Change the picture format

for i in *.jpg; do convert $i ${i%jpg}png; done

Export?

$ MYVAR=1729
$ export MYVAR=1729

The first definition creates a variable named MYVAR and assigns it the value 1729. This is a shell variable. The second definition with the export command is another way of defining a variable. It creates a variable named MYVAR, assigns it the value 1729, and marks it for export to all child processes created from that shell. This is an environment variable. Defining a Bash Variable With or Without ‘export’

When to use () vs. {} in bash?

Parentheses() cause the commands to be run in a subshell. Braces{} cause the commands to be grouped together but not in a subshell. When to use () vs. {} in bash? How to use double or single brackets, parentheses, curly braces

Manipulating Strings

String Length

${#string}

stringZ=abcABC123ABCabc
echo ${#stringZ}                 # 15

Substring Extraction

${string:position} Extracts substring from $string at $position. ${string:position:length} Extracts $length characters of substring from $string at $position.

stringZ=abcABC123ABCabc
#       0123456789.....
#       0-based indexing.
echo ${stringZ:0}                            # abcABC123ABCabc
echo ${stringZ:1}                            # bcABC123ABCabc
echo ${stringZ:7:3}                          # 23A

Delete string

${string%substring} Deletes shortest match of $substring from front of $string. ${string%%substring} Deletes longest match of $substring from front of $string.

# Make the volume of all mp3 files (having a space in the file name" by 12dB

#!/bin/bash
for f in *.mp3; do
    echo "$f" ....converting
    sox "$f" "${f%.mp3}"_12dB.mp3 gain -n -12
done

extBefore="JPG"
extAfter="PNG"

for f in *.${extBefore}; do
mv -- $f "${f%.${extBefore}}.${extAfter}"
done

Substring Replacement

${string/substring/replacement} Replace first match of $substring with $replacement. ${string//substring/replacement} Replace all matches of $substring with $replacement.

stringZ=abcABC123ABCabc
echo ${stringZ/abc/xyz}       # xyzABC123ABCabc
                              # Replaces first match of 'abc' with 'xyz'.
echo ${stringZ//abc/xyz}      # xyzABC123ABCxyz
                              # Replaces all matches of 'abc' with # 'xyz'.

Manipulating Strings

Caret(^) in shell script

Beginning of line ( ^ ) caret Symbol ^ matches the expression at the start of a line.

$ grep "^Nov 10" messages.1
Nov 10 01:12:55 gs123 ntpd[2241]: time reset +0.177479 s
Nov 10 01:17:17 gs123 ntpd[2241]: synchronized to LOCAL(0), stratum 10
Nov 10 01:18:49 gs123 ntpd[2241]: synchronized to 15.1.13.13, stratum 3

Exception in the character class If you want to search for all the characters except those in the square bracket, then use ^ (Caret) symbol as the first character after open square bracket. The following example searches for a line which does not start with the vowel letter from dictionary word file in linux. Here, the brackets means the characters in the bracket will be found individually.

grep -i  "^[^aeiou]" /usr/share/dict/linux.words

Regular Expressions in Grep Command