Bash is by default installed on most Linux distributions like AlmaLinux, CentOS, Kali, and Ubuntu. The following cheat sheet outlines some important features of bash scripting.
Bash Script Header
#!/usr/bin/env bash
echo “Hello World”
Variables
#!/usr/bin/env bash
MSG=”Hello World”
echo “$MSG Albert” # Hello World Albert
echo ‘$MSG Albert # $MSG Albert
Strings
MSG=”hello world”
Replace
echo ${MSG/w/W} # hello World
echo ${MSG//[a-zA-Z]/X} # AAAAA AAAAA
Uppercase
echo ${MSG^} # Hello world
echo ${MSG^^} # HELLO WORLD
MSG=”HELLO WORLD”
echo ${MSG,} # hELLO WORLD
echo ${MSG,,} # hello world
Substring
echo ${MSG:0:5} # hello
echo ${MSG%world} # hello
echo ${MSG#hello} # world
Alternative
echo ${MSG:-val} # HELLO WORLD
echo ${FOO:-val} # val
Collections
Arrays
names=(‘Ben’ ‘Bolt’ ‘Bob’)
names+=(‘Soto’) # Appends element
unset names[3] # Removes element
echo ${names[0]} # Ben
echo ${names[@]} # Ben Bolt Bob
echo ${#names[@]} # 3
Maps
declare -a score
score[X]=”1″
score[Y]=”2″
score[Z]=”3″
unset score[X] # Delete X entry
echo ${!score[@]} # X Y Z
echo ${score[@]} # 2 1 3
echo ${#score[@]} # 3
Functions
helloworld() {
echo “Number of arguments $#” # 2
echo “Hello World $1 from $2” # Hello World Ben from Bash
}
helloworld “Ben” “Bash
helloworld() {
echo ‘My return string!’
}
msg=$(helloworld)
echo $msg
Conditionals
if [[ $b -gt 4 ]]; then
echo “$b is greater than 4”
elif [[ $b -lt 4 ]]; then
echo “$b less than 4”
else
echo “$b is equal 4”
fi
Numeric Conditions
[[ NUM -eq NUM ]] Equal[[ NUM -ne NUM ]] Not equal
[[ NUM -lt NUM ]] Less than
[[ NUM -le NUM
a]]Less than or equal to
[[ NUM -gt NUM ]] [[ NUM -ge NUM ]] Greater than or equal to
String Conditions
[[ STRING== STRING
]]Equal [[ STRING !=
STRING ]]Not Equal [[ -z STRING
]]Empty string [[ -n STRING
]]Not empty string [[ STRING
=~ STRING
]]Regular expression match
File Conditions
[[ -f FILE ]] Is a file [[ -d FILE ]] Is a directory [[ -e FILE ]] Exists [[ -r -w -x FILE ]] Is readable, Writable, executable [[ -h FILE ]] Is symbolic linkBoolean conditions
a|[[ ! EXPR ]] |Not
a|[[ BOOL && BOOL ]] |And
a|[[ BOOL || BOOL ]] |OR
Loops
For
for ((i = 0 ; i < 10 ; i++)); do
echo “Hello World $i”
done
for i in {1..5}; do
echo “Hello World $i”
done
While
x=1;
while [ $x -le 5 ]; do
echo “Hello World”
done
Files
for i in /tmp/*.txt; do
echo $i
done
cat /tmp/hello.txt | while read line; do
echo $line
done
Print Output
printf “nn@ Writing @n”
Read Input
echo -n “Type answer: “
read ans
echo $ans
