Project84.A Bash and Tcsh Reference

Project 84. A Bash and Tcsh Reference

"What's the correct syntax for . . . ?"

This project looks at the syntax of common shell commands such as variable assignment, redirection, and shell scripting statements. It shows the syntax for Bash and Tcshthe two shells that are used most often in Mac OS X Unix.

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Project 5 compares the various shell flavors.

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Project 4 covers shell variables and environment variables, and how they differ.

Set Variables

Table 9.2 shows you how to set shell variables and environment variables.

Table 9.2. Setting Variables

Set	Bash	Tcsh
Shell variable	variable=value	set variable = value
Environment variable	ENVVAR=value; export ENVVAR	setenv ENVVAR value
Environment variable (Bash only)	declare -x ENVVAR=value

Redirection and Pipelining

Table 9.3 shows the syntax employed by both shells to express redirection and pipelining.

Table 9.3. Syntax for Redirection and Pipelining

Redirect or Pipe	Bash	Tcsh
stdout	cmd > file	cmd > file
stderr	cmd 2> file	(cmd >/dev/tty) >& file
stdout appending	cmd >> file	cmd >> file
stderr appending	cmd 2>> file	(cmd > /dev/tty) >>& file
stdout with clobber	cmd >| file	cmd >! file
stderr with clobber	cmd 2>| file	(cmd > /dev/tty) >&! file
Both to same file	cmd &> file	cmd >& file
Both to different files	cmd > out 2> err	(cmd > out) >& err
Merge stdout into stderr	cmd 1>&2	n/a
Merge stderr onto stdout	cmd 2>&1	n/a
stdin	cmd < file	cmd < file
Pipe stdout	cmd1 | cmd 2	cmd1 | cmd2
Pipe both	cmd1 2>&1 | cmd2	cmd1 |& cmd2

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Project 6 covers the concepts of redirection and pipelining.

Tee Time

To see the output of a command onscreen and redirect it to a file, use the tee command.

$ ls Sites | tee list.txt images index.html $ cat list.txt images index.html

To redirect to multiple files, just type the names of the files as arguments. Apply option -a to append to the output files rather than overwrite them.

Startup Files

The following script files are executed by the Bash shell when it starts up. For login shells (or shells started with the command bash --login), they are

• /etc/profile

• ~/.bash_profile

Note

Noninteractive Bash shells do not execute any startup files.

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Project 47 covers the shell startup sequence.

For non-login shells, they are

• /etc/bashrc (though the Bash manual claims otherwise)

• ~/.bashrc

The following script files are executed by the Tcsh shell when it starts up. For login shells (or shells started with the command tcsh -l), they are

• /etc/csh.cshrc

• /etc/csh.login

• ~/.tcshrc

• ~/.login

For non-login shells, they are

• /etc/csh.cshrc

• ~/.tcshrc

Control Constructs

Syntax for each Bash and Tcsh control construct is illustrated in the following examples. All the scripts actually work, so you can play around with them.

The if Construct

#!/bin/bash if [ "$1" = "positive" ]; then   echo "Yes" elif [ "$1" = "negative" ]; then   echo "No" else   echo "Not sure" fi #!/bin/tcsh if ("$1" == "positive") then   echo "Yes" else if ("$1" == "negative") then   echo "No" else   echo "Not sure" endif

The case/switch Construct

#!/bin/bash case "$1" in   "positive")     echo "Yes"   ;;   "negative")     echo "No"   ;;   *)     echo "Not sure"   ;; esac #!/bin/tcsh switch ("$1")   case "positive":     echo "Yes"   breaksw   case "negative":     echo "No"   breaksw   default:     echo "Not sure"   breaksw endsw

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Project 10 gives examples of control constructs in a shell script.

The for Loop

#!/bin/bash for word in hello goodbye au-revoir; do   echo $word done #!/bin/tcsh foreach word (hello goodbye au-revoir)     echo $word end

The while Loop

#!/bin/bash n=0 while [ ! $n = 10 ]; do   echo $n   n=$(expr $n + 1) done #!/bin/tcsh set n = 0 while ($n != 10)   echo $n   set n = `expr $n + 1` end