bash-reference

Command References

Bash Quick Reference

Shebang

Shebang is the '#!' in the beginning of the script.
It is used to choose the interpreter that the script will use.
If no shebang is specified, the current shell will be used.

#! /bin/bash
echo "This script is using the bash interpreter."

Variables

To access variable's data use the $ operator.
The {} is optional.

#! /bin/bash

MY_VAR="Hello World"
echo $MY_VAR
echo ${MY_VAR}ddd   # Note, this is example when to use {}

By default, all variables are global.
The local key word can be used to make function variables local.

If Else statements

#! /bin/bash

MY_VAR="var"

if [ "$MY_VAR" = "var" ]
then
    echo "This is true"
elif [ "$MY_VAR" = "Silly value" ]
then
    echo "This will not be printed"
else
    echo "This will not be printed"
fi

There are few test operators that bash provides.
For example, the -f checks if file exists.

#! /bin/bash

if [ -f $0 ]
then
    echo "File $0 exists"
fi

Loops

#! /bin/bash

FILES=$(ls .)

for FILE_NAME in ${FILES}
do
    echo $FILE_NAME
done

Positional Parameters

Each command line param can be accessed with the $ operator:

#! /bin/bash

SCRIPT_COMMAND=$0

echo "This is \$0: $SCRIPT_COMMAND "
echo "This is \$1: $1"
echo "This is \$2: $2"

The $@ variable contains list of all command line params (except for $0):

#! /bin/bash

for PARAM in $@
do
    echo "This is param value: [ $PARAM ]"
done

Exit Code

#! /bin/bash

echo "The exit code of previous command is $?"

The following script check is host is reachable:

#! /bin/bash

HOST="google.com"

ping -c 1 $HOST

if [ "$?" -eq "0" ]
then
  echo "$HOST reachable."
else
  echo "$HOST unreachable."
fi

Functions

Note that return can be used to return value from function.
If no return is used, the return value is the return value of the
last command in the function.

#! /bin/bash

#Declare hello_world
function hello_world() {
    echo "Hello World"
    return 0
}

# Call hello_world
hello_world

Functions can access their parameters via $1, $2, etc.
Note that $0 is still the script name.

#! /bin/bash

function hello_world_with_params() {
    echo "Hello World $1 $2 $3"
}

hello_world_with_params hi hey wow

Local variables can be declared using the local keyword.

#! /bin/bash

function local_var_func() {
    local MY_VAR="Hey there"
}

local_var_func

Wildcards

* - Matches zero or more characters.
? - Matches exactly one character.
[] - Match one character from the characters in [].
[!] - Match any character that is not in [!].
[a-z] - Match character that is in the range of characters.

konstantin89 / bash-reference