Shell scripting

Shell scripts should have the file extension .sh.

You must mark a file with the eXecutable permission to run it with this command:

chmod +x file.sh

To run a script, the full path must be specified. If the script is in your current working directory, use this notation:

./file.sh

The first line should have a "shebang" (#!) which points to the interpreter.

#!/bin/bash

Alternatively, #!/bin/bash -e can be used to tell bash to quit on the first error. -x will cause bash to print commands and their arguments as they are executed.

Variables

"Naked variables" are initialized with the assignment operator.

var=hello

Several things to note. There are no spaces surrounding =. Simple strings don't need ".

Use the dollar sign ($) to reference variables.

echo $var
hello

Naming convention: environment variables and constants should be IN_ALL_CAPS. Temporary variables used in scripts should be in_lower_case. Space with underlines (don't use camelCase).

Common environment variables

You can display all the current environment variables with the command printenv.

• $USER username
• $HOME home folder
• $PWD current working directory

Special script variables

• $? exit status of last command. 0 indicates success, anything else (usually 1) is abnormal.
• $0 the invoked command itself.
• $1, $2, $3... line parameters following the command.
• $* every parameter as a single string, separated by spaces.
• $@ array of every parameter

Syntax

A comment starts with #.

A command's output can be considered a string in another command's arguments using a subshell.

rm $(ls | grep "*.docx$") # deletes all *.docx files from the current directory.

Pipes (I/O redirection) and Basic Logic

• command > FILE: the output from command is written to FILE.
  • command 2> FILE: stderr from command is written to FILE.
• command >> FILE: the output from command is appended to the end of FILE.
• command < FILE: the content of FILE is used as input for command.
• command1 && command2: command2 runs if command1 succeeds.
• command1 || command2: command2 runs if command1 fails.
• command1 | command2: output from command1 is piped to the input stream of command2.

Control Flow

if [ $var = "kitty" ]; then
  echo meow
fi

• http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html
• http://tldp.org/LDP/abs/html/comparison-ops.html
• http://tldp.org/LDP/abs/html/loops1.html