Guangning Yu's Blog

• assign default value to variable

DEFAULT=5
RESULT=${VAR:-$DEFAULT}

• trap error exit

function rollback() {
  if [[ $? -ne 0 ]]; then
    echo "rollback"
  fi
}
set -ueo pipefail
trap rollback EXIT

• if file exists

#!/bin/bash
FILE="/etc/passwd"
if [[ -f $FILE ]];then
    echo "$FILE exists"
else
    echo "$FILE doesn't exist"
fi

• if directory exists

#!/bin/bash
DIR="/var/log"
if [[ -d $DIR ]]; then
   echo "$DIR exists"
else
   echo "$DIR doesn't exist"
fi

• eval

function assert_var_exists() {
    local condition="[[ ! -n \"\${$1+set}\" ]]"
    if eval $condition; then
        echo "The variable ${1} does not exist. Exit."
        exit 1
    fi
}
function export_var() {
    assert_var_exists $1
    local var_value=$(($1))
    eval "export $1=\$var_value"
    echo "$1=$var_value"
}

• Check platform

uname -m
lsb_release