Answer:

The following script demonstrates how to check if a file exist, using shell (Bash) script

#!/bin/bash

if [ -e test.txt ]
then
  echo 'The file exists.'
else
  echo 'The file does not exist.'
fi

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Answer:

To echo string to the standard error (stderr), rather than the standard output (stdout), you can define the following function in your shell (put in your ~/.bashrc file)

# function echo2() { echo "$@" 1>&2; }

Then you can execute the command like:

# echo2 test

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Answer:

A simple for loop in Bash shell would like the following. You can use it wisely to skip a lot of repetitive tasks.

#!/bin/bash

for i in {1..10}
do
   echo "I am command No. $i"
done

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Answer:

In Bash script, if you want to print out unprintable characters such as tab, you need to use -e flag together with the echo command.

E.g.

echo -e "\tfoo\tbar"

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Answer:

In Linux, the most basic way to set an environment variable is use the "=" operator.

For example:

# foo=bar
# echo $foo
bar

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Answer:

Bash support multiple line string, e.g.

#!/bin/bash
sort  <<EOT
apple
orange
banna
EOT

When you execute the script,

# bash test.sh

It gives:

apple
banna
orange

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Answer:

You can perform syntax check on a bash script, without actually running it using the following command:

# bash -n script.sh

But if your script contain execution of other program, bash will not try to run it, even if it does not exist, error of this type will not be returned.

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Answer:

To unset an environment variable in bash, you can use the unset command.

Example:

# export foo=bar
# echo $foo
bar
# unset foo
# echo $foo

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Answer:

To check the current bash shell version

# bash -version
GNU bash, version 3.2.48(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2007 Free Software Foundation, Inc.

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Answer:

The following simple bash script will run the echo command, sleep 10 seconds repeatedly.

#!/bin/bash 

while [1];
do
    echo "Hello!";
    sleep 10;
done

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