Linux shell programming : string
Here is a simple script to calculate the sum of a number from 1 to 10:
#!/bin/sh
s="12345"
len=${#s}
echo "len=$len"
Output:
5
The following extracts $length characters of substring from $string at $position:
${string:position:length}
This extracts "substring" from $string at $position:
${string:position}
Example:
#!/bin/bash
s="0123456789"
len=${#s}
echo "s=$s"
echo "s:3 => ${s:3}"
echo "s:3:len-3 => ${s:3:len-3}"
echo "s:3:4 => ${s:3:4}"
Output:
s=0123456789 s:3 => 3456789 s:3:len-3 => 3456789 s:3:4 => 3456
Indexing from the right end of the string?
#!/bin/bash
s="0123456789"
echo "s=$s"
echo "s:-3 => ${s:-3 }"
echo "s: -3 => ${s: -3 }"
echo "s:(-3) => ${s:(-3)}"
Output:
s=0123456789 s:-3 => 0123456789 s: -3 => 789 s:(-3) => 789
Here we need either a space, ' ' or parentheses, '()' to "escape" the position parameter. Otherwise, it defaults to full string, as in ${position_parameter:-default}.
To reverse a string, we can do with a command rev:
$ echo abcde | rev
To do the same thing in shell script, we need to know how to extract a single character from a string:
#!/bin/bash
s="abcde"
len=${#s}
echo "1st string at index = 0, ${s:0:1}"
echo "last string at index = 0, ${s:len-1:1}"
Output:
1st string at index = 0, a last string at index = 0, e
Note that the substring syntax looks like this:
## syntax ##
${parameter:offset:length}
Now, let's run a loop - extract a character from the end:
#!/bin/bash
s="abcde"
len=${#s}
for ((i=len-1;i>=0;i--))
do
echo "index=$i, ${s:$i:1}"
done
Output:
index=4, e index=3, d index=2, c index=1, b index=0, a
It's time to reverse the string, constructing it one by one from the end:
#!/bin/bash
s="abcde"
len=${#s}
for ((i=len-1;i>=0;i--))
do
rev="$rev${s:$i:1}"
echo "index=$i, rev=$rev"
done
Output:
index=4, rev=e index=3, rev=ed index=2, rev=edc index=1, rev=edcb index=0, rev=edcba
How to merge strings?:
#!/bin/bash
s1="Hello"
s2="Strings"
echo "$s1 $s2"
arr=($s1 $s2)
for item in ${arr[@]}
do
new_s+="$item "
done
echo $new_s
Output:
Hello Strings Hello Strings
Linux - system, cmds & shell
- Linux Tips - links, vmstats, rsync
- Linux Tips 2 - ctrl a, curl r, tail -f, umask
- Linux - bash I
- Linux - bash II
- Linux - Uncompressing 7z file
- Linux - sed I (substitution: sed 's///', sed -i)
- Linux - sed II (file spacing, numbering, text conversion and substitution)
- Linux - sed III (selective printing of certain lines, selective definition of certain lines)
- Linux - 7 File types : Regular, Directory, Block file, Character device file, Pipe file, Symbolic link file, and Socket file
- Linux shell programming - introduction
- Linux shell programming - variables and functions (readonly, unset, and functions)
- Linux shell programming - special shell variables
- Linux shell programming : arrays - three different ways of declaring arrays & looping with $*/$@
- Linux shell programming : operations on array
- Linux shell programming : variables & commands substitution
- Linux shell programming : metacharacters & quotes
- Linux shell programming : input/output redirection & here document
- Linux shell programming : loop control - for, while, break, and break n
- Linux shell programming : string
- Linux shell programming : for-loop
- Linux shell programming : if/elif/else/fi
- Linux shell programming : Test
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