BASH

Variables

Special Parameters

Variable	Meaning
`$*`	Positional parameters. Expands to a single word with spaces.
`$@`	Positional parameters. Expands to separate words.
`$#`	The number of positional parameters
`$?`	Exit status of the most recently executed foreground pipeline
`$-`	Current option flags
`$$`	Process ID of the shell
`$!`	Process ID of the most recently executed background (asynchronous) command
`$0`	Name of the shell or shell script
`$_`	Last argument(s) to the previous command

source: TLDP

String Manipulation

String Length

LENGTH=$((${#STRING}-1))

Notes:

Only works in BASH and its derivatives.
STRING must be a variable that is prepended by an octothorpe (#), not a dollar sign ($)

Substrings

# The command has the form: SUB_STRING=${STRING:start:length}
STRING='Hello World'

SUBSTR=${STRING:0:5}
echo $SUBSTR   # prints 'Hello'

SUBSTR=${STRING:3:5}
echo $SUBSTR   # prints 'lo Wo'

SUBSTR=${STRING:6:-1}
echo $SUBSTR   # prints 'Worl'

Notes:

STRING must be a variable. It is NOT prepended by a dollar sign ($)
The index of the first element is 0
Values of the last element, length, can be negative. This is how many elements to be ignored at the end of STRING

Math

Every variable in Bash is a string. To do math you need to surround the equations with $(( and )). Otherwise, operations like + will keep their string behavior, in this case, concatenation.

i=0
while [[ $i -lt 5 ]]; do
    i=$((i+1))
    echo "$i"
done

Conditionals

Format of `If Then Else`

if [[ CONDITION ]]; then
    COMMANDS
elif [[ CONDITION ]]; then
    COMMANDS
else
    COMMANDS
fi

Boolean Expressions

Also called “Test Commands” or “Primaries”

Form

[ EXPRESSION ]
# OR
[[ EXPRESSION ]]

Notes:

It is important to include spaces between the brackets ([, ], [[, and ]]) and the EXPRESSION.

Negating Boolean Expressions

if ! [[ EXPRESSION ]]; then
    # If EXPRESSION evaluates to False, the commands here will execute
fi

Single vs Double Bracket

There are two different bracket types that be used to evaluate a Boolean expression, [ EXPRESSION ] and [[ EXPRESSION ]]. While these can be used interchangeably, they handle EXPRESSION differently. The reason for this is historical. the single bracket, [ was developed in the Thompson shell, and is more limited than the double bracket, [[, operator. The double bracket was introduced in the Korn shell and adopted by others (including Bash). It is an extension of the [ operator, and adds functions like pattern matching.

The general rules I follow are:

Use single brackets, [, when you are writing scripts that might be used on older systems, or needs to be run by a variety of shells.
When you use single brackets always quote your variables: [ -n "$VARIABLE" ], (not [ -n $VARIABLE ] .)

Useful Conditionals

TLDP list of conditionals

Primaries acting on Integer Variables

Primary	Evaluates to
`[ ARG1 -eq ARG2 ]`	True if `ARG1` is equal to `ARG2`
`[ ARG1 -ne ARG2 ]`	True if `ARG1` is not equal to `ARG2`
`[ ARG1 -lt ARG2 ]`	True if `ARG1` is strictly less than `ARG2`
`[ ARG1 -le ARG2 ]`	True if `ARG1` is less than or equal to `ARG2`
`[ ARG1 -gt ARG2 ]`	True if `ARG1` is strictly greater than `ARG2`
`[ ARG1 -ge ARG2 ]`	True if `ARG1` is greater than or equal to `ARG2`

Primaries on String Variables

Primary	Evaluates to
`[ -z STRING ]`	True of the length if `STRING` is zero.
`[ -n STRING ]`	True if the length of `STRING` is non-zero.
`[ STRING ]`	Same as above.
`[ STR1 == STR2 ]`	True if the strings are equal.
`[ STR1 = STR2 ]`	Same as above but is stritly POSIX compliant.
`[ STR1 != STR2 ]`	True if the strings are not equal.
`[ STR1 '<' STR2 ]`	True if `STR1` sorts before `STR2` lexicographically.
`[ STR1 '>' STR2 ]`	True if `STR1` sorts after `STR2` lexicographically.
`[ -o OPTIONNAME ]`	True if shell option `OPTIONNAME` is enabled.

Primaries acting on files

Primary	Evaluates to
`[ -a FILE ]`	True if `FILE` exists
`[ -e FILE ]`	True if `FILE` exists
`[ -f FILE ]`	True if `FILE` exists and is a regular file
`[ -d FILE ]`	True if `FILE` exists and is a directory
`[ -L FILE ]`	True if `FILE` exists and is a symbolic link
`[ -h FILE ]`	True if `FILE` exists and is a symbolic link
`[ -p FILE ]`	True if `FILE` exists and is a named pipe (FIFO)
`[ -S FILE ]`	True if `FILE` exists and is a socket
`[ -s FILE ]`	True if `FILE` exists and has a size greater than zero
`[ -b FILE ]`	True if `FILE` exists and is a block-special file
`[ -c FILE ]`	True if `FILE` exists and is a character-special file
`[ -r FILE ]`	True if `FILE` exists and is readable
`[ -w FILE ]`	True if `FILE` exists and is writable
`[ -x FILE ]`	True if `FILE` exists and is executable
`[ -u FILE ]`	True if `FILE` exists and its SUID (set user ID) bit is set
`[ -g FILE ]`	True if `FILE` exists and its SGID bit is set
`[ -k FILE ]`	True if `FILE` exists and its sticky bit is set
`[ -O FILE ]`	True if `FILE` exists and is owned by the effective user ID
`[ -G FILE ]`	True if `FILE` exists and is owned by the effective group ID
`[ -t FD ]`	True if file descriptor `FD` is open and refers to a terminal
`[ -N FILE ]`	True if `FILE` exists and has been modified since it was last read
`[ FILE1 -nt FILE2 ]`	True if `FILE1` has been changed more recently than `FILE2`
`[ FILE1 -ot FILE2 ]`	True if `FILE1` is older than `FILE2`
`[ FILE1 -ef FILE2 ]`	True if `FILE1` and `FILE2` refer to the same device and inode numbers

Loops

For Loops

for VAR in $LIST; do
    ...
done

While Loops

while [[ CONDITION ]]; do
    ...
done

Functions

function FUNCTION_NAME = {
    ...
}

# OR
FUNCTION_NAME () = {
    ...
}

Function Arguments

Arguments can be passed to functions as follows:

FUNCTION_NAME [ARG1 [ARG2 [...]]]

Arguments are accessed the same as in bash scripts:

$1 is the function name
$2 is the first argument
$3 is the second argument …

Exit vs Return from a Function

Using return [INTEGER] will stop function execution and will set $? in the parent shell.
Using exit will stop execution and close the parent shell.

Notes on Computers

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Notes on Math & Physics

Information Theory · Linear Algebra · Solid State Physics

BASH

Variables

Special Parameters

String Manipulation

String Length

Substrings

Math

Conditionals

Format of If Then Else

Boolean Expressions

Form

Negating Boolean Expressions

Single vs Double Bracket

Useful Conditionals

Primaries acting on Integer Variables

Primaries on String Variables

Primaries acting on files

Loops

For Loops

While Loops

Functions

Function Arguments

Exit vs Return from a Function

Format of `If Then Else`