Bash Scripting: Difference between revisions

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  echo ${stringZ/abc/xyz}
  echo ${stringZ/abc/xyz}
To replace something at the ''end'' of a string, use:
echo ${stringZ/%abc/XYZ}


== Special Symbols ==
== Special Symbols ==

Revision as of 16:28, 16 November 2007

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.

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.

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;