Bash

Bash (Bourne Again SHell) is a Unix/Linux command-line shell and scripting language widely used for system administration, automation, and DevOps.

Bash scripts are plain text files interpreted by /bin/bash.

Create a script

nano script.sh

Add the shebang:

#!/bin/bash

Make it executable:

chmod +x script.sh

Run:

./script.sh

Commands

Bash commands are executables or shell built-ins.

ls
cd
pwd
echo

Piping:

ls | grep txt

Comments

# This is a comment

Variables

• Create variable:

  var="value"

• Use variable:

  echo $var

• Read-only variable:

  readonly var="value"

• Get variable information:

  declare -p var

Special variables

Variable	Description
$0	Script name
$1..$9	Positional arguments
$#	Number of arguments
$@	All arguments
$?	Exit code of last command
$$	Current process ID
$USER	Current user
$HOSTNAME	Host name
$PWD	Current directory
$HOME	Home directory

Arrays

• Declare an array:

  arr=(one two three)

• Access element:

  echo ${arr[0]}

• Array length:

  echo ${#arr[@]}

• Iterate:

  for i in "${arr[@]}"; do
  echo $i
  done

Associative arrays (hash tables)

declare -A users
users[admin]="root"
users[user]="guest"

• Access value

  echo ${users[admin]}

• Keys / values

  echo ${!users[@]}
  echo ${users[@]}

Operators

Arithmetic operators

+  -  *  /  %

Comparison operators

Operator	Description
-eq	equals
-ne	not equals
-gt	greater than
-ge	greater or equal
-lt	less than
-le	less or equal

[[ $a -gt 5 ]]

Logical operators

Operator	Description
&&	AND
||	OR
!	NOT

Condition statements

If / Else

if [[ $a -gt 10 ]]; then
  echo "Greater"
else
  echo "Smaller"
fi

Case (switch)

case $var in
  start) echo "Starting" ;;
  stop) echo "Stopping" ;;
  *) echo "Unknown" ;;
esac

Loops

For

for i in {1..5}; do
  echo $i
done

For each (array)

for x in "${arr[@]}"; do
  echo $x
done

While

while [[ $a -lt 5 ]]; do
  ((a++))
done

Do while

until [[ $a -ge 5 ]]; do
  ((a++))
done

Write / read on console

Write to console

echo "Hello world"

Without newline:

echo -n "Enter value: "

Read input

read var

With prompt:

read -p "Enter value: " var

Hidden input (password):

read -s password

Files and folder operations

Command	Description
touch file.txt	Create file
mkdir dir	Create directory
cp a b	Copy file
cp -r dir1 dir2	Copy directory
mv old new	Move / rename
rm file	Delete file
rm -r dir	Delete directory
test -f file	Check file exists
test -d dir	Check directory exists

Files I/O

File descriptors

Descriptor	Name	Description
0	stdin	Input
1	stdout	Output
2	stderr	Error output

Read line by line

while read line; do
  echo "$line"
done < file.txt

Preserve whitespaces:

while IFS= read -r line; do
  echo "$line"
done < file.txt

Write to file from script

{
  echo "Line 1"
  echo "Line 2"
} > file.txt

Append to file:

{
  echo "Line 3"
} >> file.txt

Check file extension and properties

Command	Description
test -f file.txt	Check if the file exists
test -d dir	Check if the directory exists
test -r file.txt	Check if the file is readable
test -w file.txt	Check if the file is writable
test -x file.txt	Check if the file is executable

if [[ -f file.txt ]]; then
  echo "File exists"
fi

File size & content info

wc -l file.txt   # lines
wc -w file.txt   # words
wc -c file.txt   # bytes

Pipe operators

Operator	Description
|	Send output to another command
>	Write output to file
>>	Append output to file
<	Read input from file

Named pipes (FIFOs)

• Create a pipe as a file:

  mkfifo mypipe

• Write:

  echo "hello" > mypipe

• Read:

  cat < mypipe

Pipes with loops

ps aux | while read line; do
  echo "$line"
done

• Preserve formatting:

  ps aux | while IFS= read -r line; do
  echo "$line"
  done

Output formatting

Command	Description
grep	Filter output
awk	Column processing
sed	Stream editing
cut	Extract fields
tee	Output to file and stdout

Example:

ps aux | grep root | tee output.txt