Merge pull request #429 from andreas-hofmann/master

README.md: Added default value variable expansion example

README.md

@@ -139,7 +139,7 @@ Notes:
       # continue in original dir
 ```
 
-- In Bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a Bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`.
+- In Bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a Bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Using a default value if a variable is empty: `${name:-default}`. If you want to have an additional (optional) parameter added to the previous example, you can use something like `output_file=${2:-logfile}`. If $2 is omitted and thus empty, `output_file` will be set to `logfile`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`.
 
 - Brace expansion using `{`...`}` can reduce having to re-type similar text and automate combinations of items.  This is helpful in examples like `mv foo.{txt,pdf} some-dir` (which moves both files), `cp somefile{,.bak}` (which expands to `cp somefile somefile.bak`) or `mkdir -p test-{a,b,c}/subtest-{1,2,3}` (which expands all possible combinations and creates a directory tree).
 
@@ -148,6 +148,14 @@ Notes:
       diff /etc/hosts <(ssh somehost cat /etc/hosts)
 ```
 
+- When writing scripts you may want to put all of your code in curly braces:
+```sh
+{
+      # Your code here
+}
+```
+If the closing brace is missing, your script will be prevented from executing due to a syntax error. This makes sense when your script is going to be downloaded from the web, since it prevents partially downloaded scripts from executing.
+
 - Know about "here documents" in Bash, as in `cat <<EOF ...`.
 
 - In Bash, redirect both standard output and standard error via: `some-command >logfile 2>&1` or `some-command &>logfile`. Often, to ensure a command does not leave an open file handle to standard input, tying it to the terminal you are in, it is also good practice to add `</dev/null`.