Document Outline

Command Syntax
Options
File Names As Command Arguments

Compound Commands
Wildcards and Relative Pathnames
Escapes and Quotes
Stream Redirection, Pipelines and Command Substitution

Command Syntax

Command syntax (commonly accepted convention)

$ command [options] [filenames]

As a command name we may type in:

an internal shell’s command (a

built-in

a name of a

binary

program,

a name of a

script

(an ASCII file that needs to be interpreted by

an external program).

From the user’s point of view built-ins, binaries and scripts are
undistinguishable, i.e. the same shell grammar rules apply to them.

By commonly accepted convention options can be given in any
sequence, however they should precede file names.

By the same convention two formats for options are used:

short options,
long options.

Short Options

A short option consists of a dash (

) followed by a single letter.

In the following example we list (

) the content of

the

/etc

directory using long format (

-l

Listing a directory contents using a long format

$ ls -l /etc

Short options can be grouped.

The following commands are equivalent (the

-A

option

includes hidden files for listing):

Listing all files in a directory using a long format

$ ls -l -A /etc

$ ls -A -l /etc

$ ls -lA /etc

$ ls -Al /etc

Key-value Options

Some options require additional values.

We display the

/etc/passwd

file content sorted. We

use the colon symbol as a field separator (

-t :

), we sort

by the 3

field (

-k 3

) and apply numerical sorting (

-n

A file sorted numerically by the third field (colon separated)

$ sort -t : -k 3 -n /etc/passwd

Key-value options can be grouped provided that values directly
follow the options they refer to and no ambiguity occurs.

The following commands are equivalent:

A file sorted numerically by the third field (colon separated)

$ sort -t : -k 3 -n /etc/passwd

$ sort -t: -k3 -n /etc/passwd

$ sort -n -k3 -t: /etc/passwd

$ sort -nk3 -t: /etc/passwd

Long Options

Long options are specified as:

--option-name

Most commands define equivalent short and long options. In that
case they may be used interchangeable.

The following commands are equivalent:

Listing all files in a directory

$ ls -A /etc

$ ls --almost-all /etc

Short and long options can be used together and in any sequence
but long options cannot be grouped.

In this example we use both short and long options:

Listing all files in a directory using a long format

$ ls -l --almost-all /etc

Long Key-value Options

Long key-value options are specified as:

--option-name=value

(important: no blanks must precede nor follow the ‘=’ symbol).

Mandatory values for short options are mandatory for long
options too.

The following commands are equivalent:

A file sorted numerically by the third field (colon separated)

$ sort -t : -k 3 -n /etc/passwd

$ sort --field-separator=: --key=3 --numeric-sort

/etc/passwd

Standard Options

Programmers are encouraged to define long options equivalent to
short ones.

Users may assume that two long options are defined for each
command. Those are:

--help

--version

File Names As Command Arguments

Double dash (

) may be used as an explicit end-of-options

marker in case of ambiguity.

touch

creates an empty file of a given name if it

doesn’t exist. We’d like to create a file named

--help

Double dash as an end-of-options marker

$ touch --help

help on the ‘touch’ command

$ touch -- --help # ‘--help’ is a file name

Numerous commands in lack of needed file name parameters
operate on standard input (default: keyboard) and standard
output (default: screen).

Outline of the lecture

Command Syntax
Options
File Names As Command Arguments

Compound Commands
Wildcards and Relative Pathnames
Escapes and Quotes
Stream Redirection, Pipelines and Command Substitution

Bash History

Bash keeps the history of user’s commands. By pressing the

↑

and

↓ keys users are able to quickly navigate through it.

The

history

displays enumerated list of recent commands. Then

we can recall a command by specifying its number:

number .

The shortcut:

refers to the latest command.

The last two commands are equivalent:

Recalling a command by number

$ history

1 ls -lA /etc

2 sort -t : -k 3 -n /etc/passwd

$ !2

$ sort -t : -k 3 -n /etc/passwd

Bash history can be wiped out by:

history -c

Notice: Several simultaneous sessions open for the same user
may overwrite each other’s history.

Bash Auto-completion

We can start typing a word then press the <TAB> key. Bash will
try to complete the word. The following rules apply:

the first word is completed to a valid command,
next words are completed to valid file names,
in case of ambiguity all possible completions are displayed.

The following pairs of commands give the same

results:

Using the auto-completion

$ his<TAB>

$ history

$ ls -l /et<TAB>

$ ls -l /etc

Outline of the lecture

Command Syntax
Options
File Names As Command Arguments

Compound Commands
Wildcards and Relative Pathnames
Escapes and Quotes
Stream Redirection, Pipelines and Command Substitution

Shell Variables

The following informations refer to all sh-compliant shells.
Csh-based shells use slightly different syntax, based on the C
programming language.

Variables are untyped and don’t need to be declared. Referring to
undefined variables doesn’t result in an error – ‘empty’ values are
returned.

A common practice is using capital letters for variable names.

Variables are defined by value assignment:

VARIABLE NAME=value

(important: no blanks may precede nor supersede the ‘=’).
We refer to values stored in variables by preceding variable
names with the dollar symbol:

$VARIABLE NAME

. For ambiguity

avoidance we may also use the form:

${VARIABLE NAME}

(see the

next slide).

unset

unsets a variable and removes it from the memory.

Shell Variables – Examples

echo

displays a string interpreted by the shell:

Managing variables

$ VAR1=/etc

$ echo $VAR1

/etc

$ ls -lA $VAR

listing the /etc directory contents

$ unset VAR1

$ echo $VAR1

We don’t mind the VAR2erpillar variable:

Curly braces for ambiguity avoidance

$ VAR2=Cat

$ echo ${VAR2}erpillar

Caterpillar

Environment

Every process has its own environment defined by some set of
variables.

Child processes (processes invoked by the current one) inherit
only parent’s

global

environment variables.

Newly defined variables are

local

by default. In order to make

them global they must be

exported

We can export existing variables or join exporting and

assigning a value:

Exporting variables

$ VARIABLE1=value1

$ export VARIABLE1

$ export VARIABLE2=value2

Environment Variables

printenv

and

env

print the list of global environment variables.

Examples of environment variables include:

HOME

home directory,

IFS

word separator (default: blanks),

LC *

locale settings,

PATH

colon-separated list of directories searched for
executables,

PS1

prompt string description,

SHELL

current shell.

set

prints an ordered list of both global and local shell

environment variables.

Bash provides an additional

declare

built-in which allows

viewing and managing attributes (status) of all variables.

Environment Variables – Examples

Redefining the default prompt:

Changing the default prompt

$ PS1=’Hello> ’

Hello>

Adding the home directory in front of the search path:

Modifying the search path

$ PATH=$HOME:$PATH

Selecting the interface language (it should be done

before starting the GUI):

Changing default localization to Polish

$ export LC ALL=pl PL.UTF-8

Outline of the lecture

Command Syntax
Options
File Names As Command Arguments

Compound Commands
Wildcards and Relative Pathnames
Escapes and Quotes
Stream Redirection, Pipelines and Command Substitution

Compound Commands

Commands delimited by semi-colon (

;

) on a single line will be

run in sequence.

Each command returns some value indicating success or failure.
A command can be run conditionally – depending on the success
(

) or failure (

) of previous command.

copies a file to a new name;

– renames (moves),

– deletes (removes). We delete file3 or move file5

depending on the result of copying:

Command sequences

$ cp file1 file2 ; rm file1

$ cp file3 file4 && rm file3

$ cp file5 file6 || mv file5 file6

Wildcards and Relative Pathnames

The following symbols can be used while specifying paths and
filenames:

dot (

)

current (working) directory,

double dot (

)

parent directory,

tilde (

)

home directory (the same as

$HOME

question mark (

)

any letter,

asterisk (

)

any sequence (string).

Let’s assume that the current directory is our home

directory, that is

/home/smithj

. The following commands

are equivalent:

Using wildcards and relative paths

$ ls -lA /e?c

$ ls -lA /e*

$ ls -lA ./../../etc

$ ls -lA ~/../../etc

Literal Meaning of Single Characters

Any single character preceded by a backslash (

) is considered by

a shell literally – backslash removes its special meaning. It’s often
referred as

escaping

It is possible (but highly not recommended) to name a

file

or even

’

. Quoting or escaping is necessary to refer

to such files. It may be also used for protecting blanks in
filenames.
Note that

starts comments in this examples:

Escaping special characters

$ rm * # remove all files

$ rm

\* # remove a single file ‘*’

$ rm filename with spaces # remove three files

$ rm filename

\ with\ spaces # remove a single file

Double Quotes

Double quotes (

) preserve literal meaning of all quoted

characters except:

and

(on the tilde key). Quotes matching

is requisite – placing unmatched quotes allows spanning
commands over several lines.

Double quotes allow variable substitution. They are

commonly used to preserve blanks and special symbols in
filenames. Note that quoted

doesn’t start a comment:

Using double quotes

$ echo "*** My home directory is: $HOME ***"

*** My home directory is: /home/smithj ***

$ ls -lA "My Documents"

file1.txt file2.odt file3.pdf

$ cp "name with spaces" "name with spaces.bak"

$ mv file1 "#"

$ rm "#"

Single Quotes

Single quotes (

’

) preserve literal meaning of all enclosed

characters.

All characters are used literally. There is no way to

place a single quote inside single quotes:

Using single quotes

$ echo ’$HOME’

$HOME

Mixing double and single quotes is allowed:

Using double and single quotes

$ echo " ’$HOME’ $HOME "

’/home/smithj’ /home/smithj

$ echo ’ "$HOME" $HOME ’

"$HOME" $HOME

Stream Redirection

For each program three standard streams are pre-defined. Those
are:

stdin

(

) – standard input,

stdout

(

) – standard output,

stdout

(

) – standard error.

Redirection of any of those streams is possible by using the

and

operators.

In the following examples the wildcard

is used to

indicate current (working) directory for listing:

Stream redirection

$ ls . > file # redirect output to file

$ ls . >> file # append output to file

$ sort < file # get input from file

$ ls . >& file # redirect output and errors to file

$ ls . &> file # same as above

$ ls . &>> file # append output and errors to file

Stream Redirection in Bash

Bash as well as other sh-compliant shells is able to use numbers
as stream identifiers.

can be used for standard error

redirection,

2>&1

– for merging standard output and standard

error. The latter one is commonly used in bash scripts.

The following commands are equivalent:

Stdout and stderr redirection

$ ls . >& file

$ ls . > file 2>&1

Pipelines

It is possible to connect stdout of one command to stdin of the
other one by using the pipe (

) symbol.

Commands used in pipelines for modifying output of other
commands are commonly referred as

filters

One may connect stdout and stderr to stdin of other command
by the

. This syntax is very rarely used.

sort -r

sorts in reverse order;

wc -l

prints the

number of lines. Note that we leave the files section empty
for filters:

Pipelines

$ ls . | sort -r

$ ls . | wc -l

Command Substitution

Commands quoted in

(on the tilde key) are replaced with their

stdout with all newlines deleted.

The

date

command displays (by default: current)

date.

touch

creates an empty file of a given name if it

doesn’t exist. Note that double quotes are necessary in the
first example to protect blanks:

Command substitution

$ date

Fri Feb 25 8:09:10 CET 2011

$ touch "`date`"

$ date +%d.%m.%Y

25.02.2011

$ touch `date +%d.%m.%Y`

Outline of the lecture

Command Syntax
Options
File Names As Command Arguments

Compound Commands
Wildcards and Relative Pathnames
Escapes and Quotes
Stream Redirection, Pipelines and Command Substitution