(Personal) Shell Cheatsheet

Things I have to look up too often for the old dusty /bin/sh (not bash).

Echo to stderr

>&2 echo "Warning message"

Line breaks with variables

With $'\n'

echo "username:$USER"$'\n'"home:$HOME

Functions

my_func() {
    local arg1="$1"
    local my_local_var="foo"

    # error
    return 1
}

Default values

arg="${0:-default}"

if [ "${myvar:-default}" = "something" ]; then
  # has something in myvar
fi

Redirect output from within the script

Send stdout to a file

exec > file

with stderr

exec > file
exec 2>&1

These will truncate the file. To append use

exec >> file
exec 2>&1

Handle exit status manually with set -e

ret=0
command_might_fail || ret=$?

File extension

filename="foo.txt"
extension="${filename##*.}" # txt
name="${filename%.*}" # foo

Multiple conditions

foo=1
bar=0

if [ "$foo" = "1" ]; then
    echo "foo"
fi

if [ "$foo" = "1" -o "$bar" = "1" ]; then
    echo "foo or bar"
fi


if [ "$foo" = "1" -a "$bar" = "1" ]; then
    echo "foo and bar"
fi

Parse CLI options

while true; do
    case "${1:-}" in
    -o|--option)
        shift
        option="$1"
        shift
       ;;
    -f|--boolean-flag)
        shift
        flag=true
       ;;
    -h|--help)
        help && exit 0
        ;;
    -*|--*)
        >&2 echo "Unknown arg $1"
        exit 2
        ;;
    *)
    "")
        break
        ;;
    *)
        argument="$1"
        shift
        ;;
    esac
done

Multiline strings

cat > config.json << WHATEV
{
    "defaultLanguage": "$lang"
}
WHATEV


curl -d @- -X POST -H 'Content-Type: application/json' http://localhost:8080/log << JSON
{
  "type": "wlan",
  "date": "1348134879",
  "wlaninterface": "wlan0",
  "event": "AP-STA-CONNECTED",
  "mac": "e4:d5:3d:testmac",
  "relay_timestamp": 1348134880
}
JSON

Shortable date

date +%Y-%m-%d:%H:%M:%S

Links

• Bash Pitfalls https://mywiki.wooledge.org/BashPitfalls hn