Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as
Bash, provides more protection from programmer mistakes than a compiled language.
Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output
of an echo command within the loop.
Example 14−2. Generating a variable from a loop
#!/bin/bash
# csubloop.sh: Setting a variable to the output of a loop.
variable1=`for i in 1 2 3 4 5
do
echo −n "$i"
done`
# The 'echo' command is critical
#+ to command substitution.
echo "variable1 = $variable1" # variable1 = 12345
i=0
variable2=`while [ "$i" −lt 10 ]
do
echo −n "$i"
# Again, the necessary 'echo'.
let "i += 1"
# Increment.
done`
echo "variable2 = $variable2" # variable2 = 0123456789
exit 0
Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of
writing a program or script that outputs to stdout (like a well−behaved UNIX tool should) and assigning
that output to a variable.
#include
/* "Hello, world." C program */
int main()
{
printf( "Hello, world." );
return (0);
}
bash$ gcc −o hello hello.c
#!/bin/bash
# hello.sh
greeting=`./hello`
echo $greeting
Chapter 14. Command Substitution 239