Using Command Prompt Instructions Effectively

Running Shell Scripts

When you run a shell script, you have two primary options:

1. ./script.sh: This command runs the script in a new shell. Any variables set or changed in the script do not affect the parent shell.
2. source script.sh: This command runs the script in the current shell (no new shell is created). Any variables set or changed in the script affect the parent shell.

Variables: Local vs. Environment

• Local Scope Variable: var=ali defines a variable that is only available within the current shell or script.
• Environment Variable: export name=ali sets an environment variable that is available to the current shell and any child processes or scripts.

Special Variables

• $@ vs. $*:
• $@ creates a collection of all the positional parameters, maintaining each parameter as a separate entity.
• $* creates a single string containing all the positional parameters.
• $0: This variable contains the name of the script or command being executed.
• $#: This variable contains the number of positional parameters passed to the script.

Script Arguments

When executing a script with arguments like ./script.sh val1 val2, val1 and val2 are passed as positional parameters $1 and $2 respectively.

Exit Status

• exit 0: Indicates successful execution.
• exit 1: Indicates an error occurred.

Testing with test

• if test 1 -lt 2 is equivalent to if [ 1 -lt 2 ]: Both test if 1 is less than 2.

File Tests

• -a FILE: True if file exists.
• -b FILE: True if file is a block special file.
• -c FILE: True if file is a character special file.
• -d FILE: True if file is a directory.
• -e FILE: True if file exists.
• -f FILE: True if file exists and is a regular file.
• -g FILE: True if file has the set-group-id flag set.
• -h FILE: True if file is a symbolic link.
• -L FILE: True if file is a symbolic link.
• -k FILE: True if file has its sticky bit set.
• -p FILE: True if file is a named pipe.
• -r FILE: True if file is readable by you.
• -s FILE: True if file exists and is not empty.
• -S FILE: True if file is a socket.
• -t FD: True if file descriptor FD is opened on a terminal.
• -u FILE: True if the file has the set-user-id flag set.
• -w FILE: True if the file is writable by you.
• -x FILE: True if the file is executable by you.
• -O FILE: True if the file is effectively owned by you.
• -G FILE: True if the file is effectively owned by your group.
• -N FILE: True if the file has been modified since it was last read.

Arithmetic Tests

Use one of the following operators:

• -eq: Equal
• -ne: Not equal
• -lt: Less than
• -le: Less than or equal
• -gt: Greater than
• -ge: Greater than or equal

Variable Testing

• if [[ $var ]] vs. if [ "$var" ]: Both check if the variable var is set and not empty.

Command Substitution

Store the result of a command in a variable using:

• var=$(id -u): Preferred modern syntax.
• var=\id -u“: Older syntax using backticks.