Advanced Bash−Scripting Guide

# Piping an empty "echo" to awk gives it dummy input,
#+ and thus makes it unnecessary to supply a filename.
exit 0
Advanced Bash−Scripting Guide
9.2.2. Further Discussion
For more on string manipulation in scripts, refer to Section 9.3 and the relevant section of the expr command
listing. For script examples, see:
1. Example 12−6
2. Example 9−15
3. Example 9−16
4. Example 9−17
5. Example 9−19
9.3. Parameter Substitution
Manipulating and/or expanding variables
${parameter}
Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less
ambiguous ${parameter} form works.
May be used for concatenating variables with strings.
your_id=${USER}−on−${HOSTNAME}
echo "$your_id"
#
echo "Old \$PATH = $PATH"
PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script.
echo "New \$PATH = $PATH"
${parameter−default}, ${parameter:−default}
If parameter not set, use default.
echo ${username−`whoami`}
# Echoes the result of `whoami`, if variable $username is still unset.
${parameter−default} and ${parameter:−default} are almost
equivalent. The extra : makes a difference only when parameter has been declared,
but is null.
#!/bin/bash
username0=
# username0 has been declared, but is set to null.
echo "username0 = ${username0−`whoami`}"
# Will not echo.
echo "username1 = ${username1−`whoami`}"
# username1 has not been declared.
# Will echo.
Chapter 9. Variables Revisited 84