Linux commands will be executed

Why does the «at» command always warn me that commands will be executed via sh?

It serves as a good warning to those of us that don’t use bash as our shell, because we we’ll forget that a feature that’s in our day-to-day shell isn’t going to be available when this code is run at the appointed time.

username@hostname$ at 23:00 warning: commands will be executed using /bin/sh at> rm **/*.pyc at> job 1 at 2008-10-08 23:00 

The use of ‘**’ there is perfectly valid zsh, but not /sbin/sh! It’s easy to make these mistakes if you’re used to using a different shell, and it’s your responsibility to remember to do the right thing.

@Spudd86 — Yes, but bash emulates Bourne with it’s called as /bin/sh, so even if /bin/sh is just a symlink it still behaves differently than «normal» Bash.

Does the warning have any harmful effect aside from being annoying? The man page doesn’t mention any way of turning it off, so I don’t think you can stop it from being emitted without rebuilding your at from source.

Now, if you want to just not see it, you can use at [time] 2>/dev/null to send it off to oblivion, but, unfortunately, the at> prompts are printed to STDERR for some reason (a bug, IMO — they really should go to STDOUT), so they’re also hidden by this.

It may be possible to work up some shell plumbing which will eliminate the warning without also eating the prompts, but

1. my attempt at this ( at [time] 2>&1 | grep -v warning ) doesn’t work and
2. even if you can find a combination that works, it won’t be suitable for aliasing (since the time goes in the middle rather than at the end), so you’ll need to either type it in full each time you use it or else write a wrapper script around at to handle it.

So, unless it causes actual problems, I’d say you’re probably best off just ignoring the warning like the rest of us.

Источник

Combine and Execute Multiple Linux Commands

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1. Overview

In this tutorial, we’ll see the different ways in which we can combine and execute multiple Linux commands efficiently. We’ll be using Bash for our examples, so there could be slight differences with other shells.

2. Why Combine Multiple Commands?

Executing commands one after the other in a command line is a regular activity for a Linux administrator. But, by doing so, we may face the following issues:

• The first command can take a long time to complete – one has to wait until then to run the second command
• We may miss a particular command when multiple commands are run one-by-one
• We may misspell a particular command, which may lead to unintended consequences
• It’s time-consuming and annoying to run multiple commands one-by-one

To prevent such situations and obtain the expected result, we can combine and execute multiple commands in the command line.

3. Concatenate Commands With “;”

The “;” operator executes all commands regardless of whether the previous ones failed or not.

After logging into the server, we can execute the following commands:

• Change to logs directory (using cd)
• List the latest ten log files (using ls and head)
• Print the disk usage of logs files (using du)
• Print the disk usage of all file systems (using df)

Nevertheless, we can concatenate all of the above tasks in a single line using the “;“ operator:

$ cd logs; ls -lt | head; du -sh ; df -h

However, we need to keep in mind that if one command in the command chain fails, the remaining commands will execute anyways. And this may produce unexpected outputs.

4. Concatenate Commands Conditionally

The above approach is not suitable for all situations. Such as executing commands based on the success or failure of the previous one. In such cases, we can use one of the following approaches.

4.1. Concatenate Commands With “&&“

The “&&” or AND operator executes the second command only if the preceding command succeeds.

Let’s say we’re in the home directory of the server. We have a particular folder archive_old, which has old and unnecessary files that consume a lot of disk space. Therefore, we’ll need to remove the contents of that folder to free disk space.

To do that, we can execute the following commands one-by-one:

This works fine if the folder name is correct. But, let’s assume that we misspelled the folder name accidentally, then the required directory won’t exist. Consequently, we’ll be still working in the home directory. Then, the rm command will execute even though the cd command failed. As a result, all contents of the home directory will be removed. This would be fatal.

Let’s see what happens in such a situation when we replace the “;“ operator with the “&&” operator:

This way the rm command won’t be executed. This is because the “&&” operator executes the rm command only if the cd command succeeds.

4.2. Concatenate Commands With “||”

The “||” or OR operator executes the second command only if the precedent command returns an error.

Let’s assume we’re creating a new shell script to archive log files. Before executing a new shell script, we need to make it executable. Let’s see how we can achieve this using the “||” operator.

First, we’ll check if it is executable, if it’s not we’ll make it executable using the chmod command:

$ [ -x "archive_logs.sh" ] || chmod +x archive_logs.sh

In this example, the chmod command executes only if the file archive_logs.sh is not executable.

5. Group Commands

In Bash, we can use both “ ” and “( )” operators to group commands.

$ < echo "Hi there"; pwd; uptime; date; >Hi there /tmp/test/baeldung 20:27:11 up 30 days, 5:36, 1 user, load average: 0.26, 0.59, 0.77 Wed 19 Aug 2020 08:27:11 PM CEST 

$ ( echo "Hi there"; pwd; uptime; date ) Hi there /tmp/test/baeldung 20:34:05 up 30 days, 5:43, 1 user, load average: 0.97, 0.86, 0.81 Wed 19 Aug 2020 08:34:05 PM CEST 

$ VAR="1"; < VAR="2"; echo "Inside group: VAR=$VAR"; >; echo "Outside: VAR=$VAR" Inside group: VAR=2 Outside: VAR=2 

$ VAR="1"; ( VAR="2"; echo "Inside group: VAR=$VAR" ); echo "Outside: VAR=$VAR" Inside group: VAR=2 Outside: VAR=1 

$ pwd; < cd /etc; pwd; >; pwd /tmp/test/baeldung /etc /etc $ pwd; ( cd /etc; pwd ); pwd /tmp/test/baeldung /etc /tmp/test/baeldung

$ ( echo "Hi there"; pwd; uptime; date ) > /tmp/output $ cat /tmp/output Hi there /tmp/test/baeldung 23:07:59 up 30 days, 8:17, 1 user, load average: 1.55, 1.95, 2.09 Wed 19 Aug 2020 11:07:59 PM CEST

$ ping -c1 "some.website" 1>/dev/null 2>&1 || ( sendSMS && writeLog )

$ alias sendSMS='echo "sending SMS..(site is down!)..DONE"' $ alias writeLog='echo "writing logs..(site is down!)..DONE"' 

$ ping -c1 "site.can.never.reach" 1>/dev/null 2>&1 || ( sendSMS && writeLog ) sending SMS..(site is down!)..DONE writing logs..(site is down!)..DONE $ ping -c1 "www.google.com" 1>/dev/null 2>&1 || ( sendSMS && writeLog )

$ ping -c1 "site.can.never.reach" 1>/dev/null 2>&1 || sendSMS && writeLog sending SMS..(site is down!)..DONE writing logs..(site is down!)..DONE $ ping -c1 "www.google.com" 1>/dev/null 2>&1 || sendSMS && writeLog writing logs..(site is down!)..DONE 

$ date; ./execute_backup_db.sh; date & > execute_backup_db.log

$ date && ./execute_backup_db.sh && date & > execute_backup_db.log

$ (date ; ./execute_backup_db.sh ; date) & > execute_backup_db.log