Owner's manual

In the following example, the shell runs each command only if the previous command has executed

successfully:

$ cmd1 && cmd2 && cmd3 && cmd4 && cmd5

If cmd1 succeeds, the shell runs cmd2. If cmd2 succeeds, the shell runs cmd3, and so on through

the series until a command fails or the last command has been run. (If any command fails, the shell

stops executing the command line).

In the following example, the shell runs each command only if the previous command has failed:

$ cmd1 || cmd2

If cmd1 fails, the shell runs cmd2. If cmd1 succeeds, the shell stops executing the command line.

For example, suppose that the command mysort runs a sorting program that creates a temporary

file (mysort.tmp) during its sorting process. When mysort finishes successfully, it deletes the

mysort.tmp temporary file. If, on the other hand, mysort fails, it may neglect to delete

mysort.tmp. To ensure deletion of mysort.tmp, use the following command line:

$ mysort || rm mysort.tmp

The second command, which removes mysort.tmp, executes only if mysort fails.

Using Pipes and Filters

A pipe is a one-way connection between two related commands. One command writes its output

to the pipe, and the other command reads its input from the pipe. When two or more commands

are connected by the | (pipe) operator, they form a pipeline. The output of the first command is

the input for the second command; the output of the second command is the input for the third

command, and so on.

A filter is a command that reads its standard input file, transforms that input, and then writes the

transformed input to the standard output file. Filters are typically used as intermediate commands

in pipelines; that is, they are connected by a | (pipe) operator. For example,

ls -R | sort

causes the ls command to list recursively (-R) the contents of all directories from the current

directory to the bottom of the hierarchy; the sort command sorts the results alphabetically and

displays the alphabetized list on the screen. The sort command is a filter because it alphabetizes

the list created by the ls -R command and displays the modified (alphabetized) list on the screen.

Certain commands that are not filters have a flag that causes them to act like filters. For example,

the diff (compare files) command ordinarily compares two files and writes their differences to

the standard output file. The usual format for diff follows:

diff file1 file2

However, if you use the - (dash) flag in place of one of the filenames, diff reads the standard

input file and compares it to the named file.

In the following pipeline, ls writes the contents of the current directory to the standard output file.

The diff command compares the output of ls with the contents of a file named dirfile and

then writes the differences to the standard output file (with the more command):

$ ls | diff - dirfile | more

In the following example, another kind of filter program (grep) is used:

$ ls -l | grep r-x | wc -l

In this example, the following takes place:

1. The ls -l command lists in long format the contents of the current directory.

2. The output of ls -l becomes the input to grep r-x, which acts as a filter that searches the

files in its standard input file for patterns with permissions of r-x, and writes all lines that

contain the pattern to its standard output file.

36 The OSS Shell