last revised 9 May 2009
| CSC 161 | Grinnell College | Spring, 2010 |
| Imperative Problem Solving and Data Structures | ||
The Bash shell allows users to interact with the computer through a terminal window, organizing commands that tie various programs together. First developed by Brian Fox in 1987, the Bash shell seeks to incorporate numerous features from earlier shells for the Unix operating system. In particular, the Bash shell draws upon the Bourne shell (developed by Ken Thompson and then Stephen Bourne around 1977), the Korn shell (developed by David Korn in the early 1980s), and the C shell (developed by Bill Joy, based on Thompson's earlier shells).
Within a terminal window, you already have used several commands within the Bash shell. For example, the lab on Basic Linux Commands discussed Bash commands, such as:
In this lab, we consider several additional commands, and we explore how various capabilities can be packaged to accomplish various tasks. The approach is not unlike programming in C (or Scheme), although the programming elements often are entire programs.
Bash includes a huge range of commands. Here are a few more useful capabilities:
| Command | Description | Example |
|---|---|---|
| cal | display a calendar for a given year | cal 2010 |
| date | print the current time and date in various formats many options documented with man date | date +'%I:%M %p on %A, %B %d, %Y' |
| diff | display differences in lines between two files | diff file1 file2 |
| echo | print specified text | echo hello world! |
| env | print the environmental variables currently set | env |
| grep | scan a file or other input for a specified text | env | grep home |
| hostname | return name of current workstation | hostname |
| sort | sort lines of a file | sort -n -k 12 (sort by numbers in column 12) |
| users | simple list of users | users |
| who | print list of users currently logged in | who and who -a |
| whoami | print the username of the person logged in | whoami |
| ypcat password | print user information from the password file | ypcat password | grep walker |
More information on each of these commands may be found with the man facility (e.g., man date). Beyond this basic documentation included here an in previous labs, many resources are available on line. Here are a few places to begin:
In addition to basic commands, the Bash shell includes capabilities for conditionals (if) and loops (for, while). The long history of Bash is particularly evident in the syntax allowed for if expressions. Bash allows interprets each syntax properly — but do not try to mix and match the different versions. The following examples illustrate acceptable Bash syntax, following two different ancestors.
| Task | Bourne shell style | Korn shell style |
|---|---|---|
| numeric test a > 0 |
if [ $a -gt \0 ]; then echo positive fi |
if (( $a > 0 )); then echo positive fi |
| numeric test a ≤ 0 |
if [ $a -le 0 ]; then echo non-positive else echo positive fi |
if (( $a <= 0 )); then echo non-positive else echo positive fi |
| numeric test a < 0 and a == 0 |
if [ $a -lt 0 ]; then echo negative elif [ $a -eq 0 ]; then echo zero else echo positive fi |
if (( $a <= 0 )); then echo non-positive elif (( $a == 0 )); then echo zero else echo positive fi |
| numeric test 0 ≤ a &le 10 |
if [ 0 -le $a -a $a -le 10 ]; then echo between 0 and 10 else echo not between 0 and 10 fi |
if (( (0 <= $a) && ($a <= 10) )); then echo between 0 and 10 else echo not between 0 and 10 fi |
For reference, all of these code segments are available in the Bash shell bash-example-1
This document is available on the World Wide Web as
http://www.cs.grinnell.edu/~walker/courses/161.sp10/readings/reading-linux-scripts.shtml
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created 21 April 2008 last revised 9 May 2009 |
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| For more information, please contact Henry M. Walker at walker@cs.grinnell.edu. |