The eval command is a shell builtin command used to construct a command by concatenating arguments passed to eval. A concatenated command is further executed by shell and returns a result. If no arguments are given to eval, it returns 0.
The syntax of the eval command is as follows:
eval [arg …]
The following example shows the expansion of a variable to the name of another variable using eval:
$ name=foo $ foo="Welcome to foo world" $ echo $name foo $ new_name='$'$name #new_name just stores string value $foo $ echo $new_name $foo $ eval new_name='$'$name # eval processes $foo string into variable and prints # foo variable value Welcome to foo world
Another example where eval can be useful is as follows:
$ pipe="|" $ df $pipe wc # Will give error because df: '|': No such file or directory df: 'wc': No such file or directory $ eval df $pipe wc # eval executes it as shell command 12 73 705
Here, the df command shows a system disk's usage:
A shell script showing the use of eval is as follows: #!/bin/bash #Filename: eval.sh #Description: Evaluating string as a command using eval cmd="ls /usr" echo "Output of command $cmd -" eval $cmd #eval will treat content of cmd as shell command and execute it cmd1="ls /usr | wc -l" echo "Line count of /usr -" eval $cmd1 expression="expr 2 + 4 * 6" echo "Value of $expression" eval $expression
Running the script will give you the following result:
Output of command ls /usr - avr bin games include lib lib64 libexec local sbin share src tmp Line count of /usr - 12 Value of expr 2 + 4 * 6 26
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