Bash Scripting

String Manipulations

• To replace all substrings by another, use the following syntax:

echo ${stringZ//abc/xyz}

This would replace all occurences of abc in stringZ by xyz. The following replaces only the first match:

echo ${stringZ/abc/xyz}

To replace something at the end of a string, use:

echo ${stringZ/%abc/XYZ}

Special Symbols

• "$@" expands to all command-line parameters.
• "\n" in a variable does not necessarily works as expected. Eg, no newline is created.
• \ before a newline actually escapes the newline. Thus, you can create multi-line strings or commands just by terminating a line with the \ symbol.
• Positional parameters can be accessed with $1, $2 etc. You only need brackets {} after the 9th parameter ("${10}").

Tests

• -n checks if a string is not empty, -z if it is empty.
• [ is not a keyword but a command (a program!). It is recommended to use [[ in tests which is a keyword.
• Note that [[ myVariable ]] will output true even if myVariable is equal to 0. The test must be explicit.

Quoting

• If you need to expand special characters such as *, you cannot quote.
• Use single quotes rather than double quotes, especially in sed. If you use double quotes, the \ itself won't be taken as a \ for escaping, thus causing problems.
• If you write \ and then a newline, the newline will be escaped which allows you to write multiline strings.

Expanding

• The shell expands stuff like aaa* as soon as it sees this expression. Thus if you define a custom function myFunc(), and call it like that:

myFunc stuff*

If there are two files stuff1 and stuff2 in the current directory, $1 and $2 will be set to stuff1 and stuff2. Even if they are quoted ("$1", "$2") since the expansion took place before.

Arrays

• The length of an array can be obtained via "${#array[@]}".

Arithmetic context

• When within double parenthesis (( )) or after the let keyword, Bash enters arithmetic context. You don't need to quote variables or precede them with a $ sign. Tests also work as expected, eg more like in their C counterpart. For example,

if (( myVariable )) 

will return false if myVariable is equal to 0.

• Arithmetic context also applies when in an array [].

Subshell

• To launch a command in a subshell, use (). It's usually used with $(), since that allows a variable to capture stdout of the launched process. Note that using backticks is deprecated.

Standard Input / Output

• "<<<" can be used to feed a string as standard input.

Command Line Utilities

• sdiff -s will generate a formatted output of the differences between two files. Very useful.

sed

• sed is a stream editor. It is extremely powerful and can do almost everything under the sun. It can:
  • delete a line with command d;
  • append with command a;
  • use multiline strings if needed, with the standard Bash mechanism;
  • use several replacements on one line, adding the option -e to all changes;