shell basics (bash)

echo "foobar"

echo foobar

echo echo # second echo not executed but printed!

echo foobar | xargs echo
# same output as
echo foobar

echo foo > test.txt # pipe into file, replacing the content
echo bar >> test.txt # append to file
# warning: 
cat test.txt > test.txt # defined as generating an empty file!

echo foobar | sed s/foo.*/foo/ | xargs echo
# same output as 
echo foo

echo foo | grep bar # empty
echo foobar | grep oba # foobar, oba higlighted

foo=1 # no spaces around the equal sign!
echo ${foo} # "$foo" == "1", "$foobar" == "", "${foo}bar" == "1bar"

echo $(echo foobar)
# equivalent to
echo foobar | xargs echo

for i in a b c; do 
    echo $i
done
# ; can replace a linebreak
for i in a b c; do echo $i; done

for i in {1..5}; do # 1 2 3 4 5
    echo $i
done

while true; do 
    break; 
done
# break: stop
# continue: start the loop again

foo=1
echo "${foo}" # 1
echo '${foo}' # ${foo} <- literal string
for i in "a b c"; do # quoted: one argument
    echo ${i}; 
done 
# => a b c
for i in a b c; do # unquoted: whitespace is separator!
    echo ${i}; 
done 
# a
# b
# c

# string equality
a="foo"
b="bar"
if [[ x"${a}" == x"${b}" ]] ; then
    echo a
else
    echo b
fi

# other tests
if test -z ""; then 
    echo empty
fi
if [ -z "" ]; then
    echo same check
fi
if [ ! -z "not empty" ]; then
    echo inverse check
fi
if test ! -z "not empty"; then
    echo inverse check with test
fi
if test 5 -ge 2; then
    echo 5 is greater or equal 2
fi

#!/usr/bin/env bash
echo "Hello World"

chmod +x hello.sh
./hello.sh

echo 1
echo $? # 0: success
grep 1 /dev/null # fails
echo $? # 1: failure
exit 0 # exit a script with success value (no further processing of the script)
exit 1 # exit with failure (anything but 0 is a failure)

# info about this script
version="shell option parsing example 0.1"
# check for the kind of getopt
getopt -T > /dev/null
if [ $? -eq 4 ]; then
    # GNU enhanced getopt is available
    eval set -- `getopt --name $(basename $0) --long help,verbose,version,output: --options hvo: -- "$@"`
else
    # Original getopt is available
    eval set -- `getopt hvo: "$@"`
fi

# # actually parse the options
# PROGNAME=`basename $0`
# ARGS=`getopt --name "$PROGNAME" --long help,verbose,version,output: --options hvo: -- "$@"`
# if [ $? -ne 0 ]; then
#   exit 1
# fi
# eval set -- $ARGS

# default options
HELP=no
verbose=no
VERSION=no
OUTPUT=no

# check, if the default wisp exists and can be executed. If not, fall
# back to wisp.py (which might be in PATH).
if [ ! -x $WISP ]; then
    WISP="wisp.py"
fi

while [ $# -gt 0 ]; do
    case "$1" in
        -h | --help)        HELP=yes;;
        -o | --output)      OUTPUT="$2"; shift;;
        -v | --verbose)     VERBOSE=yes;;
        --version)          VERSION=yes;;
        --)              shift; break;;
    esac
    shift
done
# all other arguments stay in $@
<<using-options>>

# Provide help output

if [[ $HELP == "yes" ]]; then
    echo "$0 [-h] [-v] [-o FILE] [- | filename]
        Show commandline option parsing.

        -h | --help)        This help output.
        -o | --output)      Save the executed wisp code to this file.
        -v | --verbose)     Provide verbose output.
        --version)          Print the version string of this script.
"
    exit 0
fi

if [[ x"$VERSION" == x"yes" ]]; then
    echo "$version"
    exit 0 # script ends here
fi

if [[ ! x"$OUTPUT" == x"no" ]]; then
    echo writing to $OUTPUT
fi

# just output all other arguments
if [ $# -gt 0 ]; then
    echo $@
fi

prog [OPTIONAL_FLAG] [OPTIONAL_ARGUMENT VALUE] REQUIRED_ARGUMENT...
# ... means that you can specify something multiple times
# short and long options
prog [-h | --help] [-v | --verbose] [--version] [-f FILE | --file FILE] 
# concatenated short options
hg help [-ec] [THEMA] # hg help -e -c == -ec

prog --help # provide help output. Often also -h
prog --version # version of the program. Often also -v
prog --verbose # often to give more detailed information. Also --debug

echo 1 ; echo 2 ; echo 3 # sequential
echo 1 & echo 2 & echo 3 # backgrounding: possibly parallel

grep foo test.txt && echo foo is in test.txt # conditional: Only if grep is successful
grep foo test.txt || echo foo is not in test.txt # conditional: on failure

echo $((1+2)) # 3
a=2
b=3
echo $((a*b)) # 6
echo $((a**$(echo 3))) # 8

man [command]

info [topic]
info [topic subtopic]
# emacs: C-h i

function i()
{
    if [[ "$1" == "info" ]]; then
        info --usage -f info-stnd
    else
        # check for usage from fast info, if that fails check man and if that also fails, just get the regular info page.
        info --usage -f "$@" 2>/dev/null || man "$@" || info "$@"
    fi
}