Linux: Basic Shell Commands

date: Show current date and time
date --rfc-3339=seconds: Show time stamp in this format: “yyyy-mm-dd hh:mm:ss-07:00” the last are time offset to UTC.
uptime: Show how long the system's been running.

source fname: Execute a file fname. source fname is equivalent to . fname
bash: Start a new bash. Ctrl+d to exit when done.
echo $PATH: View value of a environment variable PATH.
env: Show all environment variables
alias str=cmd_str;: Make str as shortcut for cmd_str. For example, alias l="ls -al --color"

Image Processing

cmd *.txt: A asterisk “*” means any character. *.txt means all files ending in “.txt”. Can be used for any command that take list of files or dir. See man 7 glob.
cmd1 | cmd2: Pass the output of cmd1 to the input of cmd2
cat fileName | cmd: Feed the content of fileName to the input of cmd
cmd > fileName: Write the output to file
cmd >> fileName: Append output to file
cat fileName1 fileName2 > newFileName: Join contents of fileName1 fileName2 to newFileName
cmd1; cmd2; …: Run several commands.
cmd1 && cmd2: Run cmd1, if success, then run cmd2 (otherwise stop.) (the && is a logical “and” operator. Unix commands returns 0 if success, else a integer error code.)
… `cmd` …: Generate the output of cmd and use it in your whole command. For example, ls -l `which more`
cmd &: Run the command cmd in background.
echo $?: Show exit status of previous command.