Small, sweet shell things.
I don't know anything about shell scripts. Have always been scared and confused. Currently I am having to process a ton of audio files and had to not repeat myself doing same things over and over again. So here I am, learning shell things!
- While assigning a value to a variable, don’t use space around
=.
# Good
APPLE='is a fruit'
# Bad
APPLE = 'is a fruit'- Print variables within quotes to preserve their whitespace if they contain any.
$greeting='Hello World'
echo $greeting # Hello World
echo "$greeting" # Hello World- To not have $ treated as a special character denoting a variable inside echo, use backslash
\
echo "\$greeting" # $greetingUseful for printing things like 'Amount : $200.00'
-
Within quotes, variables can also be printed like
${MY_VARIABLE}. This is to reduce ambiguity. -
The output of commands can be assigned to variables directly. Use back ticks or
$()to wrap around the command.
FILES_LIST=`ls`
FILES_LIST=$(ls)- Getting arguments passed to a script is easy.
$1is the first argument.$2is the second and so on.- TIP: assign the arguments to properly named variables inside scripts. Example -
DIRECTORY=$1
- TIP: assign the arguments to properly named variables inside scripts. Example -
# Define an array
MY_ARRAY=('apple' 'banana' 'orange')
# All items of the array
echo ${MY_ARRAY[@]} # apple banana orange
# Length of an array
echo ${#MY_ARRAY[@]} # 3
# Get value by index
echo ${MY_ARRAY[2]} # banana# Define
MY_STRING="Hi! I am a String"
# Print entire string
echo $MY_STRING
echo $MY_STRING[@]
# Length
echo $#MY_STRING # 17
# Get substring
# Syntax: ${STRING:START_POSITION:LENGTH}
echo ${MY_STRING:0:3} # Hi!
# If LENGTH is removed, substring till end of string.
echo ${MY_STRING:4} # I am a String
# Replacing
# First occurence
echo ${MY_STRING[@]/a/o} # Hi! I om a String
# All occurences
echo ${MY_STRING[@]//a/o} # Hi! I om o String
# with another variable or expression
TEMP='an Idiot'
echo ${MY_STRING/a String/${TEMP}} # Hi! I am an Idiot
echo ${MY_STRING/String/String on $(date)} # Hi! I am a String at Wed Jan 9 17:03:09 IST 2019
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