source script (or: . script)

Read and execute commands from filename in the current shell environment and return the exit status of the last command executed from filename If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged From man bash

exec command

replaces the shell. No new process is created (man bash) replaces the current (shell)-process with command (abs-guide)

command

when the shell encounters a command, it forks off a child process to actually execute the command (external commands only; from abs-guide) : Is this true? Isn't this command &?

eval command …

man bash: Command ist read and executed by the shell

For me useful if command contains spaces witch are protected by single quotes, eg wget -S --header'Content-Type: application/json'. Without eval, --header'Content-Type: and application/json' would be interpreted as 2 single arguments

#!/bin/bash
 
# first index is 0
app=( XMMS XINE XAWTV )
appcount=${#app[*]} # 3
appix=1
echo ${app[appix]} # XINE

while
    read -p 'Search term (English or German): ' inp
    test "$inp"
do
    query $inp
done
 
for (( ix=10 ; ix<21 ; ix++ )) ; do
    echo $ix
done
 
for arg in "$@"; do  # $@ sees arguments as separate words.
    echo $arg
done
 
# Print each word (separated by whitespace) in a textfile into one line (tested):
for token in $(cat textfile); do
    echo $token
done
 
# Loop through an array...
quote=('Die Mutter' 'der Dummen' 'ist immer' schwanger)
 
# without index:
for token in "${quote[@]}"; do
    echo $token
done
 
# with index:
for ((i=0; i<${#quote[@]}; i++)); do
    echo "$i: ${quote[i]}"
done

$meli is a shortcut for ${meli}

# get a value from a variable (meli) over a 'pointer' (who): (from abs-guide)
meli=Melanie
who=meli    # who is used as pointer to meli
adr=${!who} # now $adr contains Melanie
 
# set a value into a variable (meli) over a 'pointer' (who): (tried myself)
meli=""
who=meli
eval $(echo $who)=Melanie # now $meli contains Melanie

$# # number of command line arguments
$* # Arguments as one word
$@ # Arguments as separate words
$$ # Pid of script
$? # return value of command (& function?)
$0 # full name of script
$1 # first argument of script

Following a shift , the $@ holds the remaining command-line parameters, lacking the previous $1, which was lost¹⁾.

aVar="Otto"
echo -e "$aVar\nnext line"
echo -e '$aVar\nnext line'

Result:

Otto
next line
$aVar
next line

This means:

• Backslashes are always interpreted
• Variables are ony substituted between double quotes

query() {
    echo $*; echo $@; echo $1
}
 
query # call the function

Example:

abort() {
    echo "ERROR: $1!" >&2
    if [ "$2" ]; then errcode=$2 ; else errcode=1 ; fi
    exit $errcode
}
# usage:
[ -d "$SOURCE" ] || abort "Source \"$SOURCE\" is not a directory"

Be aware of this:

if true;  then echo "true=$?"; else echo "false=$?"; fi # prints true=0  !
if false; then echo "true=$?"; else echo "false=$?"; fi # prints false=1 !

Examples:

if [ -d $arg ]; then # a test (see below); mind the spaces!
    ...
elif ! command; then
    ...
elif [ "$a" == "$b" ] && [ "c" != "d" ]; then # also ||
    ...
elif [ ! "$aString" ]; then # true if length==0 or not specified (tested)
    ...
fi

-d file True if file exists and is a directory.
-e file True if file exists.
-f file True if file exists and is a regular file.
-h file True if file exists and is a symbolic link.
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
# see also "man bash" -> "Conditional expressions"
# Example:
if [ ! -r "$sorcefile" ]; then
    echo "Error: Cant read $sorcefile!"
    exit 2
fi

command > file.txt   # stdout 2 file
command 2> file.txt  # stderr 2 file
command 1>&2         # stdout 2 stderr
command >&2      # stdout 2 stderr ?
command 2>&1         # stderr 2 stdout
command &> /dev/null # stderr & stdout 2 file (suppress output here)

command
if [ $? -ne 0 ]; then
    echo "got an error!"
fi
#or:
command
if [ $? -eq 0 ]; then
    echo "command was successful"
fi