CODEWITHSUNDEEP
bashshell
user3809572
user3809572

Reputation: 41

Understanding exec command

Looking for some basic help in shell programming.

Suppose we have a command known as foobar, then what is the effect of shell invocation

  1. exec foobar
  2. exec 2> /var/log/foobar.log

Upvotes: 0

Views: 132

Answers (2)

Jonathan Leffler
Jonathan Leffler

Reputation: 754090

The first exec command should only be used in a script — not at a command line terminal. It replaces the shell with the program foobar, instead of running it as a separate child process. Any commands in the script after the exec foobar will not be executed (even if the shell fails to find foobar to execute); if it is an interactive terminal session, it will report the error and continue.

exec [-cl] [-a name] [command [arguments]]

If command is supplied, it replaces the shell without creating a new process. If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what the login program does. The -c option causes command to be executed with an empty environment. If -a is supplied, the shell passes name as the zeroth argument to command. If command cannot be executed for some reason, a non-interactive shell exits, unless the execfail shell option is enabled. In that case, it returns failure. An interactive shell returns failure if the file cannot be executed.

The second exec (with I/O redirection but no command) changes things so that the standard error stream goes to the file /var/log/foobar.log. Any further error messages from the shell, or from commands executed by the shell, go to the log file (unless there's another lot of I/O redirection).

If no command is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero.

Upvotes: 3

shiv
shiv

Reputation: 1952

exec foobar

will replace your shell process with foobar. I do not think you mean exec 2>/var/log/foobar.log but rather exec foobar 2>/var/log/foobar.log. This will do the same with sending 2 i.e. standard error messages to specified log file. You can read man page here.

exec(1) command is similar to exec(3) call. It replaces the code segment of calling process from that of called program. 1 and 3 signify man page sections.

Upvotes: 1

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