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Saturday, November 30, 2019
 
Shell Scripting: Few Tidbits #2

Shell Scripting Tidbits: #1

Extracting the filename and extension from an absolute path

eg.,

Input: abspath="/var/tmp/localuserinfo.out"

Filename with extension:

$ basename ${abspath}
localuserinfo.out

Extension:

$ basename ${abspath##*.}
        -OR-
$ echo ${abspath##*.}
out

${variable##pattern} trims the longest match from the beginning of a string. Likewise ${variable#pattern} trims the shortest match from the beginning of a string.

Filename without extension:

$ basename ${abspath%.*}
localuserinfo

${variable%pattern} trims the shortest match from the end of a string. Similarly ${variable%%pattern} trims the longest match from the end of a string.

Ref:

Replacing commas with white space in a string

eg.,

Input: dummystr="Python,C,Java"

To replace all matches:

$ echo ${dummystr//,/ }
Python C Java

To replace only the first match:

$ echo ${dummystr/,/ }
Python C,Java

The difference is the extra '/' in pattern in first expression. If pattern begins with '/', all matches of pattern are replaced with string. Otherwise, only the first match is replaced.

Ref: Bash Reference Manual -> Shell Parameter Expansion -> ${parameter/pattern/string}

Sending array as argument to a shell function

Expand array before passing as argument to a function in a shell script. Sending the array name only passes the first element of the array.

Since $@ holds all arguments passed to the function, access the array argument with the help of $@ in the function definition. Using variables $1, $2, .., $n gives access to individual elements in the array.

eg.,
$ cat numbers.sh 

#!/bin/bash

# print_array() .. prints all elements as expected
function print_array() {
 int_array=("$@")
 for elem in "${int_array[@]}"
 do
  echo $elem
 done
}

# print_array2() .. prints only one element .. first one in the array
function print_array2() {
 int_array=($1)
 for elem in "${int_array[@]}"
 do
  echo $elem
 done
}

numbers=(1 2 3 4)
echo "Attempting to print numbers array .."
print_array ${numbers[@]}
echo
echo "Attempt #2 with a different function .."
print_array2 ${numbers[@]}
echo
echo "Attempt #3 with only the array name as argument to print_array function .."
# following call prints only the first element of the array
print_array ${numbers}

$ ./numbers.sh 
Attempting to print numbers array ..
1
2
3
4

Attempt #2 with a different function ..
1

Attempt #3 with only the array name as argument to print_array function ..
1

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