cit160-notes/sep19-more-commands.md

September 19 - More Commands

• Simple Commands
• List of Commands
• Compound Commands
• Pipelining commands
• I/O (Input Output) Redirection
• Asynchronous Commands

Simple Commands

• name
• location?
• PATH=

Where are Commands?

$ ls      # ls is a command

$ /bin/ls # ls is a program in /bin

$ ./ls    # ls can be run from a relative path

Command in Path

$ echo $PATH

/bin /sbin /usr/bin /usr/sbin [output varies per user]

When a command is run, the system checks these locations for them.

Executing a list of commands

$ cmd1; cmd2 # do cmd1, then do cmd2

$ echo $?    # echo the status of cmd2 ($? is the status if the last command)

$ cmd1       # do cmd1

$ echo $?    # echo the exit status of cmd1

$ cmd1 && cmd2 # do cmd1, if cmd1 worked, do cmd2

Compound Commands

{ ls; exit 0; }

$ cd /tmp && { ls; exit 0; }

$ cd /tmp && ( ls; exit 0 ) # run ls and exit in subshell

( ls; exit $? ) # exit subshell the same exit status of ls

Math Commands

$ $((5+3)) # do 5+3

$ echo $?  # exit status is 0

$ $((3-3)) # do 3+3

$ echo $?  # exit status is 1 (because result is zero)

“ ” Commands

$ if [[ cd /tmp ]]; then