Bash-Cheatsheet

A collection of useful code for Bash.

Variables

Declare a variable and set its value, but only if the variable is empty or undefined. It uses indirect parameter expansion. The ! means set the variable name, not just the value.

# Required arguments:
#   1. Variable name
#   2. Variable value
#
# Usage:
#
#   setvar foo bar
#
# In the above example, if a variable named foo
# exists and is not empty it will be set to bar.
# Otherwise its value will not be altered.
setvar() {
    eval "${1}=\"${!1:-${2}}\"";
}

Paths

Find the full path of a script

When a script gets invoked via an alias $PWD returns the directory where the alias resides, not the script being called. To find the full canonical path to the current script use:

BASEPATH=$(dirname $0)

echo "$BASEPATH"

To remove the trailing slash from a directory:

if [[ $DIR =~ .*/$ ]]; then
    DIR="${DIR:0:-1}"
fi

Extract the filename from the path

filename="${filepath##*/}"

Remove the file extension, leaving only the name:

name="${filename%%.*}"

Determine if a package is installed

This looks for the existence of its shell command:

if [[ $(type curl) ]]; then
    echo " Curl installed!"
fi

For other app types use command with the -v flag to prevent the application from being launched:

if command -v inkscape &>/dev/null; then
    echo "Inkscape installed!"
fi

Arrays

Read the each line of a file into an array:

# -t means remove trailing newlines

readarray -t MYARRAY < /path/to/filename

Read a directory and put all filenames with a particular extension into an array:

i=0
declare -A MY_ARRAY
for file in /path/to/*.txt; do
    if [[ -f "$file" ]]; then
        MY_ARRAY[${i}]="$file"
        ((i++))
    fi
done

Loop through an array in reverse order

MYARRAY=('one' 'two' 'three' 'four' 'five' 'six')

for (( i=${#MYARRAY[@]}-1 ; i>=0 ; i-- )) ; do
    echo "${MYARRAY[i]}"
done

Count how many items are in an array:

MYARRAY=(1, 2, 3, 4, 5, 6)

count="${#MYARRAY[@]}"

echo "$count"

Validate an array

if [[ ${#MY_ARRAY[@]} -eq 0 ]]; then
    echo "Array empty"
fi

Loops

Iterate through all files in a directory that have a certain extension

for filepath in /path/to/*.png; do
    echo "$filepath"
done

Modulus to alternate in a loop

for ((i=1; i<=10; i++))
{
    if (( $i%2 == 0 )); then
        echo "Even"
    else
        echo "Odd"
    fi
}

Regular Expressions

Remove newlines:

MYVAR=${MYVAR//$'\n'/}

Remove spaces

Using sed

str="Hello, my name is Foo"

str=$(echo $str | sed "s/[ ]//g")

echo $str # Hello,mynameisFoo

Parameter expansion

str="John, at the Bar is a friend-of-mine"

str=${str,,}    # lowercase
str=${str//,/}  # remove commas
str=${str//-/}  # remove dashes
str=${str// /}  # remove spaces

echo $str # johnatthebarisafriendofmine

Remove everything after a certain character

str="Nancy Davis # Will bring jello to the potluck"

str=$(echo $str | sed "s/#.*//")

echo $str # Nancy Davis

Change case

MYVAR=${MYVAR,,} # Lowercase all characters
MYVAR=${MYVAR^^} # Uppercase all characters

Truncate a string to x characters

MYSTRING="foobarbaz"

$MYSTRING="${MYSTRING:0:3}" # returns "foo"
$MYSTRING="${MYSTRING:6:3}" # returns "baz"

Does the variable contian only integers?

if [[ $MYVAR =~ ^?[0-9]+$ ]]; then
    echo "Only integers allowed!"
fi

Extract the hex color value from a string:

MYSTRING=$(echo "$MYSTRING" | sed 's/.*[[:space:]]\(#[a-zA-Z0-9]\+\)[[:space:]].*/\1/')

Escape certain characters:

MYVAR=${MYVAR//&/\\&} # Excape ampersands

To find a value in a string:

SEARCHFOR="bar"

if echo "$MYSTRING" | egrep -q "(^|\s)${SEARCHFOR}($|\s)"; then
    echo "$SEARCHFOR exists!"
fi

Delete everything until the first slash

MYVAR="this is some text /path/to/foo.jpg"

MYVAR=$(echo $MYVAR | sed 's/^[^/]*\///g')

Delete everything after a colon:

MYVAR="foo:bar"

MYVAR=$(echo $MYVAR | sed "s/[:].*//")

Math

To perform arithmatic use:

echo $(( 5+2 ))

To assign the result to a variable

n=5
n=$(( n-1 ))

Generate a random number between two values:

RND=$(shuf -i 1-20 -n 1)
echo "$RND"

Killing a process

By name:

pkill foo

To look up a process by name

pgrep foo

To look up all process IDs associated with a package and kill them:

PID=$(pgrep foo)
if ! [ -z "$PID" ]; then
    for id in $PID; do
        kill "$id"
    done
fi

Misc.

Fixes echo for POSIX compatibility

echo() (
    fmt=%s
    end=\\n 
    IFS=" "

    while [ $# -gt 1 ] ; do
        case "$1" in
            [!-]*|-*[!ne]*) break       ;;
            *ne*|*en*)      fmt=%b
                            end=''      ;;
            *n*)            end=''      ;;
            *e*)            fmt=%b      ;;
        esac
        shift
    done
    printf "$fmt$end" "$*"
)

Change the input field separator from a space to a null

IFS=$'\n'

Get the returned value of the last command

ls >/dev/null 2>&1

echo $?  # Returns 0

fofofofo >/dev/null 2>&1

echo $?  # Returns 127 (command not found)