while – Programming & Mustangs!

In this tutorial I will show a few useful commands when working with linux shell.

Check Directory Exists:

if [ -d /opt/test/ ]; then
    echo 'Directory Exists'
fi

Check Directory Not Exists:

if [ ! -d /opt/test/ ]; then
    echo 'Directory Does Not Exist'
fi

Check File Exists:

if [ -f /opt/test/test.log ]; then
    echo 'File Exists'
fi

Check File Not Exists:

if [ ! -f /opt/test/test.log ]; then
    echo 'File Does Not Exist'
fi

Lowercase Variable:

val='TEXT'
echo "${val,,}"

Echo Variable:
This will print the value of “test”. Notice we use double quotes.

test='My Test Val'
echo "$test"

Echo String:

echo 'My test string'

Split:
This will split on the comma into an array list and then loop through it.

test='test 1,test 2'
split_test=(${test//,/ })

for val in "${split_test[@]}"
do
    echo $val
done

Date:
This will print the date in the format YYYY-MM-dd

my_date="$(date +Y-%m-%d)"
echo "$my_date"

Remove Space From Variable:

VAL='test string'
echo "${VAL//\ /}"

Increment Variable:

index=0
index=$((index+1))

Substring

VAL='test string'
echo "${VAL:4:4}"

If value is equal to

VAL='hello'
if [ "$VAL" == 'hello' ] ; then
    echo 'Is Equal'
fi

If with OR

VAL='hello'
if [ "$VAL" == 'hello' ] || [ "$VAL" != 'hi' ] ; then
    echo 'Is Hello'
fi

If Variable is Empty

VAL=''
if [ -z "$VAL" ] ; then
    echo 'Is Empty'
fi

Append to File

echo 'Hi' >> file_to_log_to.log

Write to File

echo 'Hi' > file_to_log_to.log

While Loop: Increment to 10

This will loop till the value is 9 then exit.

i=0
while [ $i -lt 10 ];
do
    echo "$i"
done

whoami

USER=$(whoami)

If Variable Contains Text

VAL='my Test String'
if [[ "${VAL,,}" == *"test"* ]] ; then
    echo "Found test"
fi

Color Coding

NoColor=$'\033[0m'
READ=$'\033[0;31m'
GREEN=$'\033[0;32m'
YELLOW=$'\033[1;33;40m'

printf "%s Variable Not Set %s\n" "${RED}" "${NoColor}"

Get Log to a logfile and console

SOME_COMMAND 2>&1 | tee -a "${LOG_FILE_PATH}"

Read a JSON config

JSON=$(cat "/path/to/json/file.json")
export MY_VAR=$(echo "${JSON}" | python -c 'import json,sys;obj=json.load(sys.stdin);print(obj["MyKey"])')

Extract tar to Folder

sudo tar -xvf /the/location/file.tar -C /to/location/ --force-local --no-same-owner

Update Certificates

This will update certificates. After you put a certificate in /usr/local/share/ca-certificates/

update-ca-certificates

PipeStatus

somecommand
RETURN_CODE=${PIPESTATUS[0]}

Tag: while