When writing shell scripts, it’s often useful to accept some command line parameters. \u00a0It’s even more useful to have some defaults for those parameters. \u00a0Until now I’ve been using if statements to check if the parameter was empty, and if it was, to set it to the default value. \u00a0Something like this:<\/p>\n
\r\n#!\/bin\/bash\r\n\r\nDB_HOST=$1\r\nDB_NAME=$2\r\nDB_USER=$3\r\nDB_PASS=$4\r\n\r\nif [ -z "$DB_HOST" ]\r\nthen\r\n DB_HOST="localhost"\r\nfi\r\n\r\nif [ -z "$DB_NAME" ]\r\nthen\r\n DB_NAME="wordpress"\r\nfi\r\n\r\nif [ -z "$DB_USER" ]\r\nthen\r\n DB_USER="root"\r\nfi\r\n\r\necho "Connecting to the database:"\r\necho "Host: $DB_HOST"\r\necho "Name: $DB_NAME"\r\necho "User: $DB_USER"\r\necho "Pass: $DB_PASS"\r\n<\/pre>\nIt turns out there is a much more elegant way to do this with shell parameter expansion<\/a>. \u00a0Here is how it looks rewritten:<\/p>\n
\r\n#!\/bin\/bash\r\n\r\nDB_HOST=${1-localhost}\r\nDB_NAME=${2-wordpress}\r\nDB_USER=${3-root}\r\nDB_PASS=$4\r\n\r\necho "Connecting to the database:"\r\necho "Host: $DB_HOST"\r\necho "Name: $DB_NAME"\r\necho "User: $DB_USER"\r\necho "Pass: $DB_PASS"\r\n<\/pre>\nThis is so much better. Not only the script itself is shorter, but it’s also much more obvious what is going on. \u00a0Copy-paste errors are much less likely to happen here too.<\/p>\n
I wish I learned about this sooner.<\/p>\n\n","protected":false},"excerpt":{"rendered":"\n
When writing shell scripts, it’s often useful to accept some command line parameters. \u00a0It’s even more useful to have some defaults for those parameters. \u00a0Until now I’ve been using if statements to check if the parameter was empty, and if it was, to set it to the default value. \u00a0Something like this: #!\/bin\/bash DB_HOST=$1 DB_NAME=$2 … Continue reading Shell parameter expansion : default values for shell script parameters<\/span><\/a><\/p>\n\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"Shell parameter expansion : default values for shell script parameters #Linux #shell #bash #programming #SysAdmin","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false,"_links_to":"","_links_to_target":""},"categories":[1,6,18,133,62],"tags":[3098,1960],"keyring_services":[],"class_list":["post-26923","post","type-post","status-publish","format-standard","hentry","category-general","category-linux","category-programming","category-sysadmin","category-technology","tag-bash","tag-command-line"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t