Apuntes Bash Guide for Beginners

6 01 2013

Both echo and printf are Bash built-in commands. The first always exits with a 0 status, and simply prints arguments followed by an end of line character on the standard output, while the latter allows for definition of a formatting string and gives a non-zero exit status code upon failure.

cat /usr/share/audio/at_your_service.au > /dev/audio

bash -x script_name.sh
The specified shell will start as a subshell of your current shell and execute the script. This is done when you want the script to start up with specific options or under specific conditions which are not specified in the script.

You can switch debugging mode on and off as many times as you want within the same script.
The table below gives an overview of other useful Bash options:

Table 2-1. Overview of set debugging options

Short notation Long notation Result
set -f set -o noglob Disable file name generation using metacharacters (globbing).
set -v set -o verbose Prints shell input lines as they are read.
set -x set -o xtrace Print command traces before executing command.

Brace expansion is a mechanism by which arbitrary strings may be generated. Patterns to be brace-expanded take the form of an optional PREAMBLE, followed by a series of comma-separated strings between a pair of braces, followed by an optional POSTSCRIPT. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right. Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved:

franky ~> echo sp{el,il,al}l
spell spill spall

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string “${” is not considered eligible for brace expansion.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma. Any incorrectly formed brace expansion is left unchanged.

Table 3-4. Arithmetic operators

Operator Meaning
VAR++ and VAR– variable post-increment and post-decrement
++VAR and –VAR variable pre-increment and pre-decrement
– and + unary minus and plus
! and ~ logical and bitwise negation
** exponentiation
*, / and % multiplication, division, remainder
+ and – addition, subtraction
<< and >> left and right bitwise shifts
<=, >=, < and > comparison operators
== and != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr ? expr : expr conditional evaluation
=, *=, /=, %=, +=, -=, <<=, >>=, &=, ^= and |= assignments
, separator between expressions

Table 4-1. Regular expression operators

Operator Effect
. Matches any single character.
? The preceding item is optional and will be matched, at most, once.
* The preceding item will be matched zero or more times.
+ The preceding item will be matched one or more times.
{N} The preceding item is matched exactly N times.
{N,} The preceding item is matched N or more times.
{N,M} The preceding item is matched at least N times, but not more than M times.
represents the range if it’s not first or last in a list or the ending point of a range in a list.
^ Matches the empty string at the beginning of a line; also represents the characters not in the range of a list.
$ Matches the empty string at the end of a line.
\b Matches the empty string at the edge of a word.
\B Matches the empty string provided it’s not at the edge of a word.
\< Match the empty string at the beginning of word.
\> Match the empty string at the end of word.

Contrary to [[[ prevents word splitting of variable values. So, if VAR="var with spaces", you do not need to double quote $VAR in a test – eventhough using quotes remains a good habit. Also,[[ prevents pathname expansion, so literal strings with wildcards do not try to expand to filenames. Using [[== and != interpret strings to the right as shell glob patterns to be matched against the value to the left, for instance: [[ "value" == val* ]].

case $space in
[1-6]*)
  Message="All is quiet."
  ;;
[7-8]*)
  Message="Start thinking about cleaning out some stuff.  There's a partition that is $space % full."
  ;;
9[1-8])
  Message="Better hurry with that new disk...  One partition is $space % full."
  ;;
99)
  Message="I'm drowning here!  There's a partition at $space %!"
  ;;
*)
  Message="I seem to be running with an nonexistent amount of disk space..."
  ;;
esac

File input and output are accomplished by integer handles that track all open files for a given process. These numeric values are known as file descriptors. The best known file descriptors are stdinstdout andstderr, with file descriptor numbers 0, 1 and 2, respectively. These numbers and respective devices are reserved. Bash can take TCP or UDP ports on networked hosts as file descriptors as well.

Note that each process has its own view of the files under /proc/self, as it is actually a symbolic link to /proc/<process_ID>.

The /dev/fd directory contains entries named 012, and so on. Opening the file /dev/fd/N is equivalent to duplicating file descriptor N. If your system provides /dev/stdin/dev/stdout and/dev/stderr, you will see that these are equivalent to /dev/fd/0/dev/fd/1 and /dev/fd/2, respectively. 


yum install $1 << CONFIRM
y
CONFIRM

The select construct allows easy menu generation. The syntax is quite similar to that of the for loop:
select WORD [in LIST]; do RESPECTIVE-COMMANDS; done

LIST is expanded, generating a list of items. The expansion is printed to standard error; each item is preceded by a number. If in LIST is not present, the positional parameters are printed, as if in $@ would have been specified. LIST is only printed once.

The shift command is one of the Bourne shell built-ins that comes with Bash. This command takes one argument, a number. The positional parameters are shifted to the left by this number, N. The positional parameters from N+1 to $# are renamed to variable names from $1 to $# - N+1.

 
 

Table 10-1. Options to the declare built-in

Option Meaning
-a Variable is an array.
-f Use function names only.
-i The variable is to be treated as an integer; arithmetic evaluation is performed when the variable is assigned a value (see Section 3.4.6).
-p Display the attributes and values of each variable. When -p is used, additional options are ignored.
-r Make variables read-only. These variables cannot then be assigned values by subsequent assignment statements, nor can they be unset.
-t Give each variable the trace attribute.
-x Mark each variable for export to subsequent commands via the environment.

The readonly built-in marks each specified variable as unchangeable. The syntax is:
readonly OPTION VARIABLE(s)

 

The unset built-in is used to destroy arrays or member variables of an array:

[bob in ~] unset ARRAY[1]

[bob in ~] echo ${ARRAY[*]}
one three four

[bob in ~] unset ARRAY

[bob in ~] echo ${ARRAY[*]}
<--no output-->

 

 

Table A-1. Common Shell Features

Command Meaning
> Redirect output
>> Append to file
< Redirect input
<< “Here” document (redirect input)
| Pipe output
& Run process in background.
; Separate commands on same line
* Match any character(s) in filename
? Match single character in filename
[ ] Match any characters enclosed
( ) Execute in subshell
` ` Substitute output of enclosed command
” “ Partial quote (allows variable and command expansion)
‘ ‘ Full quote (no expansion)
\ Quote following character
$var Use value for variable
$$ Process id
$0 Command name
$n nth argument (n from 0 to 9)
# Begin comment
bg Background execution
break Break from loop statements
cd Change directories
continue Resume a program loop
echo Display output
eval Evaluate arguments
exec Execute a new shell
fg Foreground execution
jobs Show active jobs
kill Terminate running jobs
newgrp Change to a new group
shift Shift positional parameters
stop Suspend a background job
suspend Suspend a foreground job
time Time a command
umask Set or list file permissions
unset Erase variable or function definitions
wait Wait for a background job to finish

 

Fuente: http://www.tldp.org/LDP/Bash-Beginners-Guide/html/Bash-Beginners-Guide.html

 

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