What is Bash?

Bash stands for Bourne-Again Shell, it is a command line scripting interpretor for UNIX distribution and a shell language.

Why learn bash?

most popular shell
easy line commands
comes with linux and other OS so no need pre-installation

Author

OURZIK jugurta.

Ressources

freeCodeCamp.org-Bash Scripting Tutorial for Beginners.

⏩ Let's start with shell basics !

Providing shell interpretor path

Bash is the default shell on almost all the Linux distributions but this doesn't mean that linux is going to assume every file we want run is also on bash, that's why we should explicitly specify the shell interpretor path at the beginning of our shell script. See the code below:

#!/bin/bash

##your shell code

In fact, the character sequence "#!" is called Shebang. It is used to tell the system which script interpreter it should use to execute the code contained in the file. This value does not necessarily correspond to Bash, it all depends on the script.We follow up the shebang by providing the full path to the shell.

If you don't know in which shell you are running, type the following line command:

echo $SHELL

it prints the shell interpretor path in which your files are running.

Before we proceed to run shell file,we should give it permission to be executed and then we do:

./filename.sh <arg1>...<argn>

1 Variables

1.1-Environment variables

Assignment (and definition if necessary): name=value

Note: ther is no space around "=" sign while assigning value to a variable name, otherwise the system will conside rit as a command name and try to excute it and ends with an error.

number=10
firstName="Jugurta"
count = 0 #error it shouldn't be space around the "=" sign

Access: $name

firstName="Jugurta"

echo "hello" $firstName

Output:

hello Jugurta

1.2-Automatic variables (shell variables)

In a script, the parameters or arguments, set by the user executing the script, are automatically and always stored in “automatic variables” (automatically filled in by the Shell). These variables are:

$0 : script name. The content of this variable is invariable.
$? : return code of the last command (0 if ok)
$1 to $9 : the possible first 9 arguments passed to the script
$# : number of arguments
$* : list of arguments (from $1 separated with space)
$@ : list of arguments (from $1)

#!/bin/sh

#EXAMPLE
#On suppose qu'on est dans le repértoire courant
currentPath=`pwd`;
if [ ! -d ~/sauvegarde ] 
then
    mkdir ~/sauvegarde
fi

#pour chaque fichier en argument 
for file in $@ 
do
    c=`echo $file| cut -d"." -f2`
    #si file est un fichier source c
    if [ $c = "c" ] 
    then
        #s'il n'est pas dans le repértoire sauvegarde on le copie
        if [ ! -f ~/sauvegarde/$file ]
        then 
            cp  $file ~/sauvegarde
    #sinon on le compare avec sa copie dèja présente et on le copie en cas de différence
        else
            numberOfLines=`diff $file ~/sauvegarde/$file | wc -l`   
            if [ $numberOfLines -ne 0 ]
            then 
                cp  $file ~/sauvegarde
            else
                echo $file already exists
            fi
        fi

    fi

done

2 Expanding metacharacters

2.1 In correspondence with the file system

asterisk(*) replaced by any number of any characters;
? replaced by exactly any character;
[ set ] replaced with exactly one character from the given set. The whole is a sequence of characters and/or character ranges. A character range is one character, followed by a dash followed by a character greater than the first in the order of ASCII codes.

2.2 Of substitution

$ replaced with the name that follows by the value of the corresponding variable;
$( command ) replaced with the standard output obtained when executing the command.

2.3 Protection

* protects the following character from expansion;
'string' protects the string from expansion;
"string" protects the string from expansion except for the characters $, $() and .

3 I/O redirection

< file : read standard input from file;
> file : write standard output to file;
2> file : write standard error to file;
>> file : concatenate standard output to end of file;
2>> file : concatenate standard error output to end of file;
command_1 | command_2 : redirect standard output of command_1 to standard input of command_2.

  #!/bin/sh
  cd dir
  ls > file (change file content with new content)
  ls >> file (concatenate old file content with new one)

#!/bin/sh
if [ -e ~/trash ]
then 
   ls ~/trash 2> /dev/null
else
      cd ~   
      mkdir trash
fi

4 The tests

Syntax: [ expression ] (or test expression).

4.1 On files and directories

-e : file exists
-s : file is not empty
-f : fich is a file
-d : fich is a directory
-r : file has permission r (read)
-w : file has permission w (write)
-x : file has permission x (execute)

FILE=$1
if [ -e $FILE -a -f $FILE] 
    then
        mv $FILE ~/trash
    fi

4.2 On integers

-eq : n1 = n2
-ne : n1 ≠ n2
-lt : n1 < n2
-gt : n1 > n2
-le : n1 <= n2
-ge : n1 >= n2

a=8
b=7
c=23
if [ a -le b ] 
    then
        echo "yes"
    fi
 
 if [c -lt a]
 then
        echo "Hola"
    fi

4.3 On strings

-z string : string is empty
-n string : string is not empty
string1 = string2 : the 2 strings are identical
string1 != string2 : the 2 strings are different

name="Jack"
str="Mount"
echo $(expr $name = $str)

Output:

4.4 Between expressions

expr1 -a expr2 : and
expr1 -o expr2 : or
! expr : not

i=0
cpt=1
if [ $( expr $i % 2 ) -eq 0 -a i -ne 10 ]
        then
            cpt=$( expr $cpt + 1 )
            i=$( expr $i + 1 )
        fi

5 Control structures

5.1 Conditional structure

if <condition>
then
# if condition true 
<sequence of commands 1>
else 
<sequence of commands 2>
fi

The condition is determined by the return code of an order. The else part is optional.

5.2 Nested conditionals

if <condition_1>
then
<sequence of commands 1>
elif <condition_2>
then
# if condition_1 est false
# and condition_2 true
<sequence of commands 2>
else
# if the two conditions are false
<sequence of commands 3>
fi

5.3 For loop

for <variable> in <list>
do
<sequence of commands>
done

5.4 While loop

while <condition>
do
<sequence of commands>
done

5.5 Functions

<fonction_name>() {
<sequence of commands>
}

6 Useful commands (small subset...)

pwd : displays the current directory (Print Working Directory)
cd : change current directory (change directory)
- cd .. : go back to parent directory
- cd / : go to the root directory
- cd : go to the current user's home directory.
- cd - : go to the previously explored directory
mkdir dir : creates the dir directory (make directory)
rmdir dir : deletes the directory dir if it is empty (remove directory)
ls : list files and directories (list)
cp : copy files and directories (copy)
- cp file1 file2
- cp file rep
- cp file.
- cp file1 ...file rep
- cp -R rep1 rep2 (rep2 exists or not, copy to rep2 or named rep2 )
rm : remove files and directories (remove)
mv : moves and/or renames a file or a directory (move)
- mv index.html home.html (renaming)
- mv index.html /home/site/ (move)
- mv index.html /home/site/home.html (move + rename)
touch : creates an empty file (or change the last access date of a existing file)
cat (concatenate) : concatenates and displays files.
- cat file
- cat file1 file2
less (or more) : displays files page by page.
- less file
file : identifies the type of a file.
- file file
vim (console), emacs, gedit. . . : text editors
echo [arguments] : displays its arguments on one line;
read name : reads a line and stores it in the variable name;
expr expression : displays the value of an arithmetic expression;
basename name [extension] : displays the private name of its path part (until the last slash) and its possible extension;
dirname name : displays the path part (before the last slash) of name;
grep pattern [files ...] : displays the lines of its entries containing the pattern.
diff, find, tr, head, tail, cut, sed, sort, ... : and many more...

Uses of some commands:

find find
```
#!/bin/bash

#1 search for the file "fileName" in the entire sub-tree of the indicated 'directory'
find ~/directory -name "fileName 

#2 Search for all files which begin whith a specific pattern "pattern*" in the entire sub-tree of the indicated directory

find ~/directory -name "pattern*" 

 #3 Search for all files which contain a pattern "pattern*" in the entire sub-tree of the indicated directory

find ~/directory -name "*pattern*
```
We can associate the exec command with find to do some processing on each file found:
```
#!/bin/bash
find path_of_targeted_directory -type -f -name "*.c" exec grep "main" {} \;

#here we are telling bash to search the pattern "main" in every c source file found by find command by associating exec 
#command to find.The curly braces {} tell bash to execute [grep "main"] when it founds a file with given name.The back slash followed by semi-column tell bash about the end of the exec command.
```
tr tr

The tr command is a Linux command-line utility that translates or deletes characters from standard input ( stdin ) and writes the result to standard output ( stdout ).

#The basic tr command syntax is: tr [options] SET1 [SET2]

#example: echo "hello world" | tr o m #output: "hellm wmrld"

#here we change caracter by another one: o->m

Some tr options:
- -C : Complements the characters in SET1, including every character in the output except the ones specified.
- -c : Complements the values in SET1. Operations apply to characters that are not in the given set.
- -d : Deletes characters from the SET1 input.
- -s : Squeezes repeated characters specified in the last operand (either SET1 or SET2) and replaces them with a single occurrence of that character.
- -t : Truncates SET1 to the length of SET2.
- -u : Ensures that any output is unbuffered.
- --help : Displays the help file with all available options.
- --version : Displays the program version information.

#More examples:
name="FloRenCE"
echo $name | tr [:upper:] [:lower:]
#output: florence

echo $name | tr [:lower:] [:upper:]
 #output: FLORENCE
 
 #the same as:
 $name | tr a-z A-Z
 #output: FLORENCE

#Remove repeated charcters with s option

echo "hello     world 234    34  35" | tr -s " "
echo "hello     world 234    34  35" | tr -s [:space:]
#output: hello world 234 34 35

#delete charcaters with d option
echo "Hello , my name is Jugurta ourzik" | tr -d 'u' 
#output: Hello , my name is Jgrta orzik

#remove all digits :
echo "your password is: Ab@224#^!" | tr -d [:digit:]
#output: your password is: Ab@#^!

sed `sed`

sed allows you to modify or delete part of a character string, for example to replace one character with another in a file, or to delete unnecessary character strings. It is a very powerful tool.

Use:

To use sed, you must provide it with a string to process. This string can come from:

- of a file
- of a variable

As a general rule the syntax is of the form sed "s/[searched_occurrence]/[substitution_occurrence]/[behavior]" A simple example of substitution in a list of files: sed "s/[searched_occurrence]/[substitution_occurrence]/g" -i mes_fichiers*

#!/bin/bash export string="this_is_a_string_of_characters" echo $string | sed -e "s/_/ /g" # Will give in response #output: this is a string of characters #g stands for glabal, that means change all occurences of _ by space (" ")

sed to replace

#!/bin/bash #replace the first occurence of the string "unix" by "linux" sed 's/unix/linux/' file.txt #Replacing string on a specific line number : You can restrict the sed command to replace the string on a specific line #number. An example is: sed '3 s/unix/linux/' file.txt #Duplicating the replaced line with /p flag : The /p print flag prints the replaced line twice on the terminal. If a line does #not have the search pattern and is not replaced, then the /p prints that line only once. sed 's/unix/linux/p' file.txt #Printing only the replaced lines : Use the -n option along with the /p print flag to display only the replaced lines. Here #the -n option suppresses the duplicate rows generated by the /p flag and prints the replaced lines only one time. sed -n 's/unix/linux/p' file.txt #Replacing string on a range of lines : You can specify a range of line numbers to the sed command for replacing a string. sed '1,3 s/unix/linux/' file.txt

sed to delete

Deleting lines from a particular file : SED command can also be used for deleting lines from a particular file. SED command is used for performing deletion operation without even opening the file Examples: 1. To Delete a particular line say n in this example Syntax: $ sed 'nd' filename.txt Example: $ sed '5d' filename.txt 2. To Delete a last line Syntax: $ sed '$d' filename.txt 3. To Delete line from range x to y Syntax: $ sed 'x,yd' filename.txt Example: $ sed '3,6d' filename.txt 4. To Delete from nth to last line Syntax: $ sed 'nth,$d' filename.txt Example: $ sed '12,$d' filename.txt 5. To Delete pattern matching line Syntax: $ sed '/pattern/d' filename.txt Example: $ sed '/abc/d' filename.txt

Cowsay-Project

JugurtaO/shellScript-sh.md

What is Bash?

Why learn bash?

Author

Ressources

⏩ Let's start with shell basics !

Providing shell interpretor path

1 Variables

1.1-Environment variables

1.2-Automatic variables (shell variables)

2 Expanding metacharacters

2.1 In correspondence with the file system

2.2 Of substitution

2.3 Protection

3 I/O redirection

4 The tests

4.1 On files and directories

4.2 On integers

4.3 On strings

4.4 Between expressions

5 Control structures

5.1 Conditional structure

5.2 Nested conditionals

5.3 For loop

5.4 While loop

5.5 Functions

6 Useful commands (small subset...)

Uses of some commands:

find `find`

We can associate the exec command with find to do some processing on each file found:

tr `tr`

Some tr options:

sed `sed`

Use:

JugurtaO/shellScript-sh.md

What is Bash?

Why learn bash?

Author

Ressources

⏩ Let's start with shell basics !

Providing shell interpretor path

1 Variables

1.1-Environment variables

1.2-Automatic variables (shell variables)

2 Expanding metacharacters

2.1 In correspondence with the file system

2.2 Of substitution

2.3 Protection

3 I/O redirection

4 The tests

4.1 On files and directories

4.2 On integers

4.3 On strings

4.4 Between expressions

5 Control structures

5.1 Conditional structure

5.2 Nested conditionals

5.3 For loop

5.4 While loop

5.5 Functions

6 Useful commands (small subset...)

Uses of some commands:

find find

We can associate the exec command with find to do some processing on each file found:

tr tr

Some tr options:

sed sed

Use:

find `find`

tr `tr`

sed `sed`