Introduction

${} (parameter substitution) is a useful way to manipulate strings in shell

it can print out the length of a string, just like len() function in python

string="hello"
echo ${#string}
→ 5

More:

• # : Delete the left part
• % : Delete the right part
• Single symbol means minimum matching
• Double symbol means maxmun matching

Extract from string

str="/dir1/dir2/my.file.txt"

# remove the first "/" and it\'s left part
echo ${str#*/}
→ dir1/dir2/my.file.txt

# remove last "/" and it\'s left part
echo ${str##*/}
→ my.file.txt

# remove the first "." and it\'s left part
echo ${str#*.}
→ file.txt

# remove last "." and it\'s left part
echo ${str##*.}
→ txt

echo ${str%/*}
→ /dir1/dir2
echo ${str%%/*}
→ (null)
echo ${str%.*}
→ /dir1/dir2/my.file
echo ${str%%.*}
→ /dir1/dir2/my

Based on character position

${str:m:n}

• m : start position
• n : end position

str="/dir1/dir2/my.file.txt"

echo ${str:0:5}
→ /dir1
echo ${str:5:5}
→ /dir2

Replace string

${str/A/B}
${str//A/B}

• replace A with B

str="/dir1/dir2/my.file.txt"

# replace the first "dir" with "path"
echo ${str/dir/path}
→ /path1/dir2/my.file.txt

# replace all "dir" with "path"
echo ${str//dir/path}
→ /path1/path2/my.file.txt

Advanced

str="/dir1/dir2/my.file.txt"

# replace the head "dir" with "path"
echo ${str/#dir/path}
→ /dir1/dir2/my.file.txt # No change
# reassign str
str="dir0/dir1/dir2/my.file.txt"
echo ${str/#dir/path}
→ path0/dir1/dir2/my.file.txt

# replace the ending "dir" with "path"
echo ${str/%dir/path}
→ dir0/dir1/dir2/my.file.txt
echo ${str/%txt/conf}
→ dir0/dir1/dir2/my.file.conf