Within a Bash script, how to judge whether its STDERR is redirected to a file in Bash on Linux? For example, ./script.sh /tmp/log 2>&1 Can script.sh detect that its STDERR is redirected? Knowing the destination file is better. To test whether a script’s STDERR (or STDOUT) is redirected to a file, check by [[ -f […]
Posts tagged Command
104 posts in total.
diff alternative on Windows to find differences between 2 files?
diff on Linux is a very handy tool. Any good diff alternatives on Windows to find differences between 2 files? A GUI program will be better on Windows. I would recommend meld for Windows users. Meld is a visual diff and merge tool open source software. meld is available on Linux, Windows and Mac OS […]
How to make sort using multiple threads to run faster?
sort supports –parallel N to run N thread. However, it seems it only uses around 100% CPU as I observed although the command specified that N threads can be used. The command is as follows cat large-file | sort –parallel `nproc` where I have 16 from nproc. How to make sort use multiple threads to […]
How to handle spaces in paths with rsync on Linux?
The common rsync commands seems not handle spaces well. For example, rsync -avxP file “user@server:/data/my dir” It reports: rsync: link_stat “/home/zma/file” failed: No such file or directory (2) How to make rsync handle spaces well? You can use the –protect-args option of rsync. $ rsync –protect-args -avxP file “user@server:/data/my dir” What does –protect-args do: -s, […]
How to allow root access to NFS?
On local filesystems, root usually has full access (read/write) to directories/files inside of it. But for NFS directory mounted from nework, root usually has no permission to write to it. How to make root act as on local directory for NFS directory? The reason that NFS directory is non-accessible to root is likely “root_squash”. It […]
In Vim, are there shortcuts to set tab widths to 2 and 4?
Tab widths may be different for different languages. Even with a common language, the tab width settings can be different for different projects. In Vim, are there shortcuts to set tab widths to 2 and 4? To quickly change the tab widths when changing projects. I am afraid no. But you create shortcuts for setting […]
How to grep a tab on Linux?
How to grep a tab on Linux? Including ‘t’ seems not working. grep can grep tabs. The problem is likely the tab is not passed to grep. If you are using Bash, you can use ANSI-C quoting to pass the “tab” to grep: $ grep $’t’ file.txt Or, use Perl-style regex (only for GNU grep) […]
How to check whether a function has been defined in Bash?
I would like to know whether a function, say f1(), has already been defined in Bash. How should I do it? You can use the type -t to get a string of the type of the name: If the -t option is used, type prints a string which is one of alias, keyword, function, builtin, […]
How to disable an option with a bash script?
It is possible to set an option inside a Bash script such as set -o errexit. But how to do the reverse thing, that is, to unset an option? Change – to + to do the reverse thing of unseting an option in bash. For example, to unset the errexit option set by set -o […]
How to get the CPU temperatur in command linux on Linux?
Most modern servers or computers have sensors to detect the temperature of various components. On Linux, how to get the CPU core temperatur in command linux? First, make sure the package “lm-sensors” is installed on your Linux and the command sensors works for you. Then, you can use this piece of script to get the […]
How to test whether PATH has a specific directory in it in Bash on Linux?
How to test whether PATH has a specific directory in it in Bash on Linux? For example, how to test whether /usr/local/mytool/bin is in $PATH? Directories in the $PATH variable are separated by :. So you can use a regular expression to test whether the $PATH contains a directory. The code for your specific example […]
How to test a file or directory exists in Python?
In Bash, the [ -f ] and [ -d ] tests can test whether a file or a directory exist. What are the corresponding python method for these tests? For -f: **os.path.isfile(path)** Return True if path is an existing regular file. This follows symbolic links. For -d: **os.path.isdir(path)** Return True if path is an existing […]
How to test whether a user already exist on Linux?
How to test whether a user account, say linuxuser, already exist on Linux? You may make use of id which tries to get user IDs. The Bash code snippet is as follows. user=hello id -u $user >/dev/null 2>&1 if [ “$?” == “0” ]; then echo “user $user already exist.” else echo “user $user doesn’t […]
How to sort lines by length in Linux?
I have a text file with many lines of text. In Linux, how to sort the lines by their length? You can make use of awk together with the sort program like awk ‘{ print length(), $0 | “sort -n” }’ /path/to/text/file Here, we use awk to calculate the text length and sort -n to […]
How to do “contains string” test in Bash?
How to test whether a string $str contains another string $needle in Bash? You can use this piece of Bash script: [[ “$str” == *”$needle”* ]] A usage example: $ str=”abcde hello” $ needle1=”deh” $ needle2=”de hello” $ [[ “$str” == *”$needle1″* ]] && echo “matched” $ [[ “$str” == *”$needle2″* ]] && echo “matched” […]
‘dd’ command cannot support calculation for its parameters
$ dd if=/dev/zero of=./4Ktest100M bs=4KB count=25000*9 dd: invalid number `25000*9′ I think ‘dd’ should support calculation for its parameters like ‘bs’, ‘count’ and so on. Answered by harryxiyou. You can get the effect by using some other tools/commands, like dd if=/dev/zero of=./4Ktest100M bs=4KB count=$((25000*9)) or dd if=/dev/zero of=./4Ktest100M bs=4KB count=$(bc <<< 25000*9) I think it […]
How to attach and mount Xen DomU’s disk partitions on Linux without Xen?
How to attach and mount Xen DomU’s disk partitions on Linux without Xen? You can use kpartx to create devices from the VM disk image (say, ./vmdisk0). To activate all the partitions in a raw VM disk image: # kpartx -av ./vmdisk0 This will output lines such as: add map loop1p1 (253:8): 0 497664 linear […]
How to use the xargs argument twice in the command on Linux?
How to use the xargs argument twice in the command on Linux? For example, to rename file to file.bak where file is from the stdin: xargs mv $file $file.bak One solution is to write a small script like move.sh file=$1 mv $file $file.bak and invoke move.sh by xargs xargs move.sh However, it is very inconvenient […]
Why does ; after & lead to unexpected token error in bash on Linux?
The command using ‘;’ after ‘&’ like ssh host1 hostname &; ssh host2 hostname & Leads to error like bash: syntax error near unexpected token `;’ Why does ; after & lead to unexpected token error in bash on Linux? And what’s the solution? The fix The quick fix is to change your command to […]
How to manually kill HDFS DataNodes?
stop-dfs.sh report that there are no datanodes running on some nodes like hdfs-node-000208: no datanode to stop However, there are DataNode process running there. How to clean these processes on many (100s) of nodes? You may use this piece of bash script: for i in `cat hadoop/etc/hadoop/slaves`; do echo $i; ssh $i ‘jps | grep […]