depending on your setup):

/Users/yourname/.composer/vendor/bin:~/.composer/vendor/bin:/usr/local/sbin:/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

This is a list of colon separated paths where unix will look for executables. Its that simple.

If you want UNIX to find an executable you can add the path to that executable to the $PATH variable.

Early entries take precedence over later entries. So if you have two executables of the same name UNIX will use the first one it finds.

In BASH to set the path just type

PATH=path:path:etc...

Setting the $PATH variable in this way (by typing it into terminal) only lasts for the current session.

To set the $PATH variable permanently, edit the .bash file directly which sets the $PATH variable each time UNIX boots up.

Note that when you use whereis to find an executable it may return a different copy of that executable than the one that is in the path and being used by UNIX. This causes some degree of pain...

4.5 System information commands

4.5.1 date

date

Returns the date in the default format unless you have changed the format in a .bash file.

4.5.2 uptime

uptime

Returns: 18:18 up 2:18, 3 users, load averages: 1.63 1.58 1.60

The number of users reflects the number of processes connected to UNIX. In this cae there are 3. The MacOS operating system Finder program is one, and one each for the 2 terminal windows which were open when I ran the command uptime

4.5.3 users

users

Returns username

4.5.4 who

who

Returns each instance of a user connecting to unix!

Do you multiple instances of your user name listed?

smerth   console  Mar 30 11:58 
smerth   ttys001  Mar 30 13:19

Username is using unix as shell (one for every open terminal window), and Username is also logged into unix as the MacOS finder.

4.5.5 uname

uname

Returns Darwin - the unix name

uname -mnrsvp

Returns: Darwin USER's-MacBook-Pro.local 15.5.0 Darwin Kernel Version 15.5.0: Tue Apr 19 18:36:36 PDT 2016; root:xnu-3328.51.21~8/RELEASE_X86_64 x86_64 i386

uname -ap

Returns same...

4.5.6 hostname

hostname

Returns localhost

4.5.7 domainname

domainname

Returns the domain name on a server

4.6 Disk information commands

4.6.1 df - disc freespace

To humanize the output use -h

df -h

Disc size is calculated using base2. To get disc size calculated using base10 use:

df -H

4.6.2 du - disc usage

du path

Gives the size of every file found at a path. Its important to restrict du to a path otherwise the burden of calculating the disc size for the whole disc is onerous.

4.6.2.1 example usage

du -ah path

When looking at directories you can specify depth...

du -hd 1

which shows the directories to a depth on one

n.b. ls reports the actual size of files, du reports the size of the blocks allocated in memory for the files.

4.7 Viewing processes

The kernal is UNIX. The shell is a text based UI. BASH is the flavor of shell most often used in MacOS to interact with UNIX.

The kernal sets aside memory, and starts a process, then outputs from the process if there is any output from that process, then stops the process when it is complete, then frees the memory it has set aside.

You can check the status of all the processes UNIX is running:

4.7.1 ps - process status

ps

Shows processes owned by user and in user's control (not background and not owned by others like the "root" user.

ps -a

shows all processes

people most commonly use:

ps aux

There is no dash in front of the options aux because it is an artifact of backwards compatibility with older UNIX versions.

4.8 Monitoring processes

4.8.1 top

Get a quick overview

top

4.8.1.1 examples:

Show 10 lines:

top -n 10

Set refresh rate set to 3 seconds:

top -s 3

Get more help for top

top ?

4.9 Stopping processes

Ctl+C is the best way to stop a process. But for some processes you must use kill them.

To kill a process you need the pid (process id)

kill -9 pid

-9 pid means: "Hey, UNIX, really, I know best, kill process with id=9"

4.10 Text file helpers

4.10.1 wc - word count

wc filename

returns:

• no. of lines
• no. of words
• no. of characters
• filename

"lines" are not lines on the screen, they are defined by line-breaks

4.10.2 sort

sort filename

sort operates on each line in a text file. It will sort the lines according to the first letter of each line and return the text to the terminal window.

Capital and lowercase letters are treated differently by sort. To sort alphabetically, independent of case, pass in the -f option

sort -f filename

For a reverse sort pass in the -r option.

4.10.3 uniq

uniq operates on the lines in a text file. It filters in/out repeated lines. It doesn't find duplicates in the list of lines. It finds duplicated lines - one immediately following the next.

It asks: "is this line the same as the next line?"

uniq -d

Returns the line that are repeated

uniq -u

Returns the unduplicated lines

4.11 Utility programs

4.11.1 cal / ncal - calendar

cal 3 1964

Returns the calendar for Mar 1964.

cal -y 1964

Returns a full year.

ncal

Rotates the calendar with the days running down the left side.

4.11.2 bc - calculator

Its a calculator! Just run bc and the program executes, then on the command line enter your calculation, for eg: 88 * 10

Or,

echo "2+5" | bc

Good overview of bc

4.11.3 expr - expression evaluator

expr 5 + 3

You need to have spaces between arguments expr 1+1 won't work but expr 1 + 1 works.

And you must escape special characters!

Good overview of expr

4.11.4 units - unit converter

units will convert from the first argument to the second.

A shortcut for entering the conversion is to put arguments into quotes on the command line:

units "190 degF" "degC"

4.12 History - working with history

The history of your interaction with bash in terminal is tracked in a file in the user driectory.

Bash writes to this history log when it quits so the current session's commands are not reflected (as you might expect...)

Bash holds the current sessions commands in memory until it quits then writes them to the file.

history

prints a list of previously executed command to the terminal - now you can use the numbers beside a command to re-issue that command instead of scrolling up with the up-arrow

!504

executes command numbered 504 in the history list

!-2

executes the command 2 back in the current history

!some alpha numeric string

executes the last command matching the string

!nano

runs the last nano you executed

!!

runs the last command

sudo !!

pulls up the last command prepended with sudo

!$

references the arguments from the last command. For example first run:

nano textfile.txt

and now:

cat !$

will open the file you nano-ed, for you to check the changes, in cat - without having the type the filename again

Delete lines from the history with -d

history -d 203

deletes line 203