Unix command line concepts

Unix/Linux command line
concepts
Jan 24, 2012 Juche 101
Artem Nagornyi

1.1 Listing files and directories
$ ls The ls command (lowercase
L and lowercase S) lists the
contents of your current
working directory.

...
$ ls -a Lists all files in the current
directory including those
whose names begin with a
dot (.) which are considered
as "hidden".

1.2 Making directories
$ mkdir dirname This will create a new sub-
directory in the current
directory.

1.3 Changing to a different directory
$ cd mydir
$ cd /local/usr/bin
Changes the current working
directory to mydir directory.
You can also use full path of
the directory.

1.4 The directories . and ..
$ cd .
$ cd ..
$ cd ./mydir/innerdir
$ ls ./mydir/innerdir
$ cd ../../anotherdir
$ ls ../../anotherdir
In UNIX, (.) means the
current directory, so typing
this command means stay
where you are (the current
directory).
(..) means the parent of the
current directory, so typing
this command will take you
one directory up the
hierarchy.
Feel free to use (.) and (..)
symbols when changing and

1.5 Pathnames
$ pwd Prints path of the current
directory (working directory).

1.6 Home directory
~
$ ls ~/mydir
This is the symbol of your
home directory. Each user of
the Unix system has its own
username and own home
directory under /home.
For example these are home
dirs: /home/artem,
/home/john
Will list the contents of mydir
sub-directory of your home
directory, no matter where
you currently are in the file
system.

Chapter 2
cp, mv, rm, rmdir, cat, less, head,
tail, grep, wc

2.1 Copying files
$ cp ./myfile /home/artem
$ cp myfile /home/artem/mf2
$ cp /usr/local/myfile .
Copies myfile file from the
current directory to
/home/artem directory.
Copies myfile file from the
/home/artem directory,
renaming it to mf2.
Copies /usr/local/myfile to the
current directory.

...
$ ln -s /usr/local/ff/firefox /usr/bin/firefox
This command will make a symbolic link /usr/bin/firefox to the
file /usr/local/ff/firefox
Symbolic links have l symbol in the beginning of 'ls -l' output
string.
$ ln /usr/local/ff/firefox /usr/bin/firefox
This will make a hard link. The difference between a symbolic
link and a hard link is that a hard link to a file is
indistinguishable from the original directory entry; just consider
it as a file alias. Hard links may not normally refer to directories.

...
$ ln myfile hlink
$ rm myfile
$ cat hlink
This experiment proves that a
hard link is just another name
for a file. Even after deleting
original file it still exists
because we haven't deleted
the hard link. Simply there is
really no such thing as "hard
link", we just create another
name for a file.

2.2 Moving files
$ mv ./myfile /home/artem
$ mv myfile /home/artem/mf2
$ mv /usr/local/myfile .
Moves myfile file from the
/home/artem directory.
Moves myfile file from the
/home/artem directory,
renaming it to mf2.
Moves /usr/local/myfile to the
current directory.

2.3 Removing files and directories
$ rm myfile
$ rm /usr/local/myfile
$ rm -R mydir
$ rmdir mydir
Removes myfile file in the
current directory.
Removes /usr/local/mydir file.
Removes mydir sub-directory
in the current directory.
Removes mydir sub-directory
in the current directory.

2.4 Displaying the contents of a file on
the screen
$ clear
$ cat myfile
$ less myfile
$ head myfile
$ head -5 myfile
$ tail -5 myfile
Will clear screen.
Will display the content of a
file on the screen.
Will display the content of a
file page-by-page.
Will display the first 10 lines
of myfile on the screen.
Will display the first 5 lines.
Will display the last 5 lines.

2.5 Searching the contents of a file
$ less myfile This will display the contents
of myfile page-be-page.
Then, still in less, type a
forward slash [/] followed by
the word to search. less finds
and highlights the keyword.
Type [n] to search for the
next occurrence of the word.

...
$ grep Science myfile
$ grep 'spinning top' myfile
$ grep -i Science myfile
This will print each line of
myfile containing the word
Science (it is case-sensitive).
To search for a phrase or
pattern, you must enclose it
in single quotes.
Key -i will ignore upper/lower
case in the search results.

...
Some of the other options of grep are:
-v display those lines that do NOT match
-n precede each matching line with the line number
-c print only the total count of matched lines

...
$ wc -w myfile
$ wc -l myfile
Will return the number of
words in myfile.
Will return the number of
lines in myfile.

Chapter 3
>, >>, <, |, sort, who

3.1 Redirection
$ cat Type cat without specifing a
file to read. Then type a few
words on the keyboard and
press the [Return] key.
Finally hold the [Ctrl] key
down and press [d] (written
as ^D for short) to end the
input.
It reads the standard input
(the keyboard), and on
receiving the 'end of file'
(^D), copies it to the standard
output (the screen).

...
$ cat > myfile Type something, then press
[Ctrl-d] to end the input.
The output will be redirected
to myfile.
If the file already contains
something, it will be
overwritten.

...
$ cat >> myfile Type something, then press
[Ctrl-d] to end the input.
The output will be redirected
and appended to myfile.
If the file already contains
something, it will be
appended.

...
$ cat myfile1 myfile2 > file3 This will join (concatenate)
myfile1 and myfile2 into a
new file called file3.
What this is doing is reading
the contents of myfile1 and
myfile2 in turn, then
outputting the text to the file
file3.

3.3 Redirecting the input
$ sort Enter this command. Then
type in the names of some
animals. Press [Return] after
each one.
dog
cat
bird
ape
[Ctrl-d]
The output will be:
ape
bird
cat
dog

...
$ sort < file1 > file2 Input redirection is <
In this command we use both
input and output redirection.
The unsorted list will be taken
from file1 and already sorted
list will be redirected to file2.

3.4 Pipes
$ who > usernames
$ sort < usernames
$ who | sort
who command returns the
list of all users currently
logged in the system. This is
a method to get a sorted list
of names by using a
temporary file usernames.
This way we can avoid
temporary file creation. Here
we connect the output of the
who command directly to the
input of the sort command.
This is exactly what pipes do.
Pipe symbol is |

...
$ who | wc -l
$ cat myfile | grep science
And this is the way to find out
how many users are logged
in. We are using a pipe
between who and wc
commands.
This displays the line of
myfile that contains 'science'
string. We are using pipe
between cat and grep
commands.

Chapter 4
*, ?, man, whatis, apropos

4.1 Wildcards
$ ls list*
$ ls *list
The character * is called a
wildcard, and will match
against none or more
character(s) in a file (or
directory) name.
This will list all files in the
current directory starting with
list...
This will list all files in the
current directory ending with
...list

...
$ ls ?ouse
The character ? will match
exactly one character.
So ?ouse will match files like
house and mouse, but not
grouse.

4.2 Unix filename conventions
Unix-legitimate filenames are any combination of these three
classes of characters:
1. Upper and lower case letters: A - Z and a - z (national
characters are also supported in Unicode and other
encodings)
2. Numbers 0 - 9
3. Periods, underscores, hyphens . _ -
Some other characters can be also supported, but they are not
recommended to use.

4.3 Getting help
$ man wc
$ whatis wc
This will display the manual
page for wc command.
Gives a one-line description
of the command, but omits
any information about options
etc.

...
$ apropos -s "1" copy If you are not sure about the
exact name of the command,
this will give you the list of
commands with keyword in
their manual page header.
-s key defines section of Unix
manual:
1. General commands
2. System calls
3. Library functions
4. Special files
5. File formats and conventions
6. Games and screensavers
7. Miscellanea
8. System administration commands and
daemons

Chapter 5
ls -lag, chmod, command &, bg,
jobs, fg, kill, ps

5.1 File system access rights
$ ls -l
● The left group of 3 gives the file
permissions for the user that owns the
file (or directory) (artem in the above
example).
● The middle group of 3 gives the
permissions for the group of people to
whom the file (or directory) belongs
(softserve in the above example);
● The rightmost group of 3 gives the
permissions for all others.
The symbol in the beginning of the string indicates whether this is a file, directory or a
link:
'd' indicates a directory, '-' indicates a file, 'l' indicates a symbolic link.
The permissions r, w, x (read, write, execute) have slightly different meanings depending
on whether they refer to a simple file or to a directory.
-rw-rw-r-- 1 artem softserve 83 Feb 3 1995 myfile
Group of
the owner
Owner of the
file
Modification
date
Filename
File
permissions
Number of
subdirectories (1
for a file)
Size

...
Access rights on files
● r indicates read permission (or otherwise), that is, the presence or absence of
permission to read and copy the file
● w indicates write permission (or otherwise), that is, the permission (or otherwise) to
change a file
● x indicates execution permission (or otherwise), that is, the permission to execute a file,
where appropriate
Access rights on directories
● r allows users to list files in the directory;
● w means that users may delete files from the directory or move files into it;
● x means the right to access files in the directory. This implies that you may read files in
the directory provided you have read permission on the individual files.
So, in order to read a file, you must have execute permission on the directory containing
that file, and hence on any directory containing that directory as a subdirectory, and so on,
up the tree.
Also file can be written if its permissions allow Write, but it can only be deleted if its
directory's permissions allow Write.

5.2 Changing access rights
Access classes:
u (user)
g (group)
o (other)
a (all: u, g and o)
Operators:
+ (add access)
- (remove access)
= (set exact access)
Examples:
chmod a+r myfile
add permission for everyone to read a
file (or just: chmod +r myfile)
chmod go-rw myfile
remove read and write permission for
group and other users
chmod a-w+x myfile
remove write permission and add
execute for all users
chmod go=r myfile
explicitly state that group and other
users' access is set only to read

...
The other way to use the chmod command is the absolute form. In this
case, you specify a set of three numbers that together determine all
the access classes and types. Rather than being able to change only
particular attributes, you must specify the entire state of the file's
permissions.
The three numbers are specified in the order: user (or owner), group,
other. Each number is the sum of values that specify: read (4), write
(2), and execute (1) access, with 0 (zero) meaning no access. For
example, if you wanted to give yourself read, write, and execute
permissions on myfile; give users in your group read and execute
permissions; and give others only execute permission, the appropriate
number would be calculated as (4+2+1)(4+0+1)(0+0+1) for the three
digits 751. You would then enter the command as:
chmod 751 myfile

5.2.1 Changing owner of the file
$ sudo chown artem myfile
$ sudo chown -hR artem
myfolder
Change the owner of myfile
to artem.
We are using sudo before
chown to temporarily give
the current user
administrative permissions
(you will need to enter the
root user password).
Change the owner of
myfolder folder to artem
including all nested sub-
directories and files
recursively.

5.2.2 Changing file timestamps
$ touch -d "2005-02-03 14:
04:25" myfile
$ touch -md "2005-02-03 14:
04:25" myfile
$ touch -ad "2005-02-03 14:
04:25" myfile
$ touch myfile
This command will set both
modification and access
date/time for the file or
directory.
Set only modification
date/time for the file or
directory.
Set only access date/time for
the file or directory.
Will create a new empty file
'myfile' if it doesn't exist.

5.3 Processes and jobs
$ ps aux
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
...
This command lists all currently running processes in the system.Output columns (this is
Linux, output on other Unixes may slightly differ):
USER = user owning the process
PID = process ID of the process
%CPU = it is the CPU time used divided by the time the process has been running
%MEM = ratio of the process’s resident memory size to the physical memory
VSZ = virtual memory usage of entire process (including swapped memory)
RSS = resident set size, the non-swapped physical memory that a task has used
TTY = controlling tty (terminal)
STAT = multi-character process state (running, sleeping, zombie, etc.)
START = starting time or date of the process
TIME = cumulative CPU time
COMMAND = command with all its arguments

...
$ ps aux | more
$ ps aux | grep pname_or_id
This will allow listing long list
of processes page-by-page.
This is the way to search for
a given process name or
process id in the list of
processes. Only the lines
containing the process name
or id will be displayed.

...
$ sleep 5
$ sleep 5 &
This will wait 5 seconds and
then return the command line
prompt.
This will run the sleep
command in background and
return the command line
prompt immediately, allowing
you do run other programs
while waiting for that one to
finish.

...
$ sleep 15 You can suspend the process
running in the foreground by
typing ^Z, i.e.hold down the
[Ctrl] key and type [z]. Then
to put it in the background,
type 'bg' and [Enter].

5.4 Listing suspended and background
processes
$ jobs
$ fg %2
$ fg %1
When a process is running,
backgrounded or stopped, it will be
entered onto a list along with a job
number.The output of this command
will be such as this:
[1] Stopped sleep 1000
[2] Running vim
[3] Running matlab
This will foreground the process
number 2.
This will resume and foreground the
stopped process number 1.

5.5 Killing/signalling a process
$ sleep 5
[Ctrl+c]
$ kill pid
$ kill -9 pid
$ kill n
[Ctrl+c] combination will kill
the foreground process.
This will kill the process using
its process id (you can get it
from the output of ps
command).
This will forcibly kill the
process even if it hanged (or
just stopped).
This will kill the job with
number n from background.

...
$ sleep 15
[Ctrl+z]
$ kill -stop pid
$ kill -cont pid
$ pkill processname
$ killall processname
This will stop (temporarily
suspend) the process.
This will stop (temporarily
suspend) the process.
This will resume the stopped
process.
To kill all processes with the
name processname.
To kill all processes with the
name processname.

Chapter 6
df, du, gzip, zcat, file, diff, find,
history

6.1 Other useful Unix commands
$ df -h
$ df -h .
$ du -ahc mydir
This will show the amount of
used/available space on all
mounted filesystems.
This will show the amount of
used/available space only on
the current filesystem.
This will show the disk usage
of each subdirectory and file
of mydir directory, and mydir
directory itself.

...
$ gzip myfile
$ gunzip myfile
$ zcat myfile.gz
$ zcat myfile.gz | less
This will compress myfile
using Gzip compressing tool.
The original file will be
deleted.
This will uncompress myfile.
This will read gzipped files
without needing to
uncompress them first.
And now you can view the
gzipped file page-by-page.

...
$ file *
$ diff file1 myfile2
Classifies the named files in
the current directory
according to the type of data
they contain, for example
ASCII text, pictures,
compressed data, directory,
etc.
Compares the contents of
two files and displays the
differences. Lines beginning
with a < denotes file1, while
lines beginning with a >
denotes file2.

...
$ find . -name "*.txt" -print
$ find . -size +1M -ls
Searches for all files with the
extension .txt, starting at the
current directory (.) and
working through all sub-
directories, then printing the
name of the file to the screen
(simple output).
To find files over 1Mb in size,
and display the result as a
long listing (similar to ls
command output).

...
$ history | less
$ set history=100
This will give an ordered list
of all the commands that you
have entered. Piping the
output to less command
allows both forward and
backward scrolling of the list
(more command only allows
forward scrolling).
This way you can change the
size of the history buffer (set
command changes runtime
parameters).

Chapter 7
export, printenv, unset, .bashrc,
source, ssh, mount, reboot,
shutdown, crontab

6.2 Environment variables
$ export MYVAR=myvalue
$ printenv
$ printenv | grep MYPAR
$ unset MYVAR
Adds a new environment
variable MYVAR with value
value myvalue (export
command works for
Debian/Ubuntu Linux).
Prints all environment
variables.
Displays the value of MYVAR
environment variable.
Unsets (deletes) environment
variable MYVAR.

...
$ export $PATH:/mydir This way we can add new
directories in the end of
PATH environment variable
(all directories are divided by
: symbol).

...
$ vi ~/.bashrc
$ source ~/.bashrc
This way we can add
environment variables on
permanent basis. Just insert
export MYVAR=myvalue in
the end of file opened in VI.
This variable will be loaded
automatically at shell start.
Force reload of environment
variables from ~/.bashrc file.
Note: This is for Bash, if you
use a different shell, you
should use another file.

6.3 Remote shell
$ ssh user@host
$ exit
This way we can connect to
another Unix machine that
has OpenSSH server running
and port 22 opened. Upon
connect you will be asked to
enter a password for user.
host parameter can be a
hostname or IP address.
You can leave the remote
shell by entering exit
command.

6.4 Mounting filesystems
$ mkdir mydir
$ sudo mount -t vfat /dev/sdc1 mydir
This way we can create a mount point and mount FAT32
filesystem to this mount point (only root user can do this).
$ umount mydir
And now we have unmounted the filesystem, the directory will
be empty.
Noticed /dev/sdc1 ? This is a device file for the filesystem. If it
exists, than the filesystem is physically present, but not
mounted until we execute the mount command.
Other fs types exist (-t option): ext3, ext4, reiserfs, ntfs, etc.

...
$ mkdir alpha
$ sudo mount -t smbfs //alpha.softservecom.com/install alpha
-o username=yourusername,password=yourpassword
This way we can mount remote SMB network filesystem,
providing credentials for authentication.
If you want the filesystem to be mounted automatically, then
you need to edit /etc/fstab file that has its own format (see the
man page for details).

6.5 Shutdown and Reboot
$ sudo reboot
$ sudo shutdown -h now
$ sudo shutdown -h 18:45
"Server is going down for
maintenance"
Reboot the system
immediately.
Shutdown the system
immediately.
Shutdown the system at 18:
45.

6.6 Scheduling
$ crontab -e
This command opens crontab file where you can schedule commands
execution. (use sudo if you need the command to be executed with
root permissions)
The format of a line is:
minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12),
weekday (0-6, 0 = Sunday), command
Example:
01 04 1 1 1 /usr/bin/somedirectory/somecommand
The above example will run /usr/bin/somedirectory/somecommand
at 4:01am on January 1st plus every Monday in January.

...
An asterisk (*) can be used so that every instance (every hour, every
weekday, every month, etc.) of a time period is used. Example:
01 04 * * * /usr/bin/somedirectory/somecommand
This command will run /usr/bin/somedirectory/somecommand
at 4:01am on every day of every month.
Comma-separated values can be used to run more than one instance
of a particular command within a time period. Dash-separated values
can be used to run a command continuously. Example:
01,31 04,05 1-15 1,6 * /usr/bin/somedirectory/somecommand
The above example will run /usr/bin/somedirectory/somecommand at
01 and 31 past the hours of 4:00am and 5:00am on the 1st
through the 15th of every January and June.

...
Usage: "@reboot /path/to/executable" will execute
/path/to/executable when the system starts.
string meaning
@reboot Run once, at startup.
@yearly Run once a year, "0 0 1 1 *".
@annually (same as @yearly)
@monthly Run once a month, "0 0 1 * *".
@weekly Run once a week, "0 0 * * 0".
@daily Run once a day, "0 0 * * *".
@midnight (same as @daily)
@hourly Run once an hour, "0 * * * *".

Unix command line concepts

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