Answer:

To trap your script before exit, you can use the trap in bash, e.g.

#!/bin/bash

set -eu

function bye {
  echo "Good bye"
}

trap bye EXIT
sleep 1000 
# Quit the script by pressing Ctrl+C

Answer:

You should always start your bash shell script with the line set -eu

The meaning is:

1. -e: cause the script to exit if an error
2. -u: cause the script to exit if any variable is not set

Answer:

Assume you have a CSV file like the following (only contain number)

1,2,3
2,3,4
3,4,5
...

How do you sum the nth column of this file? It is easy with the use of awk

e.g. Assume you want to sum the 2nd column:

# awk -F "," '{ sum += $2 } END { print sum }' test.txt

Answer:

It is not uncommon that people have abused the command `cat` for many purpose, e.g. file I/O redirection

cat data.txt | sed 's/foo/bar/s'

You can avoid the cat command by the following commands:

data.txt < sed 's/foo/bar/s'
sed 's/foo/bar/s' < data.txt

Both directions are okay.

In fact, when you better understand the usage of some commands, you might find out that redirection is completely not needed.

e.g.

sed 's/foo/bar/s' data.txt

Answer:

To echo single quote (using single quote only), you can try

# echo foo
foo
# echo \'''foo
'foo

Answer:

We have learned in the past on how to show the exit status of a given command.

But is it possible ignore the error and return normal exit code even the command has failed?

Yes, pipe "|| true" to the end of command, e.g.

# foo || true
-bash: foo: command not found
# echo $?
0

As you can see even the command foo was not found, our last exit code is still zero.

Answer:

If you want to Bash script's for loop to split on non white-space characters, you can change the IFS, e.g.

#!/bin/bash
IFS='-'

list="a-b-c-d-e"

for c in $list; do
    echo $c
done

The above script will split on the character '-'

E.g.

# ./test.sh

a
b
c
d
e

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

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

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