C&CZ's Not Frequently Enough asked questions

with answers!

Last modified: 2018-03-21

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Further reference: http://wiki.science.ru.nl/cncz/

About this FAQ

https://wiki.science.ru.nl/cncz/index.php?title=Faq_unix

Contributors

Patrick Atoon
Kees Keijzers
Jos Alsters
Peter van Campen
Simon Oosthoek

Linux shells

What is a shell?

The shell is a program that interprets the user's commands and executes them on the Operating System. There are many kinds of shells, which have different interpreted languages in which to write commands.

A shell can be used interactively, a human user typing commands on a terminal prompt, or in a shellscript. The shellscript is also called a batch file. A shell script that asks for human input can only be run from an interactive shell.

Examples of shells

On the linux machines you can use the shells:

/bin/sh : the default "Bourne" compatible shell, on Ubuntu, this is /bin/dash
/bin/dash: a light-weight Bourne/POSIX compatible shell.
/bin/bash: the Bourne Again Shell. A full fledged linux shell and the default.
/bin/zsh : the last shell you'll ever need, very configurable
/bin/tcsh: a csh compatible shell. We strongly advise not to use this for scripts!

Note that there are shells for microsoft windows, e.g. cmd.exe and powershell. Recently the bash shell is becoming available on windows as well.

What is `man`

Most commands have a manual page, you can invoke the manual page by giving the command: man <command> in an interactive shell.

prompt> man bash

will give you the manual page of the bash shell. Manual pages have a few common headers which you can find in nearly all man pages. Once you get used to this, you can quickly find out how to use the command more effectively.

prompt> man -k something

Will search for the keyword something in the summary of each man-page on the system. This may help you find the right command to do what you want, without knowing the exact command name. This is not a fool-proof solution and often a search engine will be more effective!

Typical Linux commandline

If you work in a terminal (window) you can give commands via the shell. The shell interprets your command, most importantly it splits the line you provide into words separated by whitespace.

prompt> COMMAND [arg1] [arg2] ... <enter>

prompt>	printed by the shell, usually `user@host $` , but it can be customised!
COMMAND	the command you want to execute (you type it)
[arg1]	argument for the command
[arg2]	another argument for the command
...	further arguments for the command
`<enter>`	You press the return key, or enter, to "give" the command to the shell. After this, your line will be interpreted and executed.

examples

`ls -l`	long (-l) listing of all files and directories in the current working directory
`ls -a -l`	long (-l) listing of all files and directories in the current working directory, also show hidden entries (-a)
`ls -l /tmp`	long (-l) listing of all files and directories in /tmp
`ls -l /bin/bash`	long (-l) listing of the file /bin/bash
`man man`	show manual page of the `man` command

What is the environment?

The environment is a collection of shell variables that is passed to the commands you type. Check what is in the environment with the command:

$ env

In a Bourne shell, a variable can be set like this:

prompt> MYVAR=something

to show what the variable holds (Note the $ sign):

prompt> echo $MYVAR

To make it part of the environment:

prompt> export MYVAR

Variables that are usually present are

`HOME`	your home directory, used when you give the command `cd` without arguments to get to your home directory
`PATH`	the list of directories searched for the name of the command you give without a `/` in it. NB Do not put the current directory "." at the front of the path!
`EDITOR`	The editor command used when you invoke a command that uses an editor to let you modify files. Typical editor commands are `vi`, `nano`, `joe`
`PAGER`	The pager command used when e.g. a manual page doesn't fit on a screen. Typical pagers are `less`, `more`

And many more...

What is input/output redirection

Whenever you start a command, the shell opens three "files" by default. These are stdin, stdout and stderr, also known by their numbers, 0, 1 and 2.

The (bourne) shell has the ability to redirect the flow of these files to somewhere else.

Without redirection, the three files are all connected to the terminal, so stdin to the keyboard device, stdout and stderr to the terminal window.

If you command the shell to redirect the output to a file, you can store the output of the command to a file, instead of seeing it on the window where you typed the command.

examples

`ls > mylisting`	output redirection creates or overwrites the file mylisting with the output of the `ls` command
`ls >> mylisting`	output redirection creates or appends the file mylisting with the output of the `ls` command
`ls some-file 2>myerrors`	standard error redirection creates or overwrites the file myerrors with the error message produced by the `ls` command
`wc -l <$HOME/.bash_history`	using input redirection count the number of lines in the file $HOME/.bash_history. Try the same command with and without the <! With the < the stdin is redirected from the file $HOME/.bash_history to the `wc` command. Without the < the filename is passed as an argument to wc so it can open the file itself.

Instead of redirecting to a file, it is also possible to redirect the output of a command to the stdin of another command. This is called a pipeline. Another way to look at this is to see the command after the | symbol as a filter. Multiple filters can be used in sequence to massage the output of a command into a desirable format or selection.

prompt> cat /etc/shells | wc -l

How can I run multiple commands from one commandline?

Normally, commands are separated by a newline <enter>, however, you can also use a ; semicolon to seperate commands:

prompt> ls; sleep 10; echo bla

The above will list the directory, wait 10 seconds, then print bla

How can I run a command non-interactively and without having to wait for it to finish?

A common problem is that you want to run a command that takes a long time to finish and can do so without needing any input.

You can start a command to run in the background by adding a &

prompt> ls -l &

As you will see when you try this, it is still connected to the terminal. If you prepend the command nohup you can disconnect it and even logout of the machine you run the command on. The output will typically end up in a file called nohup.out.

Alternatively: screen

If you want to run the program and be able to provide input or check on its progress now and then, you can run a virtual terminal program, like screen or tmux, these will continue running when you disconnect and log out, later you can reconnect to see how it's going. See man screen or man tmux.

What is filename expansion or wildcards?

The shell has some tools to save you typing long filenames. E.g. when you don't want to list all files in a directory:

`ls -d ?`	list all files and directories with a name of one character
`ls -d ???`	list all files and directories with a name of three character
`ls -d a*`	list all files and directories with a name starting with "a" and any number of unspecified characters after it
`ls -d .*`	list all files and directories with a name of three character
`ls -d *[0-9]`	list all files and directories ending with a number
`ls -d *[a-z]`	list all files and directories ending with a lowercase letter
`ls -d *[A-Z0-9]`	list all files and directories ending with an uppercase letter or a number
`ls -d ~/a*`	list all files and directories in my homedirectory starting with the letter a
`ls -d \` or `ls -d ''`	list all files and directories with names containing a real . Use the \' or single quotes to escape* the character following it. i.e. prevent the shell from using the * for wildcard expansion.
characters to escape	< > [ ] ( ) { } * & ! ~ ; "space" "quotes" and many more...

The option -d of ls makes sure to list the entry itself and not the contents of the directory by that name.

More information about wildcards can be found in the bash manual.

NB A special case is when you escape the newline or <enter> character. The shell will not start interpreting the command and you can add more to your command line on the next line.

prompt> ls \<enter>
-ltr<enter>
...
prompt>

Will be the same as "ls -ltr"

How do I customise my interactive shell session?

In this section we assume you are using the Bourne Again SHell (bash). You can configure your login shell via DIY. (If you are running this on your own Linux machine, you can use the chsh command to change the login shell.)

What happens when I login?

When you login, bash checks a list of possible locations for configuration files (which are full of shell commands).

Bash looks for the following files

/etc/profile
~/.bash_profile
~/.bash_login
~/.profile.

When you start a new interactive shell on the same machine, bash looks for other files:

/etc/bash.bashrc
~/.bashrc

People often want to have the same settings in all shells, so a lot of times you see the following in ~/.profile:

if [ -f ~/.bashrc ]; then
        source ~/.bashrc
fi

And all configuration can exist in ~/.bashrc

In either the system or personal bashrc files, variables and aliases are set to preference:

PATH is set to find systemwide programs, you can expand this search path to include directories in your homedir
PS1 is set to a useful prompt
PAGER

How can I change the way my prompt looks?

The variable PS1 is the prompt you normally see when you can enter a command to the shell. Various ways exist to make this more useful.

Set PS1 directly
Set PROMPT_COMMAND to something that sets PS1

To make it permanent, define PS1 or PROMPT_COMMAND in your ~/.bashrc file.

examples

PS1='\u@\h:\w\$ '
PS1='\u@\h:\w [$?]\$ '
PS1='[\e[32m]\u@\h:[\e[33m]\w[\e[32m]$[\e[0m] '

Other customizations

Scripting

Often you want to be able to reproduce a sequence of commands or even create a small program to do a lot of work in the shell.

For this purpose, you can create a batch script, shell file, or shell script. We recommend you do this in the bourne shell (sh) or (bash). Using (t)csh for shell scripts is not recommended, see Csh Programming Considered Harmful.

Basic shell script

echo start of script
ls -l
echo end of script

If you put the above 3 lines in a file myscript, you can call bash myscript and run the commands in the file.

In order to make the file a command, you can make the file executable by giving the command: chmod a+x myscript

Then you can call the script using ./myscript

Choose interpreter #! hashbang

How to provide multiple lines of input: Here Documents

How to use the output of command as parameters: Command substitution

prompt> CMD-1 `CMD-2`
or
prompt> CMD-1 $(CMD-2)

Call CMD-1 with arguments created from the output of CMD-2

example

Rename a file to have a different extension or a date in the name.

Get the date in a particular format (see man date)

prompt> date "+%Y%m%d-%H:%M"
20180327-12:41

Get the first part of the name

prompt> basename pietje.jpg .jpg
pietje

Use both commands to create a new name for move (rename)

prompt> mv -v pietje.jpg $(basename pietje.jpg .jpg)-$(date "+%Y%m%d-%H:%M").jpg
'pietje.jpg' -> 'pietje-20180327-12:45.jpg'

The `CMD-2` will be executed before `CMD-1`.
Command substitution can be invoked using `CMD-2` or $(CMD-2)
The advantage of $() is that it can be nested $(CMD-2 $(CMD-3))

TODO


1.17 Hoe vindt een shell een commando:
--------------------------------------

De shell kan een commando vinden aan de hand van:

- een absolute naam, een naam die met een '/' begint:

wn4> /usr/ucb/vi
wn4> /usr/local/datasearch/bin/telgids

- een relatieve naam, een naam die niet begint met een '/', maar
wel een '/' bevat:

wn4> bin/testprogramma
wn4> ../petervc/bin/backup-photon

- een commando of programma naam. Om te weten wat er dan opgestart
moet worden, gebruikt de shell de waarde va de PATH (sh en bash)
of path (csh) environment variabele. Deze moet een lijst van
directories bevatten waarin de naam van het betreffende commando
opgezocht kan worden:

wn4> vi
wn4> telgids
wn4> testprogramma

De lijst wordt in volgorde afgewerkt en het eerst gevonden
programma wordt opgestart. Omdat er een Unix programma met de
naam 'test' bestaat, en wanneer je zelf een programma maakt met
ook de naam 'test', bepaalt de volgorde van de directories in de
path variabele welk programma gebruikt zal worden!

De huidige directory ('.') moet ook in het path staan wil een
commando in de huidige directorie gevonden worden (dit in
tegenstelling tot MSDOS bijv.)

Je kunt het path meestal het beste veranderen door het aan de voor-
of achterzijde uit te breiden met de gewenste directorie(s):

Voor sh/bash: 

wn4> PATH=$PATH:$HOME/bin

Voor csh: 

wn4> path=(/usr/local/X11R5/bin $path)


2.7   Wat is een mailinglist?
-----------------------------

Een mailinglist is een soort email-discussie, die per email wordt
verstuurd naar de mensen die op de lijst staan. Je kunt je opgeven voor
zo'n lijst bij degene die de lijst bijhoudt (de administrator).
Iedereen die deelneemt aan de disussie stuurt zijn bericht naar de
administrator, en die zorgt ervoor dat iedereen die op de lijst staat,
ook dat bericht toegestuurd krijgt. Dit betekent vaak dat je bergen
post in je mailbox zult krijgen.

Hoe vind je uit welke mailinglists er zijn?  Welke lists er precies
allemaal zijn is niet uit te vinden, meestal kom je ze tegen `van
horen zeggen'. Als je door je nieuws bladert lees je soms iets over
zo'n mailinglist.  Daarnaast zijn er de nieuwsgroepen die onder
bit.list-serv te vinden zijn, dit zijn kopieen van mailinglists.





3.2 Hoe kan ik mijn bestanden ontoegankelijk maken voor anderen?
----------------------------------------------------------------

Daarvoor is zowel op Unix als op MSDOS met PC-NFS het 'chmod'
commando. (Het bekijken van de protecties gaat met 'ls -lg' ook op
MSDOS!)  Chmod heeft een aantal varianten om de 'bitjes' te zetten:

H:> ls -lg bestandje
-rwxr-xr-x  groep  eigenaar 112 bestandje

|||       -> eigenaar
|||    -> groep
||| -> wereld
H:>

Dus de protectie van directories en bestanden worden steeds door 3
'bitjes' gegeven: rwx met de 'waardes' 4,2,1

Het chmod commando wordt als volgt gebruikt:

wn4> chmod u=rwx,g=rx,o= bestandje
wn4> ls -lg bestandje
-rwxr-x---  groep  eigenaar 112 bestandje

Nu is het bestand alleen schrijfbaar voor de eigenaar, en lees en
executeerbaar voor de eigenaar en de groep. De rest mag niets.

Ander voorbeeld:

H:> chmod g=,o= geheim
H:> ls -lg geheim
-rwx------  groep  eigenaar 12 geheim

Nu kan alleen de eigenaar het bestand lezen, wijzigen en
executeren.  Alle anderen kunnen niets.

Ook kun je in chmod expliciet de bitjes opgeven, in octale notatie,
zoiets als:

wn4> chmod 700 bestandje
wn4> ls -lg bestandje
-rwx------  groep  eigenaar 112 bestandje

Hiermee is het bestand alleen nog te lezen, schrijven en executeren
door de eigenaar.

wn4> chmod 444 bestandje
wn4> ls -lg bestandje
-r--r--r--  groep  eigenaar 112 bestandje

Nu is het bestand nog alleen nog te lezen door iedereen.

Verder zie 'man chmod' en 'man ls' op Unix, voor PC-NFS staat e.e.a.
in de manual.

Bijbehorend is de umask variabele: deze maskeert bitjes die niet
vanzelf aan moeten gaan voor nieuwe bestanden: Als de umask 022 is
dan zullen de bitjes van de eigenaar niet beinvloed worden, het '2'
bitje (schrijven) voor de groep en de wereld wordt afgezet voor een
nieuw bestand. Ook dit is weer waar voor zowel MSDOS als Unix.
Onder MSDOS met PC-NFS zet men het umask als volgt:

C:> net umask 077

Dit zet alle toegang tot nieuwe bestanden voor anderen dan de
eigenaar af.  Onder Unix gaat het bijna identiek:

wn4> umask 022

Nu zijn voor alle nieuwe bestanden de 'schrijf bitjes' uit.

Tenslotte: de protectie van de directorie is ook belangrijk! Het
verwijderen van een file is een directorie operatie! Dus al heb ik
op de file zelf geen rechten, als ik wel mag schrijven op de
directorie kan ik het bestand gewoon verwijderen!


3.3   Help, ik ben een file op Unix kwijt! Kan ik die terug krijgen?
--------------------------------------------------------------------

Zie http://www.sci.kun.nl/cncz/procedures/backups.html

Faq unix

Contents

C&CZ's Not Frequently Enough asked questions

with answers!

About this FAQ

Contributors

Linux shells

What is a shell?

Examples of shells

What is `man`

Typical Linux commandline

examples

What is the environment?

What is input/output redirection

examples

How can I run multiple commands from one commandline?

How can I run a command non-interactively and without having to wait for it to finish?

Alternatively: screen

What is filename expansion or wildcards?

How do I customise my interactive shell session?

What happens when I login?

How can I change the way my prompt looks?

examples

Other customizations

Scripting

Basic shell script

Choose interpreter #! hashbang

How to provide multiple lines of input: Here Documents

How to use the output of command as parameters: Command substitution

example

TODO

Navigation menu

Search

Faq unix

C&CZ's Not Frequently Enough asked questions

with answers!

About this FAQ

Contributors

Linux shells

What is a shell?

Examples of shells

What is man

Typical Linux commandline

examples

What is the environment?

What is input/output redirection

examples

How can I run multiple commands from one commandline?

How can I run a command non-interactively and without having to wait for it to finish?

Alternatively: screen

What is filename expansion or wildcards?

How do I customise my interactive shell session?

What happens when I login?

How can I change the way my prompt looks?

examples

Other customizations

Scripting

Basic shell script

Choose interpreter #! hashbang

How to provide multiple lines of input: Here Documents

How to use the output of command as parameters: Command substitution

example

TODO

Navigation menu

Search

What is `man`