Bash references and shortcuts

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.

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

1. Interactive - runs /etc/profile, ~/.bashprofile and ~/.bash_login. Logout, if present runs ~/.bash_logout
2. Non interactive. Command substitution is from $BASH_ENV
3. Interactive sh login - runs /etc/profile and ~/.profile. Skipped if invoked with –noprofile
4. POSIX is started with –posix switch

$ 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 $>

$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

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)

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

$

Alises allow a long and frequently used command to be 'aliases' by a briefer one.

Lifted from a Ubuntu .bashrc:-

aliases.txt

alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'

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 $((.....))

Tested with [[...]].

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