[Chapter 45] 45.25 Shell Scripts On-the-Fly from Standard Input
Author: Jerry Peek, Tim O'Reilly & Mike Loukides
Date: 1998-08-04T21:54:55Z

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| Chapter 45 Shell Programming for the Initiated |

## 45.25 Shell Scripts On-the-Fly from Standard Input

Warning! | The shell can read commands from its standard input or from a file. To run a series of commands that can change, you may want to use one program to create the command lines automatically - and pipe that program's output to a shell, which will run those "automatic" commands.

Here's an example. [3] You want to copy files from a subdirectory and all its subdirectories into a single directory. The filenames in the destination directory can't conflict; no two files can have the same name. An easy way to name the copies is to replace each slash (`/`) in the file's relative pathname with a minus sign (`-`). [4] For instance, the file named lib/glob/aprog.c would be copied to a file named lib-glob-aprog.c. You can use sed (34.1) to convert the filenames and output cp commands like these:

> [3] This isn't recommended for systems with a 14-character filename limit.

> [4] A replacement like CTRL-a would make unique filenames (but ones that are harder to type).

>     cp from/lib/glob/aprog.c to/lib-glob-aprog.c
>     cp from/lib/glob/aprog.h to/lib-glob-aprog.h
>         ...

However, an even better solution can be developed using nawk (33.12). The following example uses find (17.1) to make a list of pathnames, one per line, in and below the copyfrom directory. Next it runs nawk to create the destination file pathnames (like `to``/lib-glob-aprog.c`) and write the completed command lines to the standard output. The shell reads the command lines from its standard input, through the pipe.

This example is in a script file because it's a little long to type at a prompt. But you can type commands like these at a prompt, too, if you want to:

>     #!/bin/sh
>     find copyfrom -type f -print |
>     nawk '{ out = $0
>     gsub("/", "-", out)
>     sub("^copyfrom-", "copyto/", out)
>     print "cp", $0, out }' |
>     sh

If you change the last line to `sh -v`, the shell's verbose option (46.1) will show each command line before executing it. If the last line has `sh -e`, the shell will quit immediately after any command returns a non-zero exit status (44.7)- that might happen, for instance, if the disk fills up and cp can't make the copy.

- JP

45.24 A Shell Can Read a Script from its Standard Input, But... |  | 45.26 Quoted hereis Document Terminators: sh vs. csh