Bash Power Tips Walkthrough

Notes

# Variables

FOO=here
FOO="here"  # equivalent, always use quotes

BAR=$FOO
BAR="$FOO"
BAR="${FOO}"
BAR='$FOO'  # single quotes, does not evaluate the variable

BAZ="$FOOsy"  # different variable name
BAZ="${FOO}sy"

# Mnemonic: # (3 on keyboard) is in FRONT and % (5 on keboard) is in BACK
echo "${BAZ%e*}"      # delete shortest occurrence at end of string
echo "${BAZ#*e}"      # ... at beginning of string
echo "${FOO##*e}"     # longest occurrence at beginning
echo "${FOO%%e*}"     # longest occurrence at end
echo "${FOO/e/a}"     # substitute a for first occurence of e
echo "${FOO//e/a}"    # substitute a for all occurrences of e
echo "${FOO/#he/ha}"  # if beginning of string matches he, replace it with ha
echo "${FOO/%re/ro}"  # if end of string ...
echo "${BAZ:0:2}"     # characters 0 through 2
echo "${#BAZ}"        # number of characters in variable

# Arithmetic
# Operators: Add(+) Subtract(-) Multiply(*) Divide(/) Modulo(%) Exponent(**)
echo "$((1+2))"
X=1
echo "$((X+1))"
echo "$((${X}+1))"

# Tests
# [ vs [[ (latter includes regex (=~) support)
# -d (is directory), -f (is file), -e (exists), -n (nonempty), -z (zero-length)
# -eq, ==
if [[ "${FOO}" =~ ^he ]]; do
    echo "Match"
fi

# Control Structures
for VARIABLE in list of values; do
    echo "${VARIABLE}"
done

while true; do
    echo date
    sleep 1
done

if [[ -n "$FOO" ]]; then
    echo "Yep"
fi

while read WORD1 WORD2; do  # reads a line from stdin, assigns first two words to variables
    echo "1: ${WORD1}, 2: ${WORD2}"
done

while : ; do  # infinite loop, ":" is a noop
    ...
    break
done

for FILE in $(ls *.jpeg); do
    mv "${FILE}" "${FILE%.jpeg}.jpg"
done

# Functions
# local, shift
my_func () {
    local VAR1="$1"; shift
    local VAR2="$1"

    echo "${VAR1}"
    echo "${VAR2}"
}

# Paths
# $BASH_SOURCE, path to the current script
# dirname $SOMEPATH, basename $SOMEPATH
# globbing
#   ?[ab]*
#   {ab*,bc*}

# Command Substitution
# Stop using backticks!
FOO=`dirname $BASH_SOURCE`
FOO="`dirname $BASH_SOURCE`"

# much better
FOO="$(dirname $(realpath "$BASH_SOURCE"))"

# settings/shopt
#!/bin/bash -x
set -x           # print every line before evaluating
set -e           # fail on errors
set -o pipefail  # fail on errors in pipelines, -o can set many options
set -u           # fail when undefined variables are encountered

# Pipelines and IO
# > overwrite file
# >> append to file
# < read from file
# << heredoc
# &0, &1, &2 stdin, stdout, stderr
# exec 6<&0; exec 0</foo/bar; ... ; exec 0<&6 6<&- # rearrange file descriptors
# | pipe stdout, |& pipe stdout+stderr
# <() subshell fifo
# <<< string redirection

# Temporary files
TEMPFILE="$(mktemp)"
trap "rm -f $TEMPFILE" exit

# Winning Combos
# curl -s | jq
# ssh somehost bash < my_local_script.sh
# find -exec

# Interactive
# ctrl-r (history search)
# !! - previous command line
# !$ - last argument of previous command
# !ssh - most recent command beginning with ssh
# host somewebsite.com; ^some^my - run previous command with substitutions
# echo hello my name is matthew; echo !!:1 !!:$ - revise and run last command