• ProgramarEnBash

Este documento es una traducción de Introduction to Bash Shell Scripting de Donald A Kassebaum, siéntase libre de adicionar más ejemplos o complementar con lo que considere de utilidad.

Introducción a la creación de guiones de Bash

Referencias

• Learning the Bash Shell, Cameron Newham & Bill Rosenblatt (O'Reilly & Associates, Inc)

• Linux in a Nutshell, Jessica Perry Hekman (O'Reilly & Associates, Inc)

• The UNIX Programming Environment, Brian W Kernigham & Rob Pike (Prenice Hall)

• Unix Power Tools, Jerry Peek, Tim O'Relly, & Mike Lookides (O'Reilly & Associates,Inc)

• Advance Bash Scripting Guide : Appendix B has some good Reference Cards

Comienzo

Obtención de Ayuda

Caracteres especiales

Redirección, tuberías y filtros

Comodines

Control de procesos

Expresiones Regulares

Definición

caracteres de patrones de texto y metacaracteres

A continuación algunos metacaracteres.

Comandos que usan expresiones regulares

awk

Pattern scanning and text processing language

ed

Line-oriented text editor

egrep

extended grep

emacs

Emacs full screen text editor

ex

Line-oriented text editor

expr

Command evaluates an expression

fgrep

Grep from patterns in a file

gawk

GNU pattern scanning and processing language

grep

Searches file for pattern (also see fgrep & egrep)

perl

Perl scripting

python

Python scripting

sed

Applies a set of user-specified editing command to a file

tcl

Tool command language

vi

Full screen text editor

Variables de Shell

Variables de shell embebidas

Variables de Ambiente

Environment variable are global to shell and subshells

Variables de usuario

Pueden ser en mayúsculas o minúsculas

Variables posicionales

• o $0 Name of function or script being called o $1 ... $9 Replace by arguments to shell or function o ${n} Replace by n-th arguments to shell or function required if number of argument is over 9

Variables especiales

• o $? Exit status or return code of last command o $# Number of arguments o $@ Argument 1 thru n with Input Field Separator o $* "$1" $2" ... $n o $! Process id of last background process o $$ Process id of shell running this script o $- The current shell flags

Algunos comandos útiles en los scripts

• basename Strip directory and option suffix
• declare Built-in command declares variable
• dirname Strip non-directory part
• echo Built-in command display message to standard out
• echo -n Built-in command display message to standard out without newline
• echo -e Builtin Command display message to standout with escape sequences
• enable Built-in command to enable/disable (-n) built-in commands
• env Built-in command displays environment variables
• eval Built-in command evaluate arguments before executing results
• exec Built-in command runs command as the shell process
• exit Built-in command exits shell
• expr Evaluates an expression and output its value
• false Built-in command always return false condition
• local Built-in command make variable local
• read Built-in command reads data into variable from standard in
• seq Print a sequence of numbers
  • o seq [OPTION]... LAST o seq [OPTION]... FIRST LAST o seq [OPTION]... FIRST INCREMENT LAST o -w equalize width by padding with leading zeroes
• sleep Command causes execution to stop for a specified number of seconds
• test Built-in command tests for various conditions, such as existence of a file, useful for controlling conditional script execution
• time Built-in command times commands
• times Print accumulated user and system times.
• true Built-in command always return true condition
• type Built-in command show what word is
• wait wait for the completion of background processing; is used to ensure that critical processing is complete before proceeding in a script

Coloreado de scripts

• tput sgr0 #Reset text attributes to normal without clearing screen
• Escape sequence to change colors #\e[${forground};${background}m example white on black sequence is \e[37;40m #Forground colors Backgrounfd Color black=30; bgblace=40 red=31; bgred=41 green=32; bggreen=42 yellow=33; bgyellow=43 blue=34; bgblue=44 magenta=35; bgmagenta=45 cyan=36; bgcyan=46 white=37; bgwhite=47
• Turn text attribute off \e[0m
• Change text attribute bold \e[1m
• Change text attribute underline \e[4m
• Change text attributer everse \e[7m

Funciones de shell y scripts

Funciones

• o Function must be sourced just like .bashrc o type function will list the function o function functionname { shell commands } o functionname () { shell commands } o Example - a helpful function function givehelp { exec $1 --help | more; }

Scripts

• o #!/bin/bash Script name run script is run if execute bit is set o #!/bin/bash -x As above, but script lines are displayed o bash -x script As above o /usr/bin/env bash Use the environment bash for the script

Opciones de depuración para los scripts

• o set -o OPTION Command line Action set -o noexec sh -n Don't run command, just check for syntax o set -o verbose sh -v Echo commands before running them o set -o xtrace sh -x Echo commands after running them o set +o OPTION Turns OPTION off

Operaciones con variables

Uso de variables

• o $var Value of variable var o "$var" Is null if variable is undefined avoids some shell syntax errors if variable is undefined o ${var} Value of variable var avoids confusion when concatenating with text o ${#var} Gives the length of the string contained in var

  o ${var:FIRST:N} Extract string from var starting at FIRST position and continuing for N-1 characters. Note FIRST starts at 0.

Paso de una variable a un programa o un script

• o echo $var | command

Arreglos

• o declare -a name Declare an array o name[index]=value Just assigning a value defines it o Index starts at zero o No maximum limit o Need not be contiguous

Establecer valores para las variables al ejecutar un comando

• o var=command var is set to output of command o var=$(command) same as above

Operaciones aritméticas

• o $(( expression )) Almost the same as 'expr expression' o $[ expression ] Same as above o ( expression ) Groups expression within $(( ... )) o expr expression Note must quote *

Operadores aritméticos

• o + add o - substract o * multiply o / divide o % remainder o # number conversion (Only in bash not expr)

Operaciones de prueba

• [ -option arg ] Same as 'test -option arg

• -option arg Same as 'test -option arg

• [ arg1 -option arg2 ] Same as 'test arg1 -option arg2'

• arg1 -option arg2 Same as 'test arg1 -option arg2'

• See test command for test complete list of options
  • o string1 = string2 String 1 equals string 2 o string1 !=string2 String 1 not equals string 2 o -n string String is not zero length o -z string String is zero length o -d FILE File is a directory o -e FILE File exists o -f FILE File exists and is a regular file o -r FILE File exists and is readable o -s FILE File exists and has length greater than zero o -w FILE File exists and is writable o -x FILE File exists and is executable o num1 -eq num2 Number 1 equals number 2 o num1 -ne num2 Number 1 not equals number 2 o num1 -lt num2 Number 1 less than number 2 o num1 -le num2 Number 1 less than or equals number 2 o num1 -gt num2 Number 1 greater than number 2 o num1 -ge num2 Number 1 greater than or equals number 2

Operaciones en cadenas

• ${varname:-word} Return var if exists and is not null, else word
• ${varname:+word} Return word if var exists and is not null, else null
• ${varname:?mess} Return var if exists and is not null, else display mess and return from script with error

Operaciones con patrones

• ${variable#pattern} If the pattern matches the beginning of the variable value, delete the shortest part that matches and return the rest
• ${variable##pattern} If the pattern matches the beginning the of variable value, delete the longest part that matches and return the rest
• ${variable%pattern} If the pattern matches the end of the variable value, delete the shortest part that matches and return the rest
• ${variable%%pattern} If the pattern matches the end of the variable value, delete the longest part that matches and return the rest

El versátil 'expr'

• expr string : regrep Anchored pattern match
• expr match string pattern Same as above
• expr substr string pos {len] Substring beginning at pos of length len
• expr length string Length of string

Control de flujo

if - Información general

• if command Test return code of command

  if command1 && command2 Test return code of command1 and command2 if command1 || command2 Test return code of command1 or command2 command can be condition i.e. [ $# -eq 0 ]

if/then/else or if/then/elif..

            if condition
              then statements...
              [else statements...]
            fi

            if condition
              then statements...
              [elif condition
                then statements...]
                [else] statements...]
            fi

Valor de retorno de una función

• return [ numeric expression or variable ] This is the return code

for

            for name [ in list ] do
              statements
            done

            for  variable = start to end do
              statements
            done

while/until

            while condition do
              statements
            done

            until condition do
              statements
            done

break/continue

• break [n] Break out of loop & select

• continue [n] Continue next iteration of loop

case

      case expression in
        pattern1[|pattern11] } statements ;;
        pattern2[|pattern21] } statements ;;
        ...
      esac

Interfaces de usuario

select

• select name [in list] do
  • statements that can use $name
  done

• Generate a menu of each item in the list formatted with a number for each choice
• Prompt the user for the number
• Store the selected choice in name. The number is stored in REPLY
• execute the statements in the do
• Repeat the process again

Opciones por la línea de comandos

• shift Shift 1st argument form the argument list
• getopts Used to process command line options
• OPTIND Variable contains number of options
• OPTARG Default variable for option
• Example while getopts ":ab:c" opt; do case $opt in a) process_option_a;; b) process_option_b $OPTARG is the option's argument;; c) process_option_c;; esac done shift $(($OPTIND - 1))

Operaciones de E/S en scripts

• printf Built-in command for formatted output
• read Built-in command to read one line into variable(s) read #Everything entered goes to REPLY read var #Everything is read into var read a b #Read 1st word in to a and rest into b read -t 300 var #Read with 300 second timeout

• Example how to use read from $file while read line; do do_something_with_line done <$file

Manejo de procesos

Señales

• Name Number Control Character

• o EXIT 0 Used to trap exiting script o HUP 1 Logout o INT 2 Control-C o QUIT 3 Control-\ o KILL 9 can not be ignored or trapped o TERM 15 Default kill o TSTP 24 Control-Z

Trampas

• o trap "" signal-list Ignore signal o trap "cmds" signal-list Execute commands if signal is caught o trap signal-list Reset signal to original condition o trap : signal-list (undocumented) ignore signal, pass to child Signal are normally not passed to subprocesses o Examples trap 'rm tmpfile; exit' 0 1 2 #remove tmpfile on exit, logout, interrupt trap "echo 'You hit Control-C'" INT while true ; do sleep 60 done o Example parent child process #!/bin/bash #parent echo parent running trap 'echo parent exiting; exit' 0 trap :2 child sleep 1000 #!/bin/bash #child echo child started. pid is $$ trap 'echo child got signal 2; exit' INT sleep 1000

Ejemplos de scripts o funciones útiles

# Function top5 Example how to set defaults # Usage top5 {n} #list n processes 
function top5 { 
  ps -ef | head -${1:-5} 
}

# Function hereis Example of HERE IS FILE and handling arguments 
# Usage hereis word1 word2 ... 
function hereis {
  for name in "$@" 
     do 
     cat <<MSG This is an example of an HERE IS FILE. One argument is ${name}. The date is `date`. 
MSG 
   done 
}

# Function pick Return selected items by user 
# Usage: .e.g var=`pick *` 
function pick { 
   for name in $@ ; do       #for each item in argument list 
       echo -n "$name (y/n/q)?" >/dev/tty #ask user to select 
       read ans #read answer from standard in 
       case $ans in #Check choices 
          y*) echo $name;; #selected 
          q*) break;; #skip rest of arguments 
          *) continue;; #skip item 
        esac 
     done 
}

# Function acal Display a nicer calendar 
# but will accept Alphabetic month 
function acal { 
    m="" 
    case $# in 
    0) cal; return;;        #no arguments 
    1) m=$1; y=`date +%Y`;; #1 argument 
    2) m=$1; y=$2;;         #2 arguments      
    esac

    case $m in 
    Jan*|jan* ) m=1;; 
    Feb*|feb* ) m=2;; 
    Mar*|mar* ) m=3;; 
    Apr*|apr* ) m=4;; 
    May|may } m=5;; 
    Jun*|jun* ) m=6;; 
    Jul*|jul* ) m=7;; 
    Aug*|aug* ) m=8;; 
    Sep*|sep* } m=9;; 
    Oct*|oct* ) m=10;; 
    Nov*|nov* ) m=11;; 
    Dec*|dec* ) m=12;; 
    [1-9]|1[0-2] ) ;; #numeric month 
    *)       ) y=$m; m="";; 
    esac 
    cal $m $y 
}

## Function selectex - Example select 
# 
function selectex () { 
   choices="/bin /usr /home" 
   select selection in $choices; do 
    if [ $selection ]; then 
      ls $selection 
      break 
    else 
      echo 'Invalid selection' 
    fi 
   done 
}

# Function fwhich Which command in $PATH is executed 
# 
function fwhich { 
   if [[ $# -eq 0 ]] ; 
     then cat << EndOfHelp 1>&2; return 2 
             Usage fwhich command #Example of parsing the $PATH 
                 Return 0 - found 
                 Return 1 - not found 
                 Return 2 - No arguments 
EndOfHelp 
    fi 
    for path in `echo $PATH | sed 's/^:/.:/ 
                                   s/::/.:/g 
                                   s/:$/:./ 
                                   s/:/ /g'`
    do 
      if [[ -x $path/$1 ]] ; # does executable file exists here? 
         then echo $path/$1 # found it 
         return 0 
      fi 
    done 
    return 1 # not found 
}

# Name: overwrite Copy standard input to output after EOF 
function overwrite { 
   if [[ $# -lt 2 ]] ; 
      then echo "Usage: overwrite file command [args]" 1>&2; return 2 
   fi

 file=$1; shift 
 new=/tmp/overwrite1$$; pld=/tmp/overwrite2$$ 
 trap 'rm -f $new $old; return 1' 1 2 15 # clean up files 
 "$@" > $new 
 if [[ $? -eq 0 ]] ; # collect output 
   then # command completed successfully 
     cp $file $old # save original file 
     trap '' 1 2 15 # we are committed; ignore signals 
     cp $new $file # copy new file into file 
     rm -f $new $old # remove temp files 
    else 
     echo "overwrite: $1 failed, $file unchanged" 1>$2 
     return 1 
    fi 
}

# Name: zgrep 
# Purpose: caseless grep of gzip files 
# Usage: zgrep text files.gz 
# 
function zgrep { 
    if [ $# -eq 0 ] ; then 
        echo "Usage: zgrep grep_text files.gz" 
        return 2 
     fi 
      text=$1 
      shift 
      while [ $# -gt 0 ]
        do 
           echo $1 gzip -cd $1 | grep -i $text 
           shift
        done
}

# Name: hgrep 
# Purpose: highlighting grep 
# Usage: hgrep pattern files 
# 
function hgrep { 
   if [ $# -lt 2 ] ; then 
      echo "Usage: hgrep pattern files" 
      return 2 
   fi 
     pattern=$1;shift 
     sep=$'\001' #note use of $' ' to create control characters 
     bold=$'\e[1m'; off=$'\e[0m' 
     underline=$'\e[4m'; reverse=$'\e[7m' #other choices of highlighting 
     sed -n "s${sep}${pattern}${sep}${reverse}&${off}${sep}gp" $* 
} 

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