Put a.txt -file to the end of b.txt -file

In a bash script such append.sh file, write the following code: 

#!/bin/bash
A=/root/tmp/test/a.txt
B=/root/tmp/test/b.txt
cat $A >> $B

The >> redirects the output of the cat command (which output file A) to the file B. But instead of overwriting contents of B, it appends to it. If you use a single >, it will instead overwrite any previous content in B.

Check if a program exists from a bash script

$ command -v foo >/dev/null 2>&1 || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

OR 
$ type foo >/dev/null 2>&1 || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

OR 
$ hash foo 2>/dev/null || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

Resize a list of images in line command

for file in *.jpg; do 

  convert -resize 800x600 -- "$file" "${file%%.jpg}-resized.jpg"; 

done

OR 

ls *.jpg|sed -e 's/\..*//'|xargs -I X convert X.jpg whatever-options X-resized.jpg 

# .jpg files, 10% size, no rename, it will overwrite the old pictures
#!/bin/bash
for i in *.jpg; do convert $i -resize 10% $(basename $i .jpg).jpg; done

How to output MySQL query results in csv format

SELECT 'Order Id', 'Product Name', 'Quantity'

UNION

(

SELECT order_id,product_name,qty
FROM orders
INTO OUTFILE 'C:\\tmp\\orders.csv'
FIELDS TERMINATED BY ','
ENCLOSED BY '"'
LINES TERMINATED BY '\n'

)

How to do something with bash when a text line appear to a file

Use command

tail -f file.log | grep --line-buffered "my pattern" | while read line
do
  echo $line
done

The --line-buffered is the key here, otherwise the read will fail.

Delete all but the 4 newest directories

ls -atrd */ | head --lines=-4 | xargs rm -rf

Bash One Liner: copy template_*.txt to foo_*.txt

Say I have three files (template_*.txt):

• template_x.txt
• template_y.txt
• template_z.txt

I want to copy them to three new files (foo_*.txt).

• foo_x.txt
• foo_y.txt
• foo_z.txt

for f in template_*.txt; do cp $f foo_${f#template_}; done

or

for file in template_*.txt ; do 

  cp $file `echo $file | sed 's/template_\(.*\)/foo_\1/'` ;  

done