Bash notes

From OriWiki

Contents 1 currently executed script path 2 debugging bash 3 redirection 4 set environment variable 4.1 export 5 Sample script 6 Submit script

currently executed script path

I wonder if there's a simpler way:

this_script_path=$(cd -P -- "$(dirname -- "$0")" && pwd -P)

echo $this_script_path

debugging bash

The following will cause a verbose execution of a script

bash -x check.sh

redirection

redirection of the error stream

$app 2> error.out

redirection of both stdout and stderr to a file

$app &> all.out 

see [1]

set environment variable

MYVAR=MY_VAR_VALUE

export MYVAR

export

export MYVAR='set and export in one command' 

exporting an environment variable make it accessible for applications that are invoked from the shell. For example, the following bash script:

#!/bin/bash
echo $MYVAR

Will print nothing if MYVAR was set but not exported

The same will happen with the following C++ application:

#include <iostream> 
#include <cstdlib>
int main() {
	char *myvar = std::getenv("MYVAR");
	std::cout << "MYVAR= " << myvar << '\n'; 
}

Sample script

#!/bin/sh

#this is a comment

#invoke a program 
#../../exercises/ex2/q3 & 

# usage of `expression`
grep `whoami` /etc/passwd

# arguments given to the script   
echo the number of arguments given to this script: $# 
echo the name of this script: $0
echo the first four arguments are: $1 $2 $3 $4 
echo

# arithmetic expressions 
echo the expression 2 + 3 equals: `expr 2 + 3` 
echo the expression 2 \* 3 equals: `expr 2 \* 3` 

#variables 
a=17  #note that a = 17 (with spaces) will not set a value to a 
b=`expr $a + 7`
echo -e a=$a "\n"b=$a+7 "\n"b=$b 

#if statement
if [ $# -ne 0 ]
then
    if [ $1 = hey ]
	then
	echo the first argument is hey
    else
	echo the first argument is not hey
    fi
fi

# while loop
a1=1
a2=1
while [ $a2 -lt 50 ] #the spaces are important 
do
  echo -n $a1 " " 
  a3=`expr $a1 + $a2`
  a1=$a2
  a2=$a3
done
echo

#until loop 
until [ 10 -gt $a1 ]
do
  echo -n $a1 ""
  a1=`expr $a1 - 1`
done
echo 
 
#for loop
names="yosi pesi moshe tzili gile"
c=1
for n in $names
do
  echo name $c: $n
  c=`expr $c + 1`
done

Submit script

#! /bin/sh
if [ $# -ne 1 ]
then
    echo -e "usage:\n" $0 "<ex num>"
    exit 1
fi

if !(test -r readme) # FILE exists and is readable
then
	echo "no readme file !"
	exit 1
fi
if !(test -r makefile)
then
	echo "no makefile file !"
	exit 1
fi

tmpName=tmp
count=1
while true
do
  tmpDir=$tmpName$count
  if !(test -r $tmpDir)
  then 
      break
  fi
  count=`expr $count + 1`
done

tarName=ex$1.tgz 

echo "----------making tar.."
tar czvf $tarName *.cpp *.h makefile readme    

echo "----------creating directory" $tmpDir ".."
mkdir $tmpDir

echo "----------copying tgz to" $tmpDir ".."
cp $tarName $tmpDir

cd $tmpDir
echo "----------extracting tgz.."
tar xzvf $tarName

echo "----------make.."
make

echo "----------run.." 
make run 
cd ..
rm -fr $tmpDir 
echo -e "\n\n ------All seems to be ok."
exit 0