Fedora Linux: Bash Programming Basics

Debugging

Start up the shell with the -x option, which will run the entire script in debug mode. Traces of each command plus its arguments are printed to standard output after the commands have been expanded but before they are executed.

#!/bin/bash -x        #on first line

Variable

a=5
export b='whatever'
c="$a $b"
d='$a $e'                                    # no substitution
local e=56

Array

list=(/tmp/m*)
echo ${#list[@]}
echo ${list[0]} 
echo ${list[@]}
echo ${list[*]} # same as @

Conditionals

Empty or unset variable

if [[ ${b:-z} == z ]]; then 
    echo "variable is empty or unset"; 
fi

String

if [ "a" = "b" ]; then
  echo yes;
else                                         #elif
  echo no
fi

multiple condition

if [ "a" = "b" -a "c" == "c" ]; then       # string = or ==
  echo "and condition true";
else                                         #elif
  echo no
fi

if [ "a" = "b" -o "c" == "c" ]; then       # string = or ==
  echo "or condition true";
else                                         #elif
  echo no
fi

if [[ $a == "b" && $b == "c" ]]; then
   echo yes;
fi

[[ -f $NATION_FILE ]] || fail "No nation file";

Numbers

if [ 5 -eq $? ]; then                         # numbers
  echo yes
fi

commands

if grep a * >/dev/null; then 
  echo yes; 
fi

Loop

For loop

for each in $(ls); do
  echo $each
done

While loop

COUNTER=0
while [  $COUNTER -lt 10 ]; do
    echo The counter is $COUNTER
    COUNTER=$((COUNTER+1)) 
done

Util

COUNTER=20
until [  $COUNTER -lt 10 ]; do
     echo COUNTER $COUNTER
     let COUNTER-=1
done

Read Each Line in a Variable

files=$(ls -1)
echo "$files"      # one line
echo $files | while read -r line; do echo $line; done    #one line
echo "$files" | while read -r line; do echo $line; done  # per line
echo "$files"|xargs  echo      #one line
echo "$files"|xargs -L1 echo   # per line

Functions

myecho()
{
  echo $*  
  echo $@
}

func_parm()
{
  [ -z $1 ] || echo parm missing && exit 1
  echo parm
}

Signal Handling

handle_int()
{
  echo "Control C"
}

trap handle_int SIGINT

Manipulating Strings

http://tldp.org/LDP/abs/html/string-manipulation.html

${#String}
${string:position}
${string:position:length}
${string#substring}
${string##substring}

Redirection

   1>filename
      # Redirect stdout to file "filename."
   1>>filename
      # Redirect and append stdout to file "filename."
   2>filename
      # Redirect stderr to file "filename."
   2>>filename
      # Redirect and append stderr to file "filename."
   &>filename
      # Redirect both stdout and stderr to file "filename."
      # This operator is now functional, as of Bash 4, final release.

   M>N
     # "M" is a file descriptor, which defaults to 1, if not explicitly set.
     # "N" is a filename.
     # File descriptor "M" is redirect to file "N."
   M>&N
     # "M" is a file descriptor, which defaults to 1, if not set.
     # "N" is another file descriptor.

Operators