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Unix Programming and Regular Expressions Workshop - Command Process Substitution

Sections:

Command Substitution the old way

Command substitution will take the text between `...` and run it as a command and then place the results back into the surrounding command

You will need to escape double quotes "

Nesting commands is difficult with the old style command substitution

You don't need to quote if you are assigning a variable

Command Substitution the new way

You can use $( ... ) to do command substitution

This has been standardized by POSIX

It is available in both bash and zsh

Advantages of Newer Command Substitution:

  • Any matching delimiters will demarcate nested instances
  • There is no need to escape nested quotes or delimiters
  • It is much easier to read and to maintain then the old styles

Command Substitution Examples

bash ; set -x

Turn on execution tracing

printf "%s marcel\n" `echo hello`

Notice that first echo hello was executed and was provided as an argument to printf

myname=`echo Jean-Marcel Belmont`
echo $myname

Here we use backticks to do command substitution and then print the variable

echo first `echo second \`echo third\` fourth` fifth

Notice here that we had to escape backticks when there are nested in other backticks

printf "%s\n" "[`echo PATH is $PATH`]"

Here we don't have to escape within double quotes

echo "hey +`echo man -\`echo \"a quote" here\`- annotate`+ what?"

Here we show a very unwield example to illustrate issues with backticks

Using the New Way

printf "%s Marcel\n" $(echo Hello)

Here is a simple example

echo first $(echo second $(echo third) fourth) fifth
echo "hey +$(echo man -$(echo "a quote" here)- annotate)+ what?"

Here is the unwieldy example using new style, cleaner right!

Expr Command Description

The expr utility evaluates expression and writes the result on standard output.

The expr command is mostly used for simple integer arithmetic but can be used for comparisons and Basic Regular Expression matching

The expr command was designed to be used inside command substutition

Replace expr command with the following:

  • You should prefer arithmetic substitution with $(( ...))

  • test [[]] command

  • sed

  • grep

  • shell pattern matching

Expr Command Examples

i=0 sum=0
while [ $i -lt 15 ]
do
        echo i is $i
        i=`expr $i + 5`
        anotherSum=$(expr $i + 5)
done

Here we assign 2 variables and use a while loop and increment i and anotherSum

Process Substitution Description

Process Substitution

In computing, process substitution is a form of inter-process communication that allows the input or output of a command to appear as a file. The command is substituted in-line, where a file name would normally occur, by the command shell. This allows programs that normally only accept files to directly read from or write to another program.

By using process substitution you can create nonlinear pipelines:

  • There can be multiple process streams that can provide input to one command
  • One comand can send output to multiple process streams
    • The tee command is good for process substitution

Process Substitution Syntax

  • A command <(a | b | ...) to be read from a pipeline

    • diff <(sort file1) <(sort file2)

  • A command >(d | e | ...) to write to a pipeline

tee >(sort | uniq > sorted.dat) \
>(do_something)
> myFile
diff -u <(sort scripts/command-process-substitution/file1) <(sort scripts/command-process-substitution/file2)

Here we sort 2 files and pass it to diff command as process substitution

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