The alias will only be available in the shell where the alias command was issued.

To persist the alias consider putting it into your .bashrc

UNIX console programs have an input file and two output files (input and output streams, as well as devices, are treated as files by the OS.) These are typically the keyboard and screen, respectively, but any or all of them can be redirected to come from — or go to — a file or other program.

STDIN is standard input, and is how the program receives interactive input. STDIN is usually assigned file descriptor 0.

STDOUT is standard output. Whatever is emitted on STDOUT is considered the "result" of the program. STDOUT is usually assigned file descriptor 1.

STDERR is where error messages are displayed. Typically, when running a program from the console, STDERR is output on the screen and is indistinguishable from STDOUT. STDERR is usually assigned file descriptor 2.

The order of redirection is important

command > file 2>&1

Redirects both (STDOUT and STDERR) to the file.

command 2>&1 > file

Redirects only STDOUT, because the file descriptor 2 is redirected to the file pointed to by file descriptor 1 (which is not the file file yet when the statement is evaluated).

Each command in a pipeline has its own STDERR (and STDOUT) because each is a new process. This can create surprising results if you expect a redirect to affect the entire pipeline. For example this command (wrapped for legibility):

$ python -c 'import sys;print >> sys.stderr, "Python error!"' \
| cut -f1 2>> error.log

will print "Python error!" to the console rather than the log file. Instead, attach the error to the command you want to capture:

$ python -c 'import sys;print >> sys.stderr, "Python error!"' 2>> error.log \
| cut -f1 

There are many comparator parameters available in bash. Not all are yet listed here.

cat can read from both files and standard inputs and concatenates them to standard output

The [[ … ]] syntax surrounds bash built-in conditional expressions. Note that spaces are required on either side of the brackets.

Conditional expressions can use unary and binary operators to test properties of strings, integers and files. They can also use the logical operators &&, || and !.

• shift shifts the positional parameters to the left so that $2 becomes $1, $3 becomes $2 and so forth.
• "$@" is an array of all the positional parameters passed to the script/function.
• "$*" is an string composed of all the positional parameters passed to the script/function.

Process substitution is a form of redirection where the input or output of a process (some sequence of commands) appear as a temporary file.

A space (" ") is required between each term (or sign) of the expression. "1+2" won't work, but "1 + 2" will work.

A common mistake is to try to execute Windows end-line formatted \r\n script files on UNIX/Linux systems, in this case the used script interpreter in the shebang is:

/bin/bash\r

And is obliviously not found but can be hard to figure out.

Character Classes

Valid character classes for the [] glob are defined by the POSIX standard:

alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit

Inside [] more than one character class or range can be used, e.g.,

$ echo a[a-z[:blank:]0-9]*

will match any file that starts with an a and is followed by either a lowercase letter or a blank or a digit.

It should be kept in mind, though, that a [] glob can only be wholly negated and not only parts of it. The negating character must be the first character following the opening [, e.g., this expression matches all files that do not start with an a

$ echo [^a]*

The following does match all files that start with either a digit or a ^

$ echo [[:alpha:]^a]*

It does not match any file or folder that starts with with letter except an a because the ^ is interpreted as a literal ^.

Escaping glob characters

It is possible that a file or folder contains a glob character as part of its name. In this case a glob can be escaped with a preceding \ in order for a literal match. Another approach is to use double "" or single '' quotes to address the file. Bash does not process globs that are enclosed within "" or ''.

Difference to Regular Expressions

The most significant difference between globs and Regular Expressions is that a valid Regular Expressions requires a qualifier as well as a quantifier. A qualifier identifies what to match and a quantifier tells how often to match the qualifier. The equivalent RegEx to the * glob is .* where . stands for any character and * stands for zero or more matches of the previous character. The equivalent RegEx for the ? glob is .{1}. As before, the qualifier . matches any character and the {1} indicates to match the preceding qualifier exactly once. This should not be confused with the ? quantifier, which matches zero or once in a RegEx. The [] glob is can be used just the same in a RegEx, as long as it is followed by a mandatory quantifier.

Equivalent Regular Expressions

Glob	RegEx
*	.*
?	.
[]	[]

bind -P show all configured shortcuts.

• Word splitting is not performed during assignments e.g newvar=$var
• Word splitting is not performed in the [[ ... ]] construct
• Use double quotes on variables to prevent word splitting

Bash configuration file:

This file is sourced whenever a new interactive Bash shell is started.

In GNU/Linux systems it's generally the ~/.bashrc file; in Mac it's ~/.bash_profile or ~/.profile

Export:

The PATH variable must be exported once (It's done by default). Once it is exported it will remain exported and any changes made to it will be applied immediately.

Apply changes:

To apply changes to a Bash configuration file, you must reload that file in a terminal (source /path/to/bash_config_file)

Using printf -v foo '%(...)T' is identical to foo=$(date +'...') and saves a fork for the call to the external program date.

Login Shell

A login shell is one whose first character of argument zero is a -, or one started with the –login option. The Initialization is more comprehensive than in an normal interactive (sub) shell.

Interactive Shell

An interactive shell is one started without non-option arguments and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.

non-interactive Shell

A non-interactive Shell is a shell in which the user can not interact with the shell. As en example, a shell running a script is always a non-interactive shell. All the same, the script can still access its tty.

Configuring a login shell

On logging in:

If '/etc/profile' exists, then source it. 
If '~/.bash_profile' exists, then source it, 
else if '~/.bash_login' exists, then source it, 
else if '~/.profile' exists, then source it. 

For non-login interactive shells

On starting up:

If `~/.bashrc' exists, then source it.

For non-interactive shells

On starting up: If the environment variable ENV is non-null, expand the variable and source the file named by the value. If Bash is not started in Posix mode, it looks for BASH_ENV before ENV.

Other files of note are:

• /etc/profile, for system-wide (not user specific) initialization code.

• .bash_logout, triggered when logging out (think cleanup stuff)

• .inputrc, similar to .bashrc but for readline.