Articles in Series "Shells and related Scripting Languages"

Bourne to Bourne Again Shell Forward Compatibility

20. August 2023 in GNU/Linux, Programmierung.

Introduction

In this article i try to find out, if Bourne Shell scripts are runnable in Bourne Again Shell without modification. If not, i advice on how to modify the code so that it runs on both Shells.

An interpreter for some variant of Bourne Shell is available as an executable /bin/sh on most Linux and UNIX systems. Writing Bourne Shell script has the possible advantage that such a script can work on all these systems with no changes to the code; be it Linux, AIX, FreeBSD, initramfs or a string passed to a C library call system(3), expressions such as

  • echo hello | wc
  • my_pid=$$

or

  • exit 1

would always work.

Recently, i used GNU Bourne Again Shell for scripting, and i wondered if all of my longstandingly Bourne Shell established practices were seamlessly portable. So i did some research, and in this article i highlight some code constructs i found that work in Bourne Shell and do not work as expected in Bourne Again Shell. Code currently interpreted by /bin/sh and containing such constructs must be reviewed if will be interpreted by /bin/bash.

As it stands, i will focus on reserved words and built-ins, brace expansion and pipeline subprocesses.

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Print XDG Desktop Definition for Application

3. November 2024 in Administration, GNU/Linux.

Update Nov 3 2024:

For an application given by „application name“ or „executable name“, output the corresponding .desktop file, if any:

#!/bin/sh
IFS=":"
xdg_data_dirs=${XDG_DATA_DIRS:-/usr/local/share:/usr/share}
search=$1

for i in $xdg_data_dirs ; do
  a="$i/applications"

  [ -d $a ] && for d in "$a"/*.desktop ; do
    grep -q -e "^Name=.*$search" -e "^Exec=.*$search" "$d" && {
      echo "# $d:"
      grep -Ev '^(Comment|GenericName|Keywords|Name\[)' "$d"
    }
  done
done

To try this code out, save it to /usr/local/bin/xdg-desktop-search, make it executable and test it, for example, as follows:

xdg-desktop-search gnome-terminal

Using sed or awk to ensure a specific last Line in a Text

20. August 2023 in GNU/Linux, Programmierung.

Given a file containing bytes of text with lines separated by the newline character (\n), one of these lines can be said to be „the last line of the file“; it is a sequence of bytes occurring in the file, for which holds:

  • The sequence contains no newline character, and
  • the sequence is followed by at most one newline character and no other bytes.

The task at hand is, using shell utilities, to write a procedure that makes sure that a given file contains a last line that contains a desired sequence of text characters.

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Make a Bourne Again Shell Script Log its Output to a File

23. November 2024 in Administration, GNU/Linux, Programmierung.

The Bourne Again Shell script presented in this article demonstrates techniques related to capturing and logging output (standard output and standard error stream) of a script into a log file while also delivering it to the regular output destinations (for example the terminal or whatever the caller has chosen to redirect to).

If you are just interested in the Bourne Again Shell code example, click here.

In the following, some questions are addressed:

  • How to „clone“ standard output and standard error stream of a shell script to a single log file?
  • Are the messages from both streams interleaved into a single file in the appropriate order?
  • Can a script perform output cloning for some portions of its procedure while skipping it for others?
  • Are there specific requirements for output that is to be cloned to a file?
  • Are there best practices when a script modifies its own output redirections?

The example provided below has been tested on GNU/Linux using GNU „bash“ version 5 and „tee“ version 8.32 from GNU coreutils. It uses the Bash-only feature of process substitution. An implementation in POSIX Shell (possibly using only POSIX tools) would be more difficult.

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Maintaining Multi-Line „stat“ Formats using Bourne Again Shell

7. November 2024 in GNU/Linux, Programmierung.

The stat command from GNU core utilities features not only a --format FORMAT option but also a --printf FORMAT one, the difference being that the latter allows for backslash escapes such as \n.

This allows for custom per-file report formats containing newlines, for example:

stat --printf 'Name: %n\nSize: %s Bytes\n' /etc/passwd

If the format string becomes more complex, the command line soon becomes unwieldy, such as:

stat --printf 'Name: %n\nOwner ID: %u\nSize: %s Bytes\nLast accessed: %x\n' /etc/passwd

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Print all indented Lines following a non-indented Line

29. April 2024 in GNU/Linux, Programmierung.

Some configuration and output text formats contain sections like the following:

foo:
   value1
   value2
bar:
   value 3

In this article, two scripts are presented that print all consecutive indented lines that follow a non-indented line that matches a search pattern given by a regular expression.

This means, given the single argument foo and the standard input above, the scripts should

  • determine the line that matches foo and
  • print the following two lines, but no other lines.

Also, the indentation of the printed lines should be removed.

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An Introduction to Programmable Completion in Bash

20. Mai 2025 in Administration, GNU/Linux, Programmierung.

I am using the terminal a lot on Linux. For my project „pulseaudio-tcp“ (see here for more information) i wanted to have more comfortable command line completion in Bourne Again Shell („Bash“). After having typed in the command name pulseaudio-tcp, when pressing the Tabulator key, i would like to see a list of all possible arguments to that command.

In this article i demonstrate how to leverage the „Programmable Completion“ feature of Bash for such purposes. A reasonably recent version of Bash (4.2 or later) is assumed. You should have a basic understanding of control flow, variables, functions and arrays in Bash. In the article, we will make use of some advanced Bash features such as array expansion with pattern matching; these will be explained „on the go“ as required.

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