====== Bash references and shortcuts ======
===== Restricted BASH shell =====
Starting BASH with ''-r'' or running ''rbash'' will start a restricted bash shell.
In this you cannot:-
* cd to change directory
* Set or reset the $PATH or $SHELL
* use absolute paths for commands (/path/to/command)
* import envoronment functions
* $SHELLOPTS not allowed
* redirecting output (no > >> | <> >& )
* use ''exec''
Don't rely on rbash for security on it's own, your login env needs to be set up correctly too.
===== Command Line arguments =====
Use these with:- ''$bash [options] [arguments] ''
-c cmd run cmd found from path search \\
-D print double quotes strings to STDOUT \\
-i run interactive mode
-r run rbash ([[linux:bashshortcut#restricted_bash_shell|Bash references and shortcuts]])
-s read in commands from STDIN - default mode
--help show help info and quit
--login behave as a login shell
--noediting don't use readline lib
--noprofile don't use any init files
--norc don't use ~/.bashrc
--posix follow POSIX standard
--rcfile - use this file not the usual ~/.bashrc
--restricted same as -r for rbash ([[linux:bashshortcut#restricted_bash_shell|Bash references and shortcuts]])
--verbose sets ''set -v'' for verbose
--version shown version to STDOUT
===== Starting BASH =====
- Interactive - runs /etc/profile, ~/.bashprofile and ~/.bash_login. Logout, if present runs ~/.bash_logout
- Non interactive. Command substitution is from $BASH_ENV
- Interactive sh login - runs /etc/profile and ~/.profile. Skipped if invoked with ''--noprofile''
- POSIX is started with --posix switch
===== Prompts =====
$ export PS1=">$ "
Primary prompt:- $PS1 \\
Secondary prompt:- $PS2 \\
\a ASCII BEL Oct 07
\d date WMD format
\e ASCII escape sequence
\h hostname to first '.'
\H full hostname
\l name of shell term device, eg ttyUSB0
\n newline
\r carriage return
\s shell name
\t time - 24hr HH:MM:SS
\T time - 12hr HH:MM:SS
\u username
\v bash version
\V bash version and patch level
\w working directory
\W base name of PWD
\! history number of command
\# command number
\$ a # for UID=0 (root) or a $ for regular user
\@ time in AM PM format
\\ backslash
\nnn Octal nnn character
\[ Start seq of non printing characters
\] end seq of non printing characters
Default PS1 id \s-\v\$ \\
$ export "PS1=\u@\H $> "
sysadmin@sysmgmt $>
===== BASH Defined variables =====
^ Variable ^ Value ^
| $n or $(n) | Argument positional variables |
| $@ | All positional variables |
| $# | Number of supplied parameters |
| $? | last return code |
| $$ | Process number of current shell |
| $BASH | file and path used to invoke BASH |
| $BASH_ENV | Path of startup file |
| $BASH_VERSION / [0], [1], [2] etc | Version of BASH and [0] is major and minor versions|
| $HOME | Used for ~ expansion |
| $HOSTNAME | Current host |
| $IFS | Imput File Seperator |
| $PATH | Search path for commands |
| $PS1 | Primary prompt - \s-\v\$ |
| $PS2 | Secondary prompt - > or # |
| $PS3 | Command prompt string #? |
| $PS4 | tracimg prompt (+) |
| $PWD | Current directory |
| $OLDPWD | Previous directory |
| $SHELL | Current Shell |
===== Variable Substitution =====
$name name variable
${name} braces delimit name
${name=word} Set name to word
${name?word} use name if set otherwise use word
${name+word} use word if name is set
${name[x]} use xth element in array name
${#name} length of variable name
${#name[*]} number of elements in name array
${name#pattern} remove shortest leading substringfrom name which matches pattern
${name##pattern} remove longest leading substringfrom name which matches pattern
${name%pattern} remove shortest trailing substringfrom name which matches pattern
${name%%pattern} remove longest trailing substringfrom name which matches pattern
${name/patt/string} replace value of name with first match of patt
FIXME - Every match
===== Command Substitution =====
This allows a command to be run in a subshell from within a script, the original format used backticks (`) to surround the command, the current way is to use $(command):-
lsoutput=`ls -1`
lsoutput=$(ls -1)
===== Bash Array =====
myarrayname=("Tom" "Dick" "Harry")
numberarray(100 200 300 400)
Accessing an array - array elements start at 0!
${ArrayName[subscript]}
echo "${myarrayname[2]}" # Harry
Number of elements:-
echo ${#myarrayname[@]} # 3
#!/bin/bash
city=("London" "Paris" "Berlin")
## number of elements in "$city" array
len=${#city[@]}
## Use bash for loop
for (( i=0; i<$len; i++ )); do echo "${city[$i]}" ; done
$ ./city.sh
London
Paris
Berlin
$
===== Aliases =====
Alises allow a long and frequently used command to be 'aliases' by a briefer one.
Lifted from a Ubuntu .bashrc:-
alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'
===== Arithmetic evaluations for expressions =====
Based on 'C' syntax, these use long integers. ''typeset -i'' will use just integers.
Evaluation is with several differnt syntax, the ''let'' built in, %%((....))%% and %%$((.....))%%
^ operator ^ explanation ^
| +, - | Unary add and subtract |
| !, ~ | Negation, locgical and bitwise |
| *, /, % | multiplication, division and modulus |
| ** | exponentiation |
| <, <=, >, => | greater than, less then or equals comparisons |
| =, != | equals and not equals |
| & | bitwise AND |
| %%|%% | bitwise OR |
| && | logical AND with short circuit |
| %%||%% | logical OR with short circuit |
===== Conditional Expressions =====
Tested with %%[[...]]%%.
^ expression ^ evaluation ^
| //string// | true if //string// is not NULL |
| -n //string// | true if //string// has a non-zero length |
| -z //string// | true if //string// has zero length |
| //string// == //pattern// | true if //string// equals //pattern// |
| //string// != //pattern// | true if //string// Not equals //pattern// |
| |
| -b //file// | true if //file// is a Block Device |
| -c //file// | true if //file// is a Character Device |
| -d //file// | true if //file// is a directory |
| -e //file// | true if //file// exists |
| -f //file// | true if //file// is a regular file |
| -h //file// | true if //file// is a symbolic link |
| -O //file// | true if owner is current UID |
| -r //file// | true if //file// is readable |
| -w //file// | true if //file// is writable |
| -x //file// | true if //file// is executable |
| |
| //file1// -nt //file2// | true if //file1// is newer than //file2// |
| //file1// -ot //file2// | true if //file1// is older than //file2// |
| //file1// -ef //file2// | true if //file1// is the same as //file2// |
| |
| //expr1// -eq //expr2// | true if //expr1// equals //expr2// |
| //expr1// -ne //expr2// | true if //expr1// not equal to //expr2// |
| //expr1// -lt //expr2// | true if //expr1// less than //expr2// |
| //expr1// -gt //expr2// | true if //expr1// greater than //expr2// |
| //expr1// -le //expr2// | true if //expr1// less than or equal to //expr2// |
| //expr1// -ge //expr2// | true if //expr1// greater than or equal to //expr2// |
| |
| (//expr//) | true if //expr// is true |
| !//expr// | true if //expr// is false |
||
| //expr1// && //expr2// | true if //expr1// AND //expr2// are true |
| //expr1// %%||%% //expr2// | true if //expr1// OR //expr2// are true |
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